respectively. Levels of serum TC, TG and LDL-c were slightly decreased and HDL-c level was a little higher by the LAB suspension treating on the normal mice. However, compared to the HFC group, levels of TC and TG were decreased extremely in lab2 + HFD and MB + HFD groups. The finding indicates that the two strains might decrease the risks for cardiovascular and arteriosclerosis diseases in various degrees, and it also could fall significantly the cholesterol level in hyperlipidemic mice. Akalin *et al.* (1997) found that consumption of acidophilus yogurt significantly lowered the values for plasma TC, LDL-c in the mice. After the male SD rats were fed high-fat diet with *Lactobacillus* ferment, there were significantly decrease on the levels of body weight, LDL-c and TC compared with the highfat diet control rats (Choi *et al.*, 2006). Researchers showed that LAB could decrease the level of cholesterol but the mechanism has not been demonstrated clearly yet. Some predicted that LAB, bile salt and cholesterol were coprecipitated, and then expelled with feces, or the cholesterol was absorpted by lactic acid bacteria (Jeun, *et al*., 2010). Otherwise, Smet *et al*. (1994) suggested that the reason of LAB reducing cholesterol might be due to the activity of bile salt hydrolysis produced by the LAB. Further research is indispensable to clarify the exact mechanism.

190 Antioxidant Enzyme

mice and control mice.

Single-colour histograms represent hepatocyte staining with anti-Nrf2 antibodies; x-axis, DTAF flurescence intensity; y-axis, frequency of cells displaying certain fluorescence intensity.

(A) (B)

(C) (D)

(E) (F)

(G) (H)

**Figure 10.** Flow cytometric analysis of Nrf2 expression in the liver tissues of lactic acid bacteria treated

Several studies have documented the relationships between increase of free radicals and blood glucose, lipid peroxidation as well as low-density lipoprotein (Tanaka et al., 2002). High fat diet could be used to induce signicant oxygen-centered free radicals and ROS generation in the mice. The liver plays a central role in the maintenance of systemic lipid homeostasis and it is especially susceptible to reactive oxygen species (ROSs) damage (Hamelet et al., 2007). Free radicals can diffuse intracellularly and result in mitochondrial enzyme damage and DNA breaks, impair cellular function (Bonnefont-Rousselot et al., 2000). SOD is a scavenger of free radicals, which has important effects on control of oxidation reactions in the body. Some LAB may enhance SOD and GSH-px activities and prevent oxidative damage (Tsai, et al., 2009). GSH is often regarded as an antioxidant agent, since it protects protein –SH groups against oxidation and can scavenge oxygen radicals and some other reactive species. It reduces different oxidants after increasing its hydrogen atom. This reaction is catalysed by enzyme GSH-px in cells (Reiter, 1995). In the research, the concentration of SOD and GSH-px in the high fat diet mice was significantly lower than those of the normal rats. Meanwhile, the activities of SOD and GSH-px were increased in various degrees in Lab1 + ND and lab2 + ND groups compared with the normal control mice after the LAB administering. The levels of SOD and GSH-px in the lab1 + HFD, lab2 + HFD and MB + HFD groups were all increased compared with HFC group, but not achieve the level of the NC group. In this study, the levels of antioxidant enzymes and the function of reducing blood lipid in MB + HFD groups were higher than the other two hyperlipidemic groups. SOD is the most important survival protein and ubiquitously induced antioxidant by various stimulants, so we hypothesized that the strains might play a more crucial role in a severe stressful condition. At the same time, supplementation of Lab also promoted expression of Nrf2 in the liver tissues of the mice. Nrf2 serves as master regular of a cellular defense system against oxidative stress (Motohashi et al., 2004; Nguyen et al., 2004). Under physiological conditions, Nrf2 is sequestered in the cytoplasm by Keap1, which facilitates its ubiquitination and proteasomic degradation (Kang, et al., 2004)Upon exposure to oxidative stress, the sequestration complex breaks down and the dissociated Nrf2 translocates into the nucleus, where it binds to cis-acting antioxidant response elements (AREs) and promotes the transcription of a large number of cytoprotective genes (Kensler et al., 2007; Vries et al., 2008). Nrf2–ARE signaling is also known to be mainly responsible for the upregulation of SOD and GSH-px gene expression and hence constitutes a crucial cellular response to environmental stresses (Surh et al., 2009). L.plantarum is further capable of activation of Nrf2 and preventing HFD-induced inhibition of antioxidant enzymes. Our Flow cytometry data clearly show that high fat diet-induced oxidative stress is associated with activation of Nrf2, as evidenced by a signicant elevation of Nrf2 in the nuclear fractions (P<0.05). Supplementation of L.plantarum markedly promoted further translocation of Nrf2 into the nucleus. Thus, L.plantarum could inhibit HFD-induced oxidative stress through the Nrf2- Keap1 signaling pathway. As expected, L. plantarum isolated from fermented cabbage upregulates antioxidative enzymes in high fat diet mice via Nrf2-dependent transcriptional activation of ARE sites. The upregulation of several antioxidative enzymes is associated with the reduced formation of ROS and enhanced survival of liver cells upon the induction of oxidative stress.

Antioxidant Therapies for Hypolipidemia and Hyperglycemia 193

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### **Author details**

Dawei Gao

*Key Laboratory of Applied Chemistry in Hebei Province, Department of Biological Engineering, Yanshan University, Qinhuangdao, China*

### **Acknowledgement**

This work was financially supported by the research grant from the Chinese Ministry of Education Doctor Degree (No.<PHONE_NUMBER>0011), a grant from Hebei Province Natural Science Fund (No. C2011203137, 11965152D), and a Chinese Postdoctoral grant (480013).

### **5. References**

Belpaeme K., Delbeke K., Zhu L., Kirsch-Volders M.Cytogenetic studies of PCB77 on brown trout (Salmo trutta fario) using the micronucleus test and the alkaline comet assay. Mutagenesis 1996, 11:485–492

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**Author details**

**Acknowledgement**

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*Yanshan University, Qinhuangdao, China*

Dawei Gao

ubiquitination and proteasomic degradation (Kang, et al., 2004)Upon exposure to oxidative stress, the sequestration complex breaks down and the dissociated Nrf2 translocates into the nucleus, where it binds to cis-acting antioxidant response elements (AREs) and promotes the transcription of a large number of cytoprotective genes (Kensler et al., 2007; Vries et al., 2008). Nrf2–ARE signaling is also known to be mainly responsible for the upregulation of SOD and GSH-px gene expression and hence constitutes a crucial cellular response to environmental stresses (Surh et al., 2009). L.plantarum is further capable of activation of Nrf2 and preventing HFD-induced inhibition of antioxidant enzymes. Our Flow cytometry data clearly show that high fat diet-induced oxidative stress is associated with activation of Nrf2, as evidenced by a signicant elevation of Nrf2 in the nuclear fractions (P<0.05). Supplementation of L.plantarum markedly promoted further translocation of Nrf2 into the nucleus. Thus, L.plantarum could inhibit HFD-induced oxidative stress through the Nrf2- Keap1 signaling pathway. As expected, L. plantarum isolated from fermented cabbage upregulates antioxidative enzymes in high fat diet mice via Nrf2-dependent transcriptional activation of ARE sites. The upregulation of several antioxidative enzymes is associated with the reduced formation of ROS and enhanced survival of liver cells upon the induction

*Key Laboratory of Applied Chemistry in Hebei Province, Department of Biological Engineering,*

This work was financially supported by the research grant from the Chinese Ministry of Education Doctor Degree (No.<PHONE_NUMBER>0011), a grant from Hebei Province Natural Science Fund (No. C2011203137, 11965152D), and a Chinese Postdoctoral grant (480013).

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**Chapter 8**

© 2012 Petrulea et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons. org/licenses/by/3. 0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

**Oxidative Stress and Antioxidant**

Mirela Petrulea, Adriana Muresan and Ileana Duncea

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51018

oxidative stress develops [7].

**1. Introduction**

DNA [5].

**Status in Hypo- and Hyperthyroidism**

Thyroid hormones are involved in the regulation of basal metabolic state and in oxidative metabolism [1]. They can cause many changes in the number and activity of mitochondrial respiratory chain components. This may result in the increased generation of reactive oxygen species (ROS) [2,3]. Oxidative stress is a general term used to describe a state of damage caused by ROS [4]. ROS have a high reactivity potential, therefore they are toxic and can lead to oxidative damage in cellular macromolecules such as proteins, lipids and

In fact, the cell contains a variety of substances capable of scavenging the free radicals, protecting them from harmful effects. Among the enzymatic antioxidants, are glutathione reductase (GR), glutathione peroxydase (GPx), catalase (CAT), superoxide dismutase (SOD), while the non-enzymatic antioxidants are glutathione (GSH), vitamin E, vitamin C, βcarotene, and flavonoids [6]. When ROS generation exceeds the antioxidant capacity of cells,

Life means a continuous struggle for energy, which is required to fight against entropy. The most effective way to obtain energy is oxidation. Oxidative processes predominantly occur in mitochondria [8]. On the other hand, mitochondria are the favorite targets of thyroid hormones. During thyroid hormone synthesis, there is a constant production of oxygenated water, which is absolutely indispensable for iodine intrafollicular oxidation in the presence of thyroid peroxidase. In recent years, the possible correlation between impaired thyroid gland

Experimental studies and epidemiological data suggest that hyperthyroidism is associated

In hypothyroidism, a decrease in free radical production is expected because of the metabolic supression brought about by the decrease in thyroid hormone levels [12,13,14].

and reproduction in any medium, provided the original work is properly cited.

function and reactive oxygen species has been increasingly taken into consideration [9].

with increase in free radical production and lipid peroxide levels [10,11].

## **Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism**

Mirela Petrulea, Adriana Muresan and Ileana Duncea

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51018

### **1. Introduction**

196 Antioxidant Enzyme

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*lucidum Ait*. Chinese Traditional Patent Medicine 1993,15: 18-19

Thyroid hormones are involved in the regulation of basal metabolic state and in oxidative metabolism [1]. They can cause many changes in the number and activity of mitochondrial respiratory chain components. This may result in the increased generation of reactive oxygen species (ROS) [2,3]. Oxidative stress is a general term used to describe a state of damage caused by ROS [4]. ROS have a high reactivity potential, therefore they are toxic and can lead to oxidative damage in cellular macromolecules such as proteins, lipids and DNA [5].

In fact, the cell contains a variety of substances capable of scavenging the free radicals, protecting them from harmful effects. Among the enzymatic antioxidants, are glutathione reductase (GR), glutathione peroxydase (GPx), catalase (CAT), superoxide dismutase (SOD), while the non-enzymatic antioxidants are glutathione (GSH), vitamin E, vitamin C, βcarotene, and flavonoids [6]. When ROS generation exceeds the antioxidant capacity of cells, oxidative stress develops [7].

Life means a continuous struggle for energy, which is required to fight against entropy. The most effective way to obtain energy is oxidation. Oxidative processes predominantly occur in mitochondria [8]. On the other hand, mitochondria are the favorite targets of thyroid hormones. During thyroid hormone synthesis, there is a constant production of oxygenated water, which is absolutely indispensable for iodine intrafollicular oxidation in the presence of thyroid peroxidase. In recent years, the possible correlation between impaired thyroid gland function and reactive oxygen species has been increasingly taken into consideration [9].

Experimental studies and epidemiological data suggest that hyperthyroidism is associated with increase in free radical production and lipid peroxide levels [10,11].

In hypothyroidism, a decrease in free radical production is expected because of the metabolic supression brought about by the decrease in thyroid hormone levels [12,13,14].

© 2012 Petrulea et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons. org/licenses/by/3. 0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The changes in the levels of the scavengers α-tocopherol, glutathione [15] and coenzyme Q[16] and activities of antioxidant enzymes [11]were found to be imbalanced and often opposite.

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 199

ageing. They in turn are a cause of development of inflammatory processes, oncogenesis

Oxidative stress is considered to play a pivotal role in the pathogenesis of aging and several degenerative diseases, such as atherosclerosis, cardiovascular disease, type 2-diabetes and cancer [30,31,32]. In order to cope with an excess of free radicals produced upon oxidative stress, humans have developed sophisticated mechanisms in order to maintain redox

These protective mechanisms either scavenge or detoxify ROS, block their production, or sequester transition metals that are the source of free radicals, and include enzymatic and non enzymatic antioxidant defenses produced in the body, namely, endogenous[34,35], and

Antioxidant enzymes act to scavenge free radicals by converting them to less harmful molecules [38]. Among the most known enzymatic antioxidants, we notice superoxide dismutase (SOD), glutathione reductase (GR), glutathione peroxydase (GPx) and catalase (CAT). SOD catalyzes the dismutation of superoxide anion radical to peroxide (H2O2) and molecular oxygen (O2). Catalase, an iron-containing hemoprotein, converts hydrogen peroxide to water and oxygen [39]. GPx is an enzyme containing a selenium ion as a cofactor [40], and for the catalyzed reaction it requires reduced glutathione (GSH), which is provided by glutathione reductase. GPx is one of the most effective antioxidants in erythrocytes. A reduction in GPx activity results in increased H2O2 levels and hence severe cellular damage

Non-enzymatic antioxidants, such as glutathione, tocopherols, retinols, and ascorbate, play

The levels of H2O2 reached physiologically in cells vary from a low 0. 001 μM to a maximum of 0. 7 μM. When H2O2 is applied to the exterior of cultured cells, the intracellular concentrations are approximately 10-fold lower than the extracellular concentrations [42, 43]. Because there are great variations in the rate of H2O2 degradation in different cell types and models, it is difficult to compare concentration-effect relations. In most cell cultures, H2O2 in the medium disappears in less than 1 h. At higher concentrations than those that have a signaling role, H2O2 induces oxidative stress, DNA oxidation and damage, and consequent mutagenesis and apoptosis [42]. For the phagocytes, H2O2 has been designated as "the enemy within" [44]. Oxidative stress involves the oxidation of various cellular components, proteins, lipids, nucleic acids, etc*.* The accumulation of oxidatively damaged proteins accelerates chaperone-mediated autophagy, which will degrade them [45]. Oxidative damage to DNA produces adducts (including 8-oxo-deoxyguanosine and thymine glycol), single-strand breaks, and at high levels double-strand breaks [46]. Positive Comet assays demonstrate these breaks. The half-life of these damages varies for the various lesions (from 9–62 min for the adducts, more for the breaks) [47]. The positive Comet assays

and impaired organ functioning [28, 29].

others supplied with the diet, namely exogenous[36,37].

homeostasis [33].

is observed [41].

an important role in scavenging ROS.

**3. Toxic effects of H2O2**

It is worth mentioning that some of the antithyroid drugs have antioxidant effects[17]. It was shown that both methimazole and propylthiouracil abolished or reduced radical production by complement attacked thyroid cells and decreased cytokine production[18].

Antioxidants treatments might be helpful in reducing the oxidative damage due to hypothyroidism and hyperthyroidism.

The available data concerning oxidative stress in both hypothyroidism and hyperthyroidism are scarce and controversial. Reviewing the most recent data on the subject, this study aims at investigating oxidative stress parameters, antioxidant status markers and their response to vitamin E supplementation in hyper- and hypothyroid rats.

### **2. Oxidative stress**

A major threat to homeostasis and therefore to the integrity of aerobic organisms arises from chemical species possessing one or more unpaired electrons in their outer orbital, called free radicals [19]. Oxygen free radicals can develop during several steps of normal metabolic events. Although free radicals (FR) have the potential to damage the organism, their generation is inevitable for some metabolic processes. The main endogenous sources of free radicals are the mitochondrial electron transport chain, the microsomal membrane electron transport chain, reactions of oxidant enzymes and auto-oxidation reactions [20,21,22].

Oxidative stress is a term that was introduced by Sies in 1985 and refers to any situation where there is a serious imbalance between the production of FR or reactive oxygen species (ROS), called the oxidative load, and the antioxidant defense system. The oxidative load is described as "a measure of the steady-state level of reactive oxygen or oxygen radicals in a biological system"[23]. Oxidative stress has been defined as "a disturbance in the prooxidant-antioxidant balance in favour of the former, leading to potential damage"[24].

Cells can tolerate moderate oxidative loads by increasing gene expression to up-regulate their reductive defense systems and restore the oxidant/antioxidant balance. But when this increased synthesis cannot be achieved due to damage to enzymes, or substrate limitations, or when the oxidative load is overwhelming, an imbalance persists and the result is oxidative stress [25]. Superoxide and hydroxyl radicals, along with non-radical oxygen species such as hydrogen peroxide (H2O2) are commonly termed reactive oxygen species (ROS) and have the highest biological activity. ROS are produced in all cells, depending on the intensity of aerobic metabolism, especially in activated neutrophils, monocytes, smooth muscle cells and in endothelial cells [26]. Disequilibrium between ROS production and inactivation leads to oxidative stress. ROS also cause injury to the basic cell structures. They readily react with macromolecules, such as lipid, protein and DNA molecules, which results in degradation of cell membranes and excessive activation or inactivation of enzymes[27]. The ultimate effects of ROS activity include mutations, metabolic dysfunction and cell ageing. They in turn are a cause of development of inflammatory processes, oncogenesis and impaired organ functioning [28, 29].

Oxidative stress is considered to play a pivotal role in the pathogenesis of aging and several degenerative diseases, such as atherosclerosis, cardiovascular disease, type 2-diabetes and cancer [30,31,32]. In order to cope with an excess of free radicals produced upon oxidative stress, humans have developed sophisticated mechanisms in order to maintain redox homeostasis [33].

These protective mechanisms either scavenge or detoxify ROS, block their production, or sequester transition metals that are the source of free radicals, and include enzymatic and non enzymatic antioxidant defenses produced in the body, namely, endogenous[34,35], and others supplied with the diet, namely exogenous[36,37].

Antioxidant enzymes act to scavenge free radicals by converting them to less harmful molecules [38]. Among the most known enzymatic antioxidants, we notice superoxide dismutase (SOD), glutathione reductase (GR), glutathione peroxydase (GPx) and catalase (CAT). SOD catalyzes the dismutation of superoxide anion radical to peroxide (H2O2) and molecular oxygen (O2). Catalase, an iron-containing hemoprotein, converts hydrogen peroxide to water and oxygen [39]. GPx is an enzyme containing a selenium ion as a cofactor [40], and for the catalyzed reaction it requires reduced glutathione (GSH), which is provided by glutathione reductase. GPx is one of the most effective antioxidants in erythrocytes. A reduction in GPx activity results in increased H2O2 levels and hence severe cellular damage is observed [41].

Non-enzymatic antioxidants, such as glutathione, tocopherols, retinols, and ascorbate, play an important role in scavenging ROS.

### **3. Toxic effects of H2O2**

198 Antioxidant Enzyme

opposite.

hypothyroidism and hyperthyroidism.

**2. Oxidative stress**

to vitamin E supplementation in hyper- and hypothyroid rats.

The changes in the levels of the scavengers α-tocopherol, glutathione [15] and coenzyme Q[16] and activities of antioxidant enzymes [11]were found to be imbalanced and often

It is worth mentioning that some of the antithyroid drugs have antioxidant effects[17]. It was shown that both methimazole and propylthiouracil abolished or reduced radical production by complement attacked thyroid cells and decreased cytokine production[18].

Antioxidants treatments might be helpful in reducing the oxidative damage due to

The available data concerning oxidative stress in both hypothyroidism and hyperthyroidism are scarce and controversial. Reviewing the most recent data on the subject, this study aims at investigating oxidative stress parameters, antioxidant status markers and their response

A major threat to homeostasis and therefore to the integrity of aerobic organisms arises from chemical species possessing one or more unpaired electrons in their outer orbital, called free radicals [19]. Oxygen free radicals can develop during several steps of normal metabolic events. Although free radicals (FR) have the potential to damage the organism, their generation is inevitable for some metabolic processes. The main endogenous sources of free radicals are the mitochondrial electron transport chain, the microsomal membrane electron

transport chain, reactions of oxidant enzymes and auto-oxidation reactions [20,21,22].

Oxidative stress is a term that was introduced by Sies in 1985 and refers to any situation where there is a serious imbalance between the production of FR or reactive oxygen species (ROS), called the oxidative load, and the antioxidant defense system. The oxidative load is described as "a measure of the steady-state level of reactive oxygen or oxygen radicals in a biological system"[23]. Oxidative stress has been defined as "a disturbance in the prooxidant-antioxidant balance in favour of the former, leading to potential damage"[24].

Cells can tolerate moderate oxidative loads by increasing gene expression to up-regulate their reductive defense systems and restore the oxidant/antioxidant balance. But when this increased synthesis cannot be achieved due to damage to enzymes, or substrate limitations, or when the oxidative load is overwhelming, an imbalance persists and the result is oxidative stress [25]. Superoxide and hydroxyl radicals, along with non-radical oxygen species such as hydrogen peroxide (H2O2) are commonly termed reactive oxygen species (ROS) and have the highest biological activity. ROS are produced in all cells, depending on the intensity of aerobic metabolism, especially in activated neutrophils, monocytes, smooth muscle cells and in endothelial cells [26]. Disequilibrium between ROS production and inactivation leads to oxidative stress. ROS also cause injury to the basic cell structures. They readily react with macromolecules, such as lipid, protein and DNA molecules, which results in degradation of cell membranes and excessive activation or inactivation of enzymes[27]. The ultimate effects of ROS activity include mutations, metabolic dysfunction and cell The levels of H2O2 reached physiologically in cells vary from a low 0. 001 μM to a maximum of 0. 7 μM. When H2O2 is applied to the exterior of cultured cells, the intracellular concentrations are approximately 10-fold lower than the extracellular concentrations [42, 43]. Because there are great variations in the rate of H2O2 degradation in different cell types and models, it is difficult to compare concentration-effect relations. In most cell cultures, H2O2 in the medium disappears in less than 1 h. At higher concentrations than those that have a signaling role, H2O2 induces oxidative stress, DNA oxidation and damage, and consequent mutagenesis and apoptosis [42]. For the phagocytes, H2O2 has been designated as "the enemy within" [44]. Oxidative stress involves the oxidation of various cellular components, proteins, lipids, nucleic acids, etc*.* The accumulation of oxidatively damaged proteins accelerates chaperone-mediated autophagy, which will degrade them [45]. Oxidative damage to DNA produces adducts (including 8-oxo-deoxyguanosine and thymine glycol), single-strand breaks, and at high levels double-strand breaks [46]. Positive Comet assays demonstrate these breaks. The half-life of these damages varies for the various lesions (from 9–62 min for the adducts, more for the breaks) [47]. The positive Comet assays for thyroid cells incubated with 50 μM H2O2 disappear by 80% in 2 h [48]. Mutagenesis, if it leads to constitutive activation of a protooncogene or to inactivation of tumor suppressor genes is carcinogenic, especially if it is combined to a proliferative effect. Thus, H2O2 is carcinogenic and has been found to play a role in several human cancers (7) even if it may not be sufficient [49]. Conversely, selenium, the essential constituent of protective enzymes, prevents tumor development in rats submitted to chemical carcinogenesis [50]. Lack of protective systems in knockout mice such as lack of peroxiredoxin or glutathione (GSH) peroxidases indeed leads to malignant cancers [51,52]. Transfection of an H2O2-generating system transforms epithelial cells [53]. High-level acute H2O2 treatment of various cells in vitro leads to apoptosis [54]. This effect has been linked to a loss of GSH and reduced glutaredoxin and consequent activation of apoptosis signal-regulating kinase (ASK) and of an apoptosis program [55]. These effects are stronger in actively proliferating cells [56]. Chronic H2O2 administration at low levels induces senescence in cultured cells in vitro in human fibroblasts [57,58]. H2O2 favors inflammation [59], and its inhibitory effect on indoleamine dioxygenase, which by depriving lymphocytes of tryptophan is immunosuppressive, would enhance immune reactions. It is therefore not astonishing that even in relatively short-lived (7 h) neutrophils [60] and macrophages, H2O2 generation is tightly regulated by a synergic two-pronged mechanism involving both intracellular calcium and diacylglycerol protein kinase C[58,61].

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 201

membrane of the follicular cell. Iodination (organification) and the coupling reaction of iodotyrosines require the presence of thyroperoxidase (TPO), a hemoprotein located in the apical plasma as well as in the adjacent cytoplasm, endoplasmic reticulum, Golgi complex,

The molecular mechanism of iodination consists of a series of successive stages, having extremely reactive free radicals as intermediate products. Following the addition of oxygenated water (H2O2) to thyroperoxidase (TPO), compound I is formed, which oxidizes iodine (I+), and the active iodine form results: the iodinium ion (I+) or the hypoiodite ion (IO- ). These remain bound to thyroperoxidase. Tyrosine residues also bind to thyroperoxidase, which favors the organification of iodine with the formation of iodotyrosines: monoiodotyrosine (MIT) and diiodotyrosine (DIT). In the absence of iodine, compound I is spontaneously converted into a stable compound, compound II, which catalyzes the coupling reaction of iodotyrosines, resulting in the formation of thyroid hormones. The excess of oxygenated water (H2O2) determines the conversion of compound II to inactive

The generation of oxygenated water, as an electron acceptor, is absolutely indispensable for thyroperoxidase activity [67]. H2O2 is produced by an NADPH-dependent process on the external aspect of the apical plasma membrane of follicular cells. Although various enzyme systems, including cytochrome reductases, can support H2O2 production in the thyroid, an NADPH-dependent, H2O2 –generating system was detected in thyroid particulate fractions that appears to be distinct from cytochrome c reductase. The activation of this NADPH

The mechanism of formation of oxygenated water (H2O2) is controversial; there are two theories: The superoxide anion is the primary product of the enzymatic conversion of oxygen which, under the action of superoxide dismutase (SOD) will be transformed into oxygenated water. The superoxide anion, produced inside the cytoplasm, close to the apical membrane, under the action of NADPH-oxidase, is released outside the thyroid follicular cell only after its transformation into oxygenated water [68]. Other data suggest that oxygenated water is the primary product of the NADPH-oxidase system, and is produced

The production of oxygenated water is stimulated by TSH-cAMP and phosphatidylinositol-Ca2+. Other enzymatic systems capable of generating oxygenated water have been

It is necessary to prevent excess or deficiency of H2O2, anything but optimal levels are linked to several thyroid diseases and disorders, such as congenital hypothyroidism,

Various reports deal with thyroid disorders and H2O2. Normal levels of H2O2 in the body vary from 0. 001 mM to 0. 7 mM, but excess "induces oxidative stress, DNA oxidation and damage, and consequent mutagenesis and apoptosis" [71]. Several selenoproteins act as a protective barrier for thyrocytes from endogenous H2O2 [72]. If DNA damage is

outside the thyroid follicular cell via the two-electron reduction of O2 [69].

evidenced: monoamine oxidase, xanthine oxidase, glucose oxidase [66].

tumorigenesis, myxedematous cretinism, thyroiditis, and cancer [70, 71].

compound III. Inactivation is prevented by iodine [66].

nuclear envelope.

oxidase requires Ca2+ ions.

### **4. Thyroid hormone synthesis**

The thyroid is a shield-shaped organ in the neck region composed of an outer layer of follicular cells and c-cells, which surrounds a lumen that contains colloid. It contributes to the body's energy output by regulation of cardiac rate and output, lipid catabolism, heat production, and skeletal growth [62], which explains the wide range of symptoms related to thyroid abnormalities. The colloid contains thyroglobulin which is converted into thyroid hormone (TH). The luminal side of the follicular cell membrane contains microvilli, which greatly increase the surface area of the cell to facilitate transfer of colloid into the follicular cell.

TH includes thyronine (T3), and thyroxine (T4). T4, and to a lesser extent T3, is synthesized in the follicular cell and is propagated by thyroid stimulating hormone (TSH) secreted by the pituitary gland. TSH synthesis is propagated by thyrotropin releasing hormone (TRH) secreted by the hypothalamus. Several of these processes deal with direct or indirect collaboration between the thyroid, hypothalamus, pituitary, or pineal glands [63, 64].

TH synthesis includes a radical intermediate, creating a need for a ROS reaction as part of the organ's function to maintain homeostasis. Iodination of tyrosine residues, catalyzed by a peroxidase enzyme, occurs on the endoplasmic reticulum of the thyroid gland cells. Coupling forms various THs [65]. H2O2 is required by peroxidase, and is formed by an enzyme from NADPH (nicotinamide adenine dinucleotide phosphate-oxidase) and Ca2+ ions.

Beginning with active transport of dietary iodide (the rate limiting substrate) into the cell by sodium-iodide symporter [65], iodide oxidation and hormone synthesis occur at the apical membrane of the follicular cell. Iodination (organification) and the coupling reaction of iodotyrosines require the presence of thyroperoxidase (TPO), a hemoprotein located in the apical plasma as well as in the adjacent cytoplasm, endoplasmic reticulum, Golgi complex, nuclear envelope.

200 Antioxidant Enzyme

for thyroid cells incubated with 50 μM H2O2 disappear by 80% in 2 h [48]. Mutagenesis, if it leads to constitutive activation of a protooncogene or to inactivation of tumor suppressor genes is carcinogenic, especially if it is combined to a proliferative effect. Thus, H2O2 is carcinogenic and has been found to play a role in several human cancers (7) even if it may not be sufficient [49]. Conversely, selenium, the essential constituent of protective enzymes, prevents tumor development in rats submitted to chemical carcinogenesis [50]. Lack of protective systems in knockout mice such as lack of peroxiredoxin or glutathione (GSH) peroxidases indeed leads to malignant cancers [51,52]. Transfection of an H2O2-generating system transforms epithelial cells [53]. High-level acute H2O2 treatment of various cells in vitro leads to apoptosis [54]. This effect has been linked to a loss of GSH and reduced glutaredoxin and consequent activation of apoptosis signal-regulating kinase (ASK) and of an apoptosis program [55]. These effects are stronger in actively proliferating cells [56]. Chronic H2O2 administration at low levels induces senescence in cultured cells in vitro in human fibroblasts [57,58]. H2O2 favors inflammation [59], and its inhibitory effect on indoleamine dioxygenase, which by depriving lymphocytes of tryptophan is immunosuppressive, would enhance immune reactions. It is therefore not astonishing that even in relatively short-lived (7 h) neutrophils [60] and macrophages, H2O2 generation is tightly regulated by a synergic two-pronged mechanism involving both intracellular

The thyroid is a shield-shaped organ in the neck region composed of an outer layer of follicular cells and c-cells, which surrounds a lumen that contains colloid. It contributes to the body's energy output by regulation of cardiac rate and output, lipid catabolism, heat production, and skeletal growth [62], which explains the wide range of symptoms related to thyroid abnormalities. The colloid contains thyroglobulin which is converted into thyroid hormone (TH). The luminal side of the follicular cell membrane contains microvilli, which greatly increase the surface area of the cell to facilitate transfer of colloid into the follicular cell.

TH includes thyronine (T3), and thyroxine (T4). T4, and to a lesser extent T3, is synthesized in the follicular cell and is propagated by thyroid stimulating hormone (TSH) secreted by the pituitary gland. TSH synthesis is propagated by thyrotropin releasing hormone (TRH) secreted by the hypothalamus. Several of these processes deal with direct or indirect

TH synthesis includes a radical intermediate, creating a need for a ROS reaction as part of the organ's function to maintain homeostasis. Iodination of tyrosine residues, catalyzed by a peroxidase enzyme, occurs on the endoplasmic reticulum of the thyroid gland cells. Coupling forms various THs [65]. H2O2 is required by peroxidase, and is formed by an enzyme from

Beginning with active transport of dietary iodide (the rate limiting substrate) into the cell by sodium-iodide symporter [65], iodide oxidation and hormone synthesis occur at the apical

collaboration between the thyroid, hypothalamus, pituitary, or pineal glands [63, 64].

NADPH (nicotinamide adenine dinucleotide phosphate-oxidase) and Ca2+ ions.

calcium and diacylglycerol protein kinase C[58,61].

**4. Thyroid hormone synthesis**

The molecular mechanism of iodination consists of a series of successive stages, having extremely reactive free radicals as intermediate products. Following the addition of oxygenated water (H2O2) to thyroperoxidase (TPO), compound I is formed, which oxidizes iodine (I+), and the active iodine form results: the iodinium ion (I+) or the hypoiodite ion (IO- ). These remain bound to thyroperoxidase. Tyrosine residues also bind to thyroperoxidase, which favors the organification of iodine with the formation of iodotyrosines: monoiodotyrosine (MIT) and diiodotyrosine (DIT). In the absence of iodine, compound I is spontaneously converted into a stable compound, compound II, which catalyzes the coupling reaction of iodotyrosines, resulting in the formation of thyroid hormones. The excess of oxygenated water (H2O2) determines the conversion of compound II to inactive compound III. Inactivation is prevented by iodine [66].

The generation of oxygenated water, as an electron acceptor, is absolutely indispensable for thyroperoxidase activity [67]. H2O2 is produced by an NADPH-dependent process on the external aspect of the apical plasma membrane of follicular cells. Although various enzyme systems, including cytochrome reductases, can support H2O2 production in the thyroid, an NADPH-dependent, H2O2 –generating system was detected in thyroid particulate fractions that appears to be distinct from cytochrome c reductase. The activation of this NADPH oxidase requires Ca2+ ions.

The mechanism of formation of oxygenated water (H2O2) is controversial; there are two theories: The superoxide anion is the primary product of the enzymatic conversion of oxygen which, under the action of superoxide dismutase (SOD) will be transformed into oxygenated water. The superoxide anion, produced inside the cytoplasm, close to the apical membrane, under the action of NADPH-oxidase, is released outside the thyroid follicular cell only after its transformation into oxygenated water [68]. Other data suggest that oxygenated water is the primary product of the NADPH-oxidase system, and is produced outside the thyroid follicular cell via the two-electron reduction of O2 [69].

The production of oxygenated water is stimulated by TSH-cAMP and phosphatidylinositol-Ca2+. Other enzymatic systems capable of generating oxygenated water have been evidenced: monoamine oxidase, xanthine oxidase, glucose oxidase [66].

It is necessary to prevent excess or deficiency of H2O2, anything but optimal levels are linked to several thyroid diseases and disorders, such as congenital hypothyroidism, tumorigenesis, myxedematous cretinism, thyroiditis, and cancer [70, 71].

Various reports deal with thyroid disorders and H2O2. Normal levels of H2O2 in the body vary from 0. 001 mM to 0. 7 mM, but excess "induces oxidative stress, DNA oxidation and damage, and consequent mutagenesis and apoptosis" [71]. Several selenoproteins act as a protective barrier for thyrocytes from endogenous H2O2 [72]. If DNA damage is

perpetuated, it can lead to carcinogenesis. Also, increased levels of H2O2 inhibit iodide uptake and organification [73]. Several genetic disorders have been shown to decrease H2O2 production by creating a partial iodide-organification defect and reducing or eliminating hormone production [74]. This led to permanent congenital hypothyroidism in non-TH producing individuals, and mild, transient hypothyroidism in low hormone level subjects. As an autoregulatory effect, H2O2 production is diminished by high iodide concentration, but mildly stimulated by low iodide levels [75].

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 203

primary actions in several cell organelles, in a coordinated succession: binding to the cell membrane as a substrate, to mitochondria, through which metabolic energy required for nuclear transcription and posttranscription is released, and the specific synthesis of structures and functions is directed. Mitochondria are particularly important for the action mechanism of thyroid hormones, representing the final step of oxygen transfer in the

Mitochondrial respiration is a complex metabolic process by which hydrogen from the reduced forms of dehydrogenases is oxidized to proton (H+) and molecular oxygen from air is reduced to anion, which allows for the formation of water. NADPH+H+-dehydrogenase, flavoproteins (FMNH2/FADH2), non-porphyrin iron-sulfur proteins, ubiquinones (Q), and certain cytochromes participate in the main oxidoreduction reactions of the respiratory chain [79]. In mitochondrial respiration, significant amounts of hydrogen superoxide and peroxide radicals are formed, probably due to the auto-oxidable nature of the enzymatic system components (coenzyme Q, NADH+H+-dehydrogenase, cytochrome b), on the one hand, and to the incomplete reduction of the oxygen molecule ("trivalent" reduction occurs instead of "tetravalent" reduction), on the other hand. Superoxide formation is continuous in the respiratory chain, approximately 1-2% of the electrons that participate in the chain

Thyroid hormones increase the concentration and activity of Na+-K+ dependent ATP-ase, as well as Na+ and K+ permeability. 15% to 40% of the basal energy used by the cell is used for the maintenance of an electrochemical gradient. Thyroid hormones concomitantly stimulate the activity of cellular anabolic and catabolic enzymes, determining in this way the

Data from in vivo and in vitro studies indicate that thyroid hormones have a considerable impact on oxidative stress [11]. The great majority of the reactive oxygen species (ROS) are generated at mitochondrial level, via oxidative phosphorylation. Thyroid hormones act on mitochondria by regulating energy metabolism, and mitochondria are a major source of intracellular free radicals [82,83]. During thyroid hormone synthesis, there is a constant production of oxygenated water, which is absolutely indispensable for iodine intrafollicular oxidation in the presence of thyroid peroxidase. In recent years, the possible correlation between impaired thyroid gland function and reactive oxygen species has been increasingly

The aim of our study was to evaluate oxidative stress parameters, antioxidant status markers and their response to vitamin E supplementation in experimental hyperthyroidism

White, male, Wistar rats weighing between 220 and 240 g were purchased from The Iuliu Hatieganu University of Medicine and Pharmacy,Cluj-Napoca biobase. All animals were kept under the same environmental conditions, at a room temperature of 23±1ºC, with an

They were divided into 5 groups of 10 animals each: group 1– controls, group 2 – animals treated with L-thyroxine 10μg/animal/day for 30 days and group 3 – L-thyroxine treated rats

artificial lighting cycle (lights on 08. 00-20. 00 h) and water ad libitum.

form superoxide and its dismutation product – hydrogen peroxide [80].

intensification of energy consumption [81].

taken into consideration [9].

and hypothyroidism.

respiratory chain [67].

As stimulation by TSH permits, monoiodotyrosine (MIT) and diiodotyrosine (DIT) are released from the lumen into the follicular cell. Here, ferric TPO product (oxidized by H2O2) reacts with DIT to form a radical stabilized by the aromatic ring. Oxidation of either MIT or another DIT, followed by coupling, yields T3 and T4, respectively. Coupling in this reaction is catalyzed by TPO. TH inhibits production of TSH and TRH, an autoregulatory effect.

After hormone synthesis, any free iodotyrosine derivative left over is deiodinated quite rapidly due to excess iodotyrosine deiodinase, avoiding formation of other iodoamino acids, and recycled back into the thyroid. Thus, only T3 and T4 can be found in the thyroid vein's blood supply [76]. T3 is more potent than T4, more rapid in its reaction, and may be the active form of excreted T4 that is deiodinized by the target cells [75]. Two general effects of TH are described. First, altering the natural level by injection or thyroidectomy showed altered metabolism rates for several organs, suggesting the need for TH for energy metabolism. This includes the diaphragm, epidermis, gastric mucosa, heart, kidney, liver, pancreas, salivary gland, and skeletal muscles. There are also effects of TH on development. Hypothyroidism proved to have an effect on the rate and result of development; yet these observations were described as quantitative rather than qualitative, and are generally more easily reversed than are developmental inadequacies [77]. Still, although TH affects many of the body's cells, it is not considered necessary to the survival of the organism, and removal is not uncommon.

### **5. Oxidative stress in experimental hyperthyroidism and hypothyroidism**

Thyroid hormones regulate several essential physiological processes such as energy metabolism, growth and formation of the central nervous system, tissue differentiation and reproduction. The molecular action of thyroid hormones is mediated via the thyroid hormone receptors which, after ligand binding, activate genes by binding to the thyroid hormone response elements [79].

Thyroid hormones control the intensity of basal metabolism. They are calorigenic and, consequently, they increase oxygen consumption and heat production. Basal metabolism decreases in hypothyroidism and increases in hyperthyroidism. In the second case, an increase in the number and size of mitochondria, particularly of their cristae, has been seen, concomitantly with the increased concentration and the intensified activity of oxidative phosphorylation enzymes. T3 and T4 have been found to stimulate in vitro protein synthesis in mitochondria, ADP capture, ATP formation and oxygen consumption. The primary ligands of T3 are the nucleus and the mitochondrion. In fact, thyroid hormones have primary actions in several cell organelles, in a coordinated succession: binding to the cell membrane as a substrate, to mitochondria, through which metabolic energy required for nuclear transcription and posttranscription is released, and the specific synthesis of structures and functions is directed. Mitochondria are particularly important for the action mechanism of thyroid hormones, representing the final step of oxygen transfer in the respiratory chain [67].

202 Antioxidant Enzyme

is not uncommon.

hormone response elements [79].

but mildly stimulated by low iodide levels [75].

perpetuated, it can lead to carcinogenesis. Also, increased levels of H2O2 inhibit iodide uptake and organification [73]. Several genetic disorders have been shown to decrease H2O2 production by creating a partial iodide-organification defect and reducing or eliminating hormone production [74]. This led to permanent congenital hypothyroidism in non-TH producing individuals, and mild, transient hypothyroidism in low hormone level subjects. As an autoregulatory effect, H2O2 production is diminished by high iodide concentration,

As stimulation by TSH permits, monoiodotyrosine (MIT) and diiodotyrosine (DIT) are released from the lumen into the follicular cell. Here, ferric TPO product (oxidized by H2O2) reacts with DIT to form a radical stabilized by the aromatic ring. Oxidation of either MIT or another DIT, followed by coupling, yields T3 and T4, respectively. Coupling in this reaction is catalyzed by TPO. TH inhibits production of TSH and TRH, an autoregulatory effect.

After hormone synthesis, any free iodotyrosine derivative left over is deiodinated quite rapidly due to excess iodotyrosine deiodinase, avoiding formation of other iodoamino acids, and recycled back into the thyroid. Thus, only T3 and T4 can be found in the thyroid vein's blood supply [76]. T3 is more potent than T4, more rapid in its reaction, and may be the active form of excreted T4 that is deiodinized by the target cells [75]. Two general effects of TH are described. First, altering the natural level by injection or thyroidectomy showed altered metabolism rates for several organs, suggesting the need for TH for energy metabolism. This includes the diaphragm, epidermis, gastric mucosa, heart, kidney, liver, pancreas, salivary gland, and skeletal muscles. There are also effects of TH on development. Hypothyroidism proved to have an effect on the rate and result of development; yet these observations were described as quantitative rather than qualitative, and are generally more easily reversed than are developmental inadequacies [77]. Still, although TH affects many of the body's cells, it is not considered necessary to the survival of the organism, and removal

**5. Oxidative stress in experimental hyperthyroidism and hypothyroidism**

Thyroid hormones regulate several essential physiological processes such as energy metabolism, growth and formation of the central nervous system, tissue differentiation and reproduction. The molecular action of thyroid hormones is mediated via the thyroid hormone receptors which, after ligand binding, activate genes by binding to the thyroid

Thyroid hormones control the intensity of basal metabolism. They are calorigenic and, consequently, they increase oxygen consumption and heat production. Basal metabolism decreases in hypothyroidism and increases in hyperthyroidism. In the second case, an increase in the number and size of mitochondria, particularly of their cristae, has been seen, concomitantly with the increased concentration and the intensified activity of oxidative phosphorylation enzymes. T3 and T4 have been found to stimulate in vitro protein synthesis in mitochondria, ADP capture, ATP formation and oxygen consumption. The primary ligands of T3 are the nucleus and the mitochondrion. In fact, thyroid hormones have Mitochondrial respiration is a complex metabolic process by which hydrogen from the reduced forms of dehydrogenases is oxidized to proton (H+) and molecular oxygen from air is reduced to anion, which allows for the formation of water. NADPH+H+-dehydrogenase, flavoproteins (FMNH2/FADH2), non-porphyrin iron-sulfur proteins, ubiquinones (Q), and certain cytochromes participate in the main oxidoreduction reactions of the respiratory chain [79]. In mitochondrial respiration, significant amounts of hydrogen superoxide and peroxide radicals are formed, probably due to the auto-oxidable nature of the enzymatic system components (coenzyme Q, NADH+H+-dehydrogenase, cytochrome b), on the one hand, and to the incomplete reduction of the oxygen molecule ("trivalent" reduction occurs instead of "tetravalent" reduction), on the other hand. Superoxide formation is continuous in the respiratory chain, approximately 1-2% of the electrons that participate in the chain form superoxide and its dismutation product – hydrogen peroxide [80].

Thyroid hormones increase the concentration and activity of Na+-K+ dependent ATP-ase, as well as Na+ and K+ permeability. 15% to 40% of the basal energy used by the cell is used for the maintenance of an electrochemical gradient. Thyroid hormones concomitantly stimulate the activity of cellular anabolic and catabolic enzymes, determining in this way the intensification of energy consumption [81].

Data from in vivo and in vitro studies indicate that thyroid hormones have a considerable impact on oxidative stress [11]. The great majority of the reactive oxygen species (ROS) are generated at mitochondrial level, via oxidative phosphorylation. Thyroid hormones act on mitochondria by regulating energy metabolism, and mitochondria are a major source of intracellular free radicals [82,83]. During thyroid hormone synthesis, there is a constant production of oxygenated water, which is absolutely indispensable for iodine intrafollicular oxidation in the presence of thyroid peroxidase. In recent years, the possible correlation between impaired thyroid gland function and reactive oxygen species has been increasingly taken into consideration [9].

The aim of our study was to evaluate oxidative stress parameters, antioxidant status markers and their response to vitamin E supplementation in experimental hyperthyroidism and hypothyroidism.

White, male, Wistar rats weighing between 220 and 240 g were purchased from The Iuliu Hatieganu University of Medicine and Pharmacy,Cluj-Napoca biobase. All animals were kept under the same environmental conditions, at a room temperature of 23±1ºC, with an artificial lighting cycle (lights on 08. 00-20. 00 h) and water ad libitum.

They were divided into 5 groups of 10 animals each: group 1– controls, group 2 – animals treated with L-thyroxine 10μg/animal/day for 30 days and group 3 – L-thyroxine treated rats

protected with 10 mg/animal/day of vitamin E administered intramuscularly, for 30 days, group 4 – animals treated with Propylthiouracil (5mg/100g animal /day), for 30 days and group 5 – Propylthiouracil treated rats protected with 10 mg/animal/day of vitamin E administered intramuscularly, for 30 days. The L-thyroxine and Propylthiouracil quantity dissolved in 2 ml of milk was administered by gavage in the morning on an empty stomach.

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 205

Glutathione concentration was determined using a calibration curve made with known concentrations of glutathione processed in the same way. The results were expressed as

Superoxide dismutase (SOD) activity of the samples was evaluated using the Flohe method [87] and expressed as U SOD per milligram of protein (U/mg protein). Dosage was performed on lysed erythrocytes at 250C. Superoxide-dismutase (SOD) catalysed the

The superoxide radical (O2•‾) reacts with C ferricytochrome, which can be continuously monitored by recording the absorbance at 550 nm. Superoxid-dismutase reduces the concentration of superoxide ions and thus inhibits the reduction of the C cytochrome and the SOD amount may be thus calculated from the degree of inhibition of the C cytochrome using a calibration curve achieved by the known SOD standards. One unit of SOD activity is defined as the amount of enzyme able to inhibit the reduction rate of cytochrome C by 50%. Serum free-thyroxine (FT4) concentrations were measured with an enzyme immunoassay kit

Significantly higher FT4 (p<0. 001) values were observed in the L-thyroxine administered group as compared with the control group. FT4 values of the L-thyroxine and vitamin Eadministered group were significantly decreased in respect to those of the L-thyroxine only

In the hyperthyroid rats, the MDA levels did not differ significantly from euthyroid values (p>0. 05) while in the thyroid tissue, the MDA levels were significantly decreased (p<0. 01) as compared with euthyroid values. We found that carbonyl proteins levels were significantly higher (1. 31±0. 33, p=0. 0001) in the serum of Thyroxin treated rats, while in the thyroid homogenates, the levels of carbonyl proteins did not differ significantly from the

Thiol groups (SH), superoxide dismutase ( SOD) and reduced glutathione (GSH) were lower in the L -thyroxine-administered group in comparison to the control group (p<0. 001).

A significantly high SH level and a significantly low GSH level were observed in the thyroids

We also investigated the relation between the mean values of FT4 and the mean values of MDA in the L-thyroxine-administered group. There was a significant positive correlation

Significantly low FT4 (p<0. 001) values were observed in the Propylthiouracil administered

In serum and thyroid tissue of the hypothyroid rats, the MDA levels did not differ

We found that carbonyl proteins levels were significantly higher (0. 99±0. 27, p<0. 05) in serum, and the thyroid tissue (1. 99±0. 61, p<0. 05) of the Propylthiouracil treated rats, as

of the L-thyroxine-administered group in comparison to the control group (p<0. 001).

between hyperthyroidism and oxidative stress. (p>0. 5; r2=0. 70).

group as compared with the control group.

significantly from euthyroid values (p>0. 05).

compared with the control group.

superoxideradical (O2•‾) dismutation in peroxide (H2O2 ) and oxygen (O2 ).

micromoles per litre (μmol/l).

(EIAgen Free T4 Kit, Adaltis Italia).

administered group.

control group.

Thirty days into the experiment, blood was taken from the retro orbital sinus and the rats were sacrificed by cervical dislocation following ether anaesthesia.

Thyroid gland was immediately dissected out and placed into ice-cold isolation medium. Tissue homogenates were used for analytical procedures.

Malondialdehyde (MDA), the marker of lipid peroxidation, carbonyl proteins, SH groups,reduced glutathion (GSH) and superoxide dismutase (SOD) were determined from the serum, while MDA, carbonyl proteins, SH groups and GSH were determined from the thyroid tissue homogenates.

The lipid peroxides level was assessed by fluorescence according to the Conti and Moran method [84], based on the reaction between malondialdehyde, the marker of lipid peroxidation and thiobarbituric acid, measured spectrophotometrically at 534nm.

Plasma or tissue homogenates were boiled in 2-thiobarbituric acid solution 10mM in K2HPO4 75mM PH3 and extracted on n-butanol consecutively. Concentration values of MDA are expressed in nmol / ml based on specific calibration curves.

Protein oxidation was determined through the estimation of carbonyl groups photometrically with dinitrophenylhydrazine according to the Reznick method[85] and expressed as nmol per mg of protein (nmol/mg protein). Serum samples were submitted to a reaction with 2,4- dinitrophenylhydrazine 10 mM in HCL 2,5N, and treated with 20% trichloracetic acid; the precipitate obtained by centrifugation was washed with a 1: 1 (v/v) mixture of ethyl acetate and absolute ethylic alcohol and dissolved in guanidine chlorhydrate 6M. In the samples thus obtained the protein concentration was determined by measuring extinction at 280 nm. The carbonyl concentration was given by the formula

$$\mathbf{C} = \mathbf{A}\mathbf{b}\mathbf{s}\_{355} \ge 45,45 \text{nmol} / \text{ml} $$

The thiol content of samples was determined with dithionitrobenzoic acid (DTNB), according to the Hu method [86]. One plasma volume was mixed with Tris (0,25M)- EDTA20mM pH 8,2 buffer, absorbance being read at 412 nm. The Ellman( DTNB)10mM reagent was added, which produces a staining reaction, and the absorption was determined again at the same wave length.

The results were expressed as nmol SH per milligram of protein (nmol/mg protein).

Fluorescence was used to determine the glutathione (GSH) values [86]. For the GSH dosage one plasma volume was mixed with TCA 10% and then centrifuged, the supernatant separated and additioned with 1. 7 ml phosphate buffer pH 8 and 1 ml o-phthalaldehyde. Emission intensity was measured at 420 nm at an excitation of 350 nm.

Glutathione concentration was determined using a calibration curve made with known concentrations of glutathione processed in the same way. The results were expressed as micromoles per litre (μmol/l).

204 Antioxidant Enzyme

protected with 10 mg/animal/day of vitamin E administered intramuscularly, for 30 days, group 4 – animals treated with Propylthiouracil (5mg/100g animal /day), for 30 days and group 5 – Propylthiouracil treated rats protected with 10 mg/animal/day of vitamin E administered intramuscularly, for 30 days. The L-thyroxine and Propylthiouracil quantity dissolved in 2 ml of milk was administered by gavage in the morning on an empty stomach. Thirty days into the experiment, blood was taken from the retro orbital sinus and the rats

Thyroid gland was immediately dissected out and placed into ice-cold isolation medium.

Malondialdehyde (MDA), the marker of lipid peroxidation, carbonyl proteins, SH groups,reduced glutathion (GSH) and superoxide dismutase (SOD) were determined from the serum, while MDA, carbonyl proteins, SH groups and GSH were determined from the

The lipid peroxides level was assessed by fluorescence according to the Conti and Moran method [84], based on the reaction between malondialdehyde, the marker of lipid

Plasma or tissue homogenates were boiled in 2-thiobarbituric acid solution 10mM in K2HPO4 75mM PH3 and extracted on n-butanol consecutively. Concentration values of

Protein oxidation was determined through the estimation of carbonyl groups photometrically with dinitrophenylhydrazine according to the Reznick method[85] and expressed as nmol per mg of protein (nmol/mg protein). Serum samples were submitted to a reaction with 2,4- dinitrophenylhydrazine 10 mM in HCL 2,5N, and treated with 20% trichloracetic acid; the precipitate obtained by centrifugation was washed with a 1: 1 (v/v) mixture of ethyl acetate and absolute ethylic alcohol and dissolved in guanidine chlorhydrate 6M. In the samples thus obtained the protein concentration was determined by measuring extinction at 280 nm. The carbonyl concentration was given by the formula

C Abs x 45,45nmol / ml <sup>355</sup>

The thiol content of samples was determined with dithionitrobenzoic acid (DTNB), according to the Hu method [86]. One plasma volume was mixed with Tris (0,25M)- EDTA20mM pH 8,2 buffer, absorbance being read at 412 nm. The Ellman( DTNB)10mM reagent was added, which produces a staining reaction, and the absorption was determined

Fluorescence was used to determine the glutathione (GSH) values [86]. For the GSH dosage one plasma volume was mixed with TCA 10% and then centrifuged, the supernatant separated and additioned with 1. 7 ml phosphate buffer pH 8 and 1 ml o-phthalaldehyde.

The results were expressed as nmol SH per milligram of protein (nmol/mg protein).

Emission intensity was measured at 420 nm at an excitation of 350 nm.

peroxidation and thiobarbituric acid, measured spectrophotometrically at 534nm.

MDA are expressed in nmol / ml based on specific calibration curves.

were sacrificed by cervical dislocation following ether anaesthesia.

Tissue homogenates were used for analytical procedures.

thyroid tissue homogenates.

again at the same wave length.

Superoxide dismutase (SOD) activity of the samples was evaluated using the Flohe method [87] and expressed as U SOD per milligram of protein (U/mg protein). Dosage was performed on lysed erythrocytes at 250C. Superoxide-dismutase (SOD) catalysed the superoxideradical (O2•‾) dismutation in peroxide (H2O2 ) and oxygen (O2 ).

The superoxide radical (O2•‾) reacts with C ferricytochrome, which can be continuously monitored by recording the absorbance at 550 nm. Superoxid-dismutase reduces the concentration of superoxide ions and thus inhibits the reduction of the C cytochrome and the SOD amount may be thus calculated from the degree of inhibition of the C cytochrome using a calibration curve achieved by the known SOD standards. One unit of SOD activity is defined as the amount of enzyme able to inhibit the reduction rate of cytochrome C by 50%.

Serum free-thyroxine (FT4) concentrations were measured with an enzyme immunoassay kit (EIAgen Free T4 Kit, Adaltis Italia).

Significantly higher FT4 (p<0. 001) values were observed in the L-thyroxine administered group as compared with the control group. FT4 values of the L-thyroxine and vitamin Eadministered group were significantly decreased in respect to those of the L-thyroxine only administered group.

In the hyperthyroid rats, the MDA levels did not differ significantly from euthyroid values (p>0. 05) while in the thyroid tissue, the MDA levels were significantly decreased (p<0. 01) as compared with euthyroid values. We found that carbonyl proteins levels were significantly higher (1. 31±0. 33, p=0. 0001) in the serum of Thyroxin treated rats, while in the thyroid homogenates, the levels of carbonyl proteins did not differ significantly from the control group.

Thiol groups (SH), superoxide dismutase ( SOD) and reduced glutathione (GSH) were lower in the L -thyroxine-administered group in comparison to the control group (p<0. 001).

A significantly high SH level and a significantly low GSH level were observed in the thyroids of the L-thyroxine-administered group in comparison to the control group (p<0. 001).

We also investigated the relation between the mean values of FT4 and the mean values of MDA in the L-thyroxine-administered group. There was a significant positive correlation between hyperthyroidism and oxidative stress. (p>0. 5; r2=0. 70).

Significantly low FT4 (p<0. 001) values were observed in the Propylthiouracil administered group as compared with the control group.

In serum and thyroid tissue of the hypothyroid rats, the MDA levels did not differ significantly from euthyroid values (p>0. 05).

We found that carbonyl proteins levels were significantly higher (0. 99±0. 27, p<0. 05) in serum, and the thyroid tissue (1. 99±0. 61, p<0. 05) of the Propylthiouracil treated rats, as compared with the control group.

Vitamin E supplementation increased significantly the carbonyl proteins levels as compared with the hypothyroid rats.

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 207

autocatalytic mechanism leading to oxidative destruction of cellular membranes. Such destruction can lead to cell death and to the production of toxic and reactive aldehyde metabolites called free radicals [103]. Lipid peroxidation is associated with a wide variety of toxic effects, including decreased membrane fluidity and function, impaired functions of the mitochondria and Golgi apparatus and inhibition of enzymes. Malondialdehyde (MDA) is an end-product of lipid peroxidation and is frequently measured as an index of these

Thyroid stimulating hormone (TSH) affect metabolism and may be affected by the thyroxine secretions. High concentrations of thyroid hormones stimulate free radical formation in mitochondria by affecting oxygen metabolism [18]. Although reactive oxygen species play an important role in physiological mechanisms, extremely reactive oxygen radicals can cause severe oxidative damage to molecules [110]. If cellular mechanisms cannot scavenge these reactive oxygen species, toxicity is found in biomembranes and lipid peroxidation

Triiodothyronine (T3) and thyroxin (T4) circulating hormones are involved in the modulation of the physiological mitochondrial respiration process [105]. These agents were reported to change the number and activity of the mitochondrial respiratory chain components. The up regulating of these hormones can result in a mitochondrial respiration perturbation and a consequent increase in ROS generation [107]. These ROS would lead to oxidative damage to biological macromolecules, including lipids, proteins and DNA [108]. In contrast, in the case of hypothyroidism, there is a suppression of the metabolic rate and decline in ROS release

Recently, increasing experimental and clinical studies have shown that free radicals play a key role in the etiology of many diseases. Thyroid hormones cause oxidative stress as they

Effects of thyroid hormones on lipid peroxidation have been subject of investigation in several laboratories but the results are rather contradictory. It was reported that hypermetabolic condition in hyperthyroidism was associated with an increase in free radical formation and lipid peroxidation levels [10, 11,110]. In previous studies, there are conflicting results about oxidative stress in hyperthyroidism. In some studies, it was demonstrated that the products of lipid peroxidation were decreased [111,112]. On the contrary, Fernandez et al. [10] and Dumitriu et al. [113] found high products of lipid peroxidation. Similarly, Iangalenko et al. [114] found that lipid peroxidation was increased in hyperthyroid patients. Asayama et al. [115] showed that the damaging effect of lipid peroxidation was increased in liver, heart and some skeletal muscles of rats, diminishing antioxidant enzymes in

Peroxidative effects elicited by thyroid hormones were found in the brain of newborn [116] and adult [117] rats. Such effects were also found in heart homogenates [110, 11, 118, 119] from young rats. However, increased lipid peroxidation in hearts from old (1. 5 years) but not from young (8 weeks) hyperthyroid rats was also reported [120]. Thyroid hormone treatment was found to increase lipid peroxidation in lymphoid organs such as mesenteric

increase ROS, while activating metabolic systems of the body in general. [10]

occurs. This damage is usually more evident in cellular membranes.

processes [104].

[109].

experimental hyperthyroidism.

Thiol groups (SH), superoxide dismutase (SOD) and reduced glutathione (GSH) levels in the hypothyroid group did not differ significantly from the control group.

Administration of Vitamin E to hypothyroid rats resulted in a significant decrease in serum antioxidant status parameters (SH, SOD, GSH) levels as compared with the Propylthiouracil treated rats.

Thyroid hormones, of which T3 is the major active form, exert a multitude of physiological effects affecting growth, development and metabolism of vertebrates [88], so that they can be considered major regulators of their homeostasis. On the other hand, elevated circulating levels of thyroid hormones are associated with modifications in the whole organism (weight loss and increased metabolism and temperature) and in several body regions. Indeed, low plasma lipid levels, tachycardia, atrial arrhythmias, heart failure, muscle weakness and wasting are commonly found in hyperthyroid animals. Plasma membrane [89], endoplasmic reticulum [90] and mitochondria [91] have been considered as potential cellular sites of action of thyroid hormone. However, it is now generally accepted that most of the actions of thyroid hormone results from influences on trascription of T3-responsive genes, which are mediated through nuclear thyroid hormone receptors [92]. It is worth noting that the idea that oxidative stress underlies dysfunctions produced by hyperthyroidism is not in contradiction with mediation of T3 action through nuclear events. Indeed, it is conceivable that some of the biochemical changes favouring the establishment of the oxidative stress (increase in mitochondrial levels of electron carriers, NOS activity and the unsaturation degree of lipids) are due to stimulation of the expression of specific genes initiated through T3 binding to nuclear receptors. Thyroid hormone induces upregulation of NOS gene expression in rat hypothalamus [93], and it is conceivable that this also happens in other tissues in which T3-induced NO• overproduction has been shown [94, 95, 96].

The superoxide anion, hydrogen peroxide and the hydroxyl radical are the major reactive oxygen species in our body. Free radicals are produced as a consequence of normal metabolism and their levels and activities are controlled by enzymatic defense mechanisms, such as the SOD, GPx and CAT, and nonenzymatic defense mechanisms, such as ascorbic acid, Vitamin E, and GSH [97,98,99]. Oxidative damage arises when an imbalance occurs in this system, i. e. over-production of free radicals and/or a decrease in antioxidant defence mechanisms [100].

Disturbances of the oxidant/antioxidant balance resulting from the increased production of ROS are causative factors in the oxidative damage of cellular structures and molecules, such as lipids, proteins and nucleic acids [101]. In particular, biological membranes rich in unsaturated fatty acids are cellular structures susceptible to free radical attack [102].

Among the mediators involved in the pathophysiology of hyperthyroidism and subsequent tissue injury in animal models, free radical-mediated lipid peroxidation plays a pivotal role. Oxygen free radicals react with all biological substances. Lipid peroxidation is an autocatalytic mechanism leading to oxidative destruction of cellular membranes. Such destruction can lead to cell death and to the production of toxic and reactive aldehyde metabolites called free radicals [103]. Lipid peroxidation is associated with a wide variety of toxic effects, including decreased membrane fluidity and function, impaired functions of the mitochondria and Golgi apparatus and inhibition of enzymes. Malondialdehyde (MDA) is an end-product of lipid peroxidation and is frequently measured as an index of these processes [104].

206 Antioxidant Enzyme

treated rats.

mechanisms [100].

with the hypothyroid rats.

Vitamin E supplementation increased significantly the carbonyl proteins levels as compared

Thiol groups (SH), superoxide dismutase (SOD) and reduced glutathione (GSH) levels in the

Administration of Vitamin E to hypothyroid rats resulted in a significant decrease in serum antioxidant status parameters (SH, SOD, GSH) levels as compared with the Propylthiouracil

Thyroid hormones, of which T3 is the major active form, exert a multitude of physiological effects affecting growth, development and metabolism of vertebrates [88], so that they can be considered major regulators of their homeostasis. On the other hand, elevated circulating levels of thyroid hormones are associated with modifications in the whole organism (weight loss and increased metabolism and temperature) and in several body regions. Indeed, low plasma lipid levels, tachycardia, atrial arrhythmias, heart failure, muscle weakness and wasting are commonly found in hyperthyroid animals. Plasma membrane [89], endoplasmic reticulum [90] and mitochondria [91] have been considered as potential cellular sites of action of thyroid hormone. However, it is now generally accepted that most of the actions of thyroid hormone results from influences on trascription of T3-responsive genes, which are mediated through nuclear thyroid hormone receptors [92]. It is worth noting that the idea that oxidative stress underlies dysfunctions produced by hyperthyroidism is not in contradiction with mediation of T3 action through nuclear events. Indeed, it is conceivable that some of the biochemical changes favouring the establishment of the oxidative stress (increase in mitochondrial levels of electron carriers, NOS activity and the unsaturation degree of lipids) are due to stimulation of the expression of specific genes initiated through T3 binding to nuclear receptors. Thyroid hormone induces upregulation of NOS gene expression in rat hypothalamus [93], and it is conceivable that this also happens in other

hypothyroid group did not differ significantly from the control group.

tissues in which T3-induced NO• overproduction has been shown [94, 95, 96].

The superoxide anion, hydrogen peroxide and the hydroxyl radical are the major reactive oxygen species in our body. Free radicals are produced as a consequence of normal metabolism and their levels and activities are controlled by enzymatic defense mechanisms, such as the SOD, GPx and CAT, and nonenzymatic defense mechanisms, such as ascorbic acid, Vitamin E, and GSH [97,98,99]. Oxidative damage arises when an imbalance occurs in this system, i. e. over-production of free radicals and/or a decrease in antioxidant defence

Disturbances of the oxidant/antioxidant balance resulting from the increased production of ROS are causative factors in the oxidative damage of cellular structures and molecules, such as lipids, proteins and nucleic acids [101]. In particular, biological membranes rich in

Among the mediators involved in the pathophysiology of hyperthyroidism and subsequent tissue injury in animal models, free radical-mediated lipid peroxidation plays a pivotal role. Oxygen free radicals react with all biological substances. Lipid peroxidation is an

unsaturated fatty acids are cellular structures susceptible to free radical attack [102].

Thyroid stimulating hormone (TSH) affect metabolism and may be affected by the thyroxine secretions. High concentrations of thyroid hormones stimulate free radical formation in mitochondria by affecting oxygen metabolism [18]. Although reactive oxygen species play an important role in physiological mechanisms, extremely reactive oxygen radicals can cause severe oxidative damage to molecules [110]. If cellular mechanisms cannot scavenge these reactive oxygen species, toxicity is found in biomembranes and lipid peroxidation occurs. This damage is usually more evident in cellular membranes.

Triiodothyronine (T3) and thyroxin (T4) circulating hormones are involved in the modulation of the physiological mitochondrial respiration process [105]. These agents were reported to change the number and activity of the mitochondrial respiratory chain components. The up regulating of these hormones can result in a mitochondrial respiration perturbation and a consequent increase in ROS generation [107]. These ROS would lead to oxidative damage to biological macromolecules, including lipids, proteins and DNA [108]. In contrast, in the case of hypothyroidism, there is a suppression of the metabolic rate and decline in ROS release [109].

Recently, increasing experimental and clinical studies have shown that free radicals play a key role in the etiology of many diseases. Thyroid hormones cause oxidative stress as they increase ROS, while activating metabolic systems of the body in general. [10]

Effects of thyroid hormones on lipid peroxidation have been subject of investigation in several laboratories but the results are rather contradictory. It was reported that hypermetabolic condition in hyperthyroidism was associated with an increase in free radical formation and lipid peroxidation levels [10, 11,110]. In previous studies, there are conflicting results about oxidative stress in hyperthyroidism. In some studies, it was demonstrated that the products of lipid peroxidation were decreased [111,112]. On the contrary, Fernandez et al. [10] and Dumitriu et al. [113] found high products of lipid peroxidation. Similarly, Iangalenko et al. [114] found that lipid peroxidation was increased in hyperthyroid patients. Asayama et al. [115] showed that the damaging effect of lipid peroxidation was increased in liver, heart and some skeletal muscles of rats, diminishing antioxidant enzymes in experimental hyperthyroidism.

Peroxidative effects elicited by thyroid hormones were found in the brain of newborn [116] and adult [117] rats. Such effects were also found in heart homogenates [110, 11, 118, 119] from young rats. However, increased lipid peroxidation in hearts from old (1. 5 years) but not from young (8 weeks) hyperthyroid rats was also reported [120]. Thyroid hormone treatment was found to increase lipid peroxidation in lymphoid organs such as mesenteric lymph nodes and thymus, without major effects in the spleen [12], a thyroid hormoneunresponsive tissue [121]. Thus, no significant change (TBARS) or decrease (HPs) were observed in lipid peroxidation level in the testis from adult hyperthyroid rats [122], and the thyroid hormone-induced increase in lipid peroxidation was found to be confined to some skeletal muscles. In both rat [11, 12] and cat [123], such an increase was found in the soleus, a red muscle mainly composed of slow-twitch oxidative glycolitic fibres (type I). Conversely, no change was found in the extensor digitorum longus (EDL) [11, 123], a white muscle mainly composed of fast-twitch glycolitic fibres (Type IIb). These results are consistent with early observations that red, but not white muscles, are sensitive to thyroid hormones [124, 125]. Lipid peroxidation was also increased by thyroid hormone in rat gastrocnemious [110,126], a mixed fibre muscle also containing fast-twitch oxidative glycolytic fibers (type IIa), but was decreased in the white portion of such a muscle [12]. On the other hand, it is surprising that in kidney from hyperthyroid rats the lipid peroxidation level does not change [127], although the tissue exhibits a calorigenic response to thyroid hormone similar to that elicited in liver [121].

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 209

Numerous oxidases in the cytosol, endoplasmic reticulum and outer mitochondrial membrane also contribute to O2 consumption and lead to O2• and H2O2 generation [133].

Joanta et al. [134] revealed an increase of the lipid peroxides content and carbonyl proteins level in blood, liver, thyroid, heart and skeletal muscle in experimental hyperthyroidism, suggesting that thyroid hyperfunction is accompanied by oxidative stress. R. Shinohara et al. [120] have investigated how thyroid function might influence the production of oxygen free radicals, the lipid peroxidation process and antioxidant activity in muscle of rat myocardium. It was found that the degree of lipid peroxidation, assessed by measuring substances that react with thiobarbituric acid, significantly increases in animals with

Also the antioxidant enzyme activity changed: increased the xanthine oxidase and superoxiddismutase and decreased the glutathione peroxidase. These changes in the prooxidant/antioxidant balance, caused by thyroid hormones excess could be involved in

Zaiton et al. [123] revealed increased concentration of lipid peroxidation products in the myocardium and solear muscle in rat, but not in liver tissue. Conflicting results obtained Fernandez et al. [10] : increased liver content in lipid peroxides induced by thyroid hormones. Tapia et al. [135] studied the influence of thyroid hormones on Kupffer cells activity in isolated liver, perfused with colloidal carbon solution. The conclusion was that hyperthyroidism increase Kupffer cells activity and the production of oxygen free radicals at this level. Therefore liver macrophages could be an alternative source of reactive species.

Retroocular fibroblast proliferation is involved in the pathogenesis of ophthalmopathy in Basedow-Graves disease. H. Burch et al. [136] studied the way in which the superoxide radical, generated by the xanthine oxidase/hypoxanthine system, can induce cell proliferation in fibroblast cultures from patients with severe ophthalmopathy, as well as from control patients, in whom the excision of retroorbital tissue was performed. The authors found that the superoxide radical determined fibroblast proliferation, the intensity of this phenomenon depending on the concentration of reactive oxygen species. The effectiveness of some pharmacological agents on retroocular fibroblast proliferation induced by the superoxide radical was also monitored. For this, retroocular tissue was incubated with methimazole, propylthiouracil (synthesis antithyroid drugs), allopurinol (a xanthine oxidase inhibitor), and nicotinamide (an antioxidant). The most effective regarding the inhibition of superoxide radical production and implicitly, that of fibroblast proliferation, were methimazole, allopurinol and nicotinamide. These results suggest the implication of reactive oxygen species

Mitochondria are particularly susceptible to ROS-induced damage because they are a major site of oxygen free radical production [138] and contain great amounts of high and low molecular weight Fe2+ complexes, which promote the oxidative damage of membrane lipids [139,140]. Thyroid state-linked changes in the balance between ROS production and

in retroocular fibroblast proliferation in Basedow-Graves disease [137].

Major complications of hyperthyroidism are the myopathy and cardiothyreosis [81].

hyperthyroidism than euthyroidiene.

myocardial dysfunction.

Studies on the mouse showed lower susceptibility to thyroid hormone-induced lipid oxidative damage. Indeed, levels of lipid peroxidation were found to be increased in hindlimb muscles [128], unchanged in heart [129] and decreased in liver [107] from hyperthyroid mice. The results concerning liver were attributed by the authors to the animal species or long-term (4-5 weeks) treatment they used, because a laboratory study describing no increase in index of lipid peroxidation in hyperthyroid rat liver used the same long-term treatment [11]. Although this may be true, it is interesting that in both mouse and rat hyperthyroidism was induced by T4, whose biological activity can differ from T3 in some tissues. Indeed, recent studies have shown that T4, but not T3, increases lipid peroxidation in rat interscapular brown adipose tissue [130].

Although the pathophysiological consequences of the accelerated lipid peroxidation are not yet fully elucidated, this biochemical change is thought to be responsible for some complications of hyperthyroidism. However, it is still to be determined whether the various target tissues of thyroid hormone undergo other biochemical changes that either predispose to free radical-mediated injury, or oppose it.

Despite some contradictory reports, the aforementioned results provide strong evidence that thyroid hormones induce oxidative stress in target tissues. Oxidative stress results from a disturbance of the normal cell balance between production of ROS and the capacity to neutralize their action.

In aerobic cells O2 is mainly consumed through its four-electron reduction to water by cytochrome c oxidase. This reaction occurs without release of any intermediate in the O2 reduction. However, despite the efficiency of the mitochondrial electron transport system, the nature of the alternating one-electron oxidation-reduction reactions it catalyses predisposes electron carriers to side reactions, in which an electron is transferred to O2 directly, instead of the next electron carrier in the chain, generating O2• [131]. This radical is then converted by spontaneous or catalysed dismutation into hydrogen peroxide (H2O2) [132], which can be turned into highly reactive hydroxyl radical (•OH).

Numerous oxidases in the cytosol, endoplasmic reticulum and outer mitochondrial membrane also contribute to O2 consumption and lead to O2• and H2O2 generation [133].

Major complications of hyperthyroidism are the myopathy and cardiothyreosis [81].

208 Antioxidant Enzyme

lymph nodes and thymus, without major effects in the spleen [12], a thyroid hormoneunresponsive tissue [121]. Thus, no significant change (TBARS) or decrease (HPs) were observed in lipid peroxidation level in the testis from adult hyperthyroid rats [122], and the thyroid hormone-induced increase in lipid peroxidation was found to be confined to some skeletal muscles. In both rat [11, 12] and cat [123], such an increase was found in the soleus, a red muscle mainly composed of slow-twitch oxidative glycolitic fibres (type I). Conversely, no change was found in the extensor digitorum longus (EDL) [11, 123], a white muscle mainly composed of fast-twitch glycolitic fibres (Type IIb). These results are consistent with early observations that red, but not white muscles, are sensitive to thyroid hormones [124, 125]. Lipid peroxidation was also increased by thyroid hormone in rat gastrocnemious [110,126], a mixed fibre muscle also containing fast-twitch oxidative glycolytic fibers (type IIa), but was decreased in the white portion of such a muscle [12]. On the other hand, it is surprising that in kidney from hyperthyroid rats the lipid peroxidation level does not change [127], although the tissue exhibits a calorigenic response to thyroid hormone similar to that elicited in liver [121].

Studies on the mouse showed lower susceptibility to thyroid hormone-induced lipid oxidative damage. Indeed, levels of lipid peroxidation were found to be increased in hindlimb muscles [128], unchanged in heart [129] and decreased in liver [107] from hyperthyroid mice. The results concerning liver were attributed by the authors to the animal species or long-term (4-5 weeks) treatment they used, because a laboratory study describing no increase in index of lipid peroxidation in hyperthyroid rat liver used the same long-term treatment [11]. Although this may be true, it is interesting that in both mouse and rat hyperthyroidism was induced by T4, whose biological activity can differ from T3 in some tissues. Indeed, recent studies have shown that T4, but not T3, increases lipid peroxidation

Although the pathophysiological consequences of the accelerated lipid peroxidation are not yet fully elucidated, this biochemical change is thought to be responsible for some complications of hyperthyroidism. However, it is still to be determined whether the various target tissues of thyroid hormone undergo other biochemical changes that either predispose

Despite some contradictory reports, the aforementioned results provide strong evidence that thyroid hormones induce oxidative stress in target tissues. Oxidative stress results from a disturbance of the normal cell balance between production of ROS and the capacity to

In aerobic cells O2 is mainly consumed through its four-electron reduction to water by cytochrome c oxidase. This reaction occurs without release of any intermediate in the O2 reduction. However, despite the efficiency of the mitochondrial electron transport system, the nature of the alternating one-electron oxidation-reduction reactions it catalyses predisposes electron carriers to side reactions, in which an electron is transferred to O2 directly, instead of the next electron carrier in the chain, generating O2• [131]. This radical is then converted by spontaneous or catalysed dismutation into hydrogen peroxide (H2O2)

[132], which can be turned into highly reactive hydroxyl radical (•OH).

in rat interscapular brown adipose tissue [130].

to free radical-mediated injury, or oppose it.

neutralize their action.

Joanta et al. [134] revealed an increase of the lipid peroxides content and carbonyl proteins level in blood, liver, thyroid, heart and skeletal muscle in experimental hyperthyroidism, suggesting that thyroid hyperfunction is accompanied by oxidative stress. R. Shinohara et al. [120] have investigated how thyroid function might influence the production of oxygen free radicals, the lipid peroxidation process and antioxidant activity in muscle of rat myocardium. It was found that the degree of lipid peroxidation, assessed by measuring substances that react with thiobarbituric acid, significantly increases in animals with hyperthyroidism than euthyroidiene.

Also the antioxidant enzyme activity changed: increased the xanthine oxidase and superoxiddismutase and decreased the glutathione peroxidase. These changes in the prooxidant/antioxidant balance, caused by thyroid hormones excess could be involved in myocardial dysfunction.

Zaiton et al. [123] revealed increased concentration of lipid peroxidation products in the myocardium and solear muscle in rat, but not in liver tissue. Conflicting results obtained Fernandez et al. [10] : increased liver content in lipid peroxides induced by thyroid hormones. Tapia et al. [135] studied the influence of thyroid hormones on Kupffer cells activity in isolated liver, perfused with colloidal carbon solution. The conclusion was that hyperthyroidism increase Kupffer cells activity and the production of oxygen free radicals at this level. Therefore liver macrophages could be an alternative source of reactive species.

Retroocular fibroblast proliferation is involved in the pathogenesis of ophthalmopathy in Basedow-Graves disease. H. Burch et al. [136] studied the way in which the superoxide radical, generated by the xanthine oxidase/hypoxanthine system, can induce cell proliferation in fibroblast cultures from patients with severe ophthalmopathy, as well as from control patients, in whom the excision of retroorbital tissue was performed. The authors found that the superoxide radical determined fibroblast proliferation, the intensity of this phenomenon depending on the concentration of reactive oxygen species. The effectiveness of some pharmacological agents on retroocular fibroblast proliferation induced by the superoxide radical was also monitored. For this, retroocular tissue was incubated with methimazole, propylthiouracil (synthesis antithyroid drugs), allopurinol (a xanthine oxidase inhibitor), and nicotinamide (an antioxidant). The most effective regarding the inhibition of superoxide radical production and implicitly, that of fibroblast proliferation, were methimazole, allopurinol and nicotinamide. These results suggest the implication of reactive oxygen species in retroocular fibroblast proliferation in Basedow-Graves disease [137].

Mitochondria are particularly susceptible to ROS-induced damage because they are a major site of oxygen free radical production [138] and contain great amounts of high and low molecular weight Fe2+ complexes, which promote the oxidative damage of membrane lipids [139,140]. Thyroid state-linked changes in the balance between ROS production and

antioxidant capacity should result in changes in the damage to mitochondrial components. Therefore, we investigated the effects of altered thyroid states on the extent of oxidative damage of mitochondrial lipids and proteins.

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 211

proline and threonine residues may also yield carbonyl derivatives. In addition, carbonyl groups may be introduced into proteins by reactions with aldehydes (4- hydroxi-2- nonenal, malondialdehyde) produced during lipid peroxidation or with reactive carbonyl derivatives( ketoamine, ketoaldehydes, deoxyosones) generated as a consequence of the reaction of reducing sugars or their oxidation products with lysine residues of proteins (glycation and glycoxidation reactions). The presence of carbonyl groups in proteins has

There are not many data regarding the effect of the thyroid state on protein oxidation. In experimental hyperthyroidism increased protein oxidation was demonstrated in different tissues [146,147]. Enhanced myocardial protein oxidation was also shown in the study of [148] by means of carbonyl group measurement. An elevation of this protein oxidation

In our study, the increased levels of protein-bound carbonyls in serum of L-thyroxinetreated rats is in agreement with the earlier reports [150,151] suggesting the role of free radicals in the pathogenesis, which demand the need for studies assessing the therapeutic

A recent study[152] found a positive association between thyroid hormones in excess and lipid peroxides correlated by linear regression which clearly suggest induction of oxidative stress. Such an effect may be related to the enhanced metabolic rate generated by thyroid

In the thyroid homogenates of the L-thyroxine administered rats, the MDA values were significantly decreased and carbonyl proteins levels did not show significant changes.

These results show that hyperthyroid state is not accompanied by oxidative stress in the thyroid gland and contradict the results of [134] who observed an increase in lipid peroxides

The synthesis of thyroid hormones crucially depends on H2O2, which works as a donor of oxidative equivalents for thyroperoxidase [154]. Because of its great toxicity, H2O2 synthesis must always remain in adequation with the hormonal synthesis and strictly contained at the apical pole of the cell. Thyrocytes possess various enzymatic systems, such as GPx, catalase, superoxide dismutases, and peroxiredoxins that contribute to limit cellular injuries when

Our findings may be explained by the fact that the external administration of thyroid hormones usually inhibits pituitary secretion of TSH and indirectly hormonal synthesis [158]. It is therefore possible that decreased oxidative stress observed in thyrocytes, is due in

Substances that neutralize the potential ill effects of free radicals are generally grouped in the so-called antioxidant defence system. Such a system includes both low molecular weight

therefore been used as a marker of ROS-mediated protein oxidation [134].

marker was demonstrated in the plasma of hyperthyroid patients [149,147].

hormone administration, leading to an accelerated ROS production [153,141].

and carbonyl proteins in the same tissue in experimental hyperthyroidism.

H2O2 or other ROS are produced in excess [155,156,157].

part to the absence of H2O2.

**6. Antioxidant status**

role of antioxidants in hyperthyroidism.

It is well known that MDA is a terminal product of lipid peroxidation. So the content of MDA can be used to estimate the extent of lipid peroxidation. The latter can indirectly reflect the status of the metabolism of free radicals, the degree to which the tissue cells are attacked by free radicals and the degree to which lipid is peroxidated.

In our study in the plasma of L-thyroxine-treated rats, the marker of lipid peroxidation (MDA) levels did not differ significantly from the euthyroid values. This result of unchanged lipid peroxidation level can be correlated with the observations of Asayama et al. [11] who found no change of MDA in liver homogenates from hyperthyroidism induced rats rendered hyperthyroid by administration of T4 to their drinking water over a 4-week period. However our results are not in concordance with the findings of Seven et al. [141] who found a significant increase in MDA levels in the plasma of rats rendered hyperthyroid by administration of T4 in their food for 24 days and Venditti et al. [110] who noticed that hyperthyroidism induced in rats by T3 daily i. p. injections for 10 days caused significantly increased MDA levels in liver, heart and skeletal muscle homogenates.

These discrepancies among results seem to reflect a dependence of peroxidative processes on various factors, such as tissue, species, the iodothyronine used and treatment duration. On the other hand, it is not possible to exclude the fact that some conflicting results depend on the different accuracies of the methods used for lipid peroxidation determination. For example the method for evaluating thiobarbituric acid reactive substances (TBARS) is inaccurate, and returns results which differ according to the assay conditions used [19].

The high increase in the level of MDA and hydroperoxides in hyperthyroidism might be due to the possible changes in the cellular respiration of target tissues, which are undoubtedly related to any alteration in the thyroid function, knowing the major role of the thyroid hormones in the control (acceleration) of the mitochondrial respiration rate [108], [2] and [28]. From a biochemical point of view, the provoked hyperthyroidism, and via a variety of mechanisms, mitochondrial respiratory chain activity is altered, leading to an increase in electrons transfer from the respiratory chain through the acceleration of the cellular metabolism rate, resulting in the increased generation of superoxide (O2•−) at the site of ubiquinone [7]. Superoxide radicals can lead to the formation of many other reactive species, including hydroxyl radicals (OH•), which can readily start the free-radical process of lipid peroxidation [3] and [6].

Proteins are also sensitive to oxidative damage which leads to alteration in their structure and ability to function [142]. Protein oxidation can lead to a loss of critical thiol groups (SH) in addition to modifications of amino acids leading to the formation of carbonyl and other oxidized moieties[143,144,145].

Oxidative cleavage of proteins by either the alpha-amidation pathway or by oxidation of glutamyl side chains leads to formation of a peptide in which the N-terminal amino acid is blocked by an alpha-ketoacyl derivative. However, direct oxidation of lysine, arginine, proline and threonine residues may also yield carbonyl derivatives. In addition, carbonyl groups may be introduced into proteins by reactions with aldehydes (4- hydroxi-2- nonenal, malondialdehyde) produced during lipid peroxidation or with reactive carbonyl derivatives( ketoamine, ketoaldehydes, deoxyosones) generated as a consequence of the reaction of reducing sugars or their oxidation products with lysine residues of proteins (glycation and glycoxidation reactions). The presence of carbonyl groups in proteins has therefore been used as a marker of ROS-mediated protein oxidation [134].

There are not many data regarding the effect of the thyroid state on protein oxidation. In experimental hyperthyroidism increased protein oxidation was demonstrated in different tissues [146,147]. Enhanced myocardial protein oxidation was also shown in the study of [148] by means of carbonyl group measurement. An elevation of this protein oxidation marker was demonstrated in the plasma of hyperthyroid patients [149,147].

In our study, the increased levels of protein-bound carbonyls in serum of L-thyroxinetreated rats is in agreement with the earlier reports [150,151] suggesting the role of free radicals in the pathogenesis, which demand the need for studies assessing the therapeutic role of antioxidants in hyperthyroidism.

A recent study[152] found a positive association between thyroid hormones in excess and lipid peroxides correlated by linear regression which clearly suggest induction of oxidative stress. Such an effect may be related to the enhanced metabolic rate generated by thyroid hormone administration, leading to an accelerated ROS production [153,141].

In the thyroid homogenates of the L-thyroxine administered rats, the MDA values were significantly decreased and carbonyl proteins levels did not show significant changes.

These results show that hyperthyroid state is not accompanied by oxidative stress in the thyroid gland and contradict the results of [134] who observed an increase in lipid peroxides and carbonyl proteins in the same tissue in experimental hyperthyroidism.

The synthesis of thyroid hormones crucially depends on H2O2, which works as a donor of oxidative equivalents for thyroperoxidase [154]. Because of its great toxicity, H2O2 synthesis must always remain in adequation with the hormonal synthesis and strictly contained at the apical pole of the cell. Thyrocytes possess various enzymatic systems, such as GPx, catalase, superoxide dismutases, and peroxiredoxins that contribute to limit cellular injuries when H2O2 or other ROS are produced in excess [155,156,157].

Our findings may be explained by the fact that the external administration of thyroid hormones usually inhibits pituitary secretion of TSH and indirectly hormonal synthesis [158]. It is therefore possible that decreased oxidative stress observed in thyrocytes, is due in part to the absence of H2O2.

### **6. Antioxidant status**

210 Antioxidant Enzyme

damage of mitochondrial lipids and proteins.

of lipid peroxidation [3] and [6].

oxidized moieties[143,144,145].

antioxidant capacity should result in changes in the damage to mitochondrial components. Therefore, we investigated the effects of altered thyroid states on the extent of oxidative

It is well known that MDA is a terminal product of lipid peroxidation. So the content of MDA can be used to estimate the extent of lipid peroxidation. The latter can indirectly reflect the status of the metabolism of free radicals, the degree to which the tissue cells are

In our study in the plasma of L-thyroxine-treated rats, the marker of lipid peroxidation (MDA) levels did not differ significantly from the euthyroid values. This result of unchanged lipid peroxidation level can be correlated with the observations of Asayama et al. [11] who found no change of MDA in liver homogenates from hyperthyroidism induced rats rendered hyperthyroid by administration of T4 to their drinking water over a 4-week period. However our results are not in concordance with the findings of Seven et al. [141] who found a significant increase in MDA levels in the plasma of rats rendered hyperthyroid by administration of T4 in their food for 24 days and Venditti et al. [110] who noticed that hyperthyroidism induced in rats by T3 daily i. p. injections for 10 days caused significantly

These discrepancies among results seem to reflect a dependence of peroxidative processes on various factors, such as tissue, species, the iodothyronine used and treatment duration. On the other hand, it is not possible to exclude the fact that some conflicting results depend on the different accuracies of the methods used for lipid peroxidation determination. For example the method for evaluating thiobarbituric acid reactive substances (TBARS) is inaccurate, and returns results which differ according to the assay conditions used [19].

The high increase in the level of MDA and hydroperoxides in hyperthyroidism might be due to the possible changes in the cellular respiration of target tissues, which are undoubtedly related to any alteration in the thyroid function, knowing the major role of the thyroid hormones in the control (acceleration) of the mitochondrial respiration rate [108], [2] and [28]. From a biochemical point of view, the provoked hyperthyroidism, and via a variety of mechanisms, mitochondrial respiratory chain activity is altered, leading to an increase in electrons transfer from the respiratory chain through the acceleration of the cellular metabolism rate, resulting in the increased generation of superoxide (O2•−) at the site of ubiquinone [7]. Superoxide radicals can lead to the formation of many other reactive species, including hydroxyl radicals (OH•), which can readily start the free-radical process

Proteins are also sensitive to oxidative damage which leads to alteration in their structure and ability to function [142]. Protein oxidation can lead to a loss of critical thiol groups (SH) in addition to modifications of amino acids leading to the formation of carbonyl and other

Oxidative cleavage of proteins by either the alpha-amidation pathway or by oxidation of glutamyl side chains leads to formation of a peptide in which the N-terminal amino acid is blocked by an alpha-ketoacyl derivative. However, direct oxidation of lysine, arginine,

attacked by free radicals and the degree to which lipid is peroxidated.

increased MDA levels in liver, heart and skeletal muscle homogenates.

Substances that neutralize the potential ill effects of free radicals are generally grouped in the so-called antioxidant defence system. Such a system includes both low molecular weight free-radical scavengers and a complex enzyme array involved in scavenging free radicals, terminating chain reactions, and removing or repairing damaged cell constituents. To provide maximum protection, these substances are strategically compartmentalized in subcellular organelles within the cell and act in concert. In examining antioxidant changes found in hyperthyroid tissues, it needs to be underscored that although thyroid hormone can directly control levels of enzymes with antioxidant activity or regulate scavenger content, antioxidant depletion could not be the cause, but the consequence of the oxidative stress. The effects of thyroid hormone on antioxidant status have been extensively investigated in rat tissues, while a few data concerning other species are available [159].

Oxidative Stress and Antioxidant Status in Hypo- and Hyperthyroidism 213

enzyme depends on the relative concentration of the reducing (GSH) and oxidized (hydroperoxides) substrates. The phospholipid hydroperoxide GPx — discovered as a factor preventing lipid peroxidation — is considered to be involved in the protection of biomembranes against oxidative stress. In general, these isoenzymes may have a role in the regulation of the delicate regional redox balance, in particular the regulation of the appropriate tone of hydroperoxides known to be involved in cellular signaling, and to evoke several cellular responses, for example, programmed cell death, proliferation, cytokine production, and so on [172]. Glutathione S-transferases are three enzyme families cytosolic, mitochondrial and microsomal — that detoxify noxious electrophilic xenobiotics, such as chemical carcinogens, environmental pollutants and antitumor agents. Moreover, they protect against reactive compounds produced in vivo during oxidative stress by inactivating endogenous unsaturated aldehydes, quinones, epoxides and hydroperoxides, all of which are produced intracellularly after the exposure to pollutants, or consumption of overcooked or mycotoxin-contaminated food, or polluted water [173]. Glutathione Stransferases exert those protective effects because they are able to catalyze the conjugation of GSH with oxidation end products and represent a second line of defense against the highly toxic spectrum of substances produced by ROS-mediated reaction. Both GPx and GST activities can eventually lower the level of total intracellular GSH. During the course of the reaction catalyzed by GPx, the exaggerated production of GSSG can lead to the formation of mixed disulfides in cellular proteins, or to the release of GSSG excess by the cell, to maintain the intracellular GSH/GSSG ratio. During the GST-mediated reactions, GSH is conjugated with various electrophiles and the GSH adducts are actively secreted by the cell. Mixed disulfide formation together with GSSG or GS-conjugated efflux can result in the depletion of cellular GSH, which can be opposed by a de novo synthesis or by reducing the formed GSSG. In the presence of oxidative stress, GSH concentration rapidly decreases while GSSG — potentially highly cytotoxic — increases because of the reduction of peroxides or as a result of free radical scavenging. This has two important consequences: (1) the thiol redox status of the cell will shift and activate certain oxidant response transcriptional elements, and (2) GSSG may be preferentially secreted from the cell and degraded extracellularly, increasing the cellular requirement for de novo GSH synthesis. Glutathione disulfide can also be reduced back to GSH by the action of glutathione reductase (GRed) utilizing NADPH as a reductant [174]. Glutathione reductase is a flavoenzyme and is represented by a single-copy gene in humans. It has been observed that exposure to agents that lead to increased oxidative stress also leads to an increase in its mRNA content. Further experimental data have shown the importance of GRed activity in GSH metabolism, demonstrating that the enzymatic activity is regulated in response to stress, and that mutations affecting GRed activity would have deleterious consequences. The recycling pathway for GSH formation is thus fundamental in the metabolism of GSH-dependent defense reactions [175]. In conclusion, the presence of GSH is essential, but not in itself sufficient, to prevent the cytotoxicity of ROS, being of fundamental importance the functionality of the glutathione-dependent enzymes, which participate in the first and

second lines of defense.

Several antioxidant enzymes exist that convert ROS into less noxious compounds, for example, superoxide dismutase (SOD), catalase, thioredoxin reductase, peroxiredoxin and glutathione peroxidase (GPx) [160,161,162,163,164]. Collectively, these enzymes provide a first line of defense against superoxide and hydrogen peroxides. They are of enormous importance in limiting ROS-mediated damages to biological macromolecules, but they are not able to be 100% effective because certain compounds generated by the interaction of ROS with macromolecules are highly reactive. It is then mandatory to detoxify these secondary products in order to prevent further intracellular damage, degradation of cell components and eventual cell death. This second line of defense against ROS is provided by enzymes such as GPx, glutathione S-transferase (GST), aldo-keto reductase and aldheyde dehydrogenase [165,166,167]. Thus, the central role of reduced GSH appears clear in intracellular endogenous antioxidant defenses as it is involved in all the lines of protection against ROS [35].