is not thought to interfere with initial heparin binding, but rather to inhibit any resulting conformational changes. even though it is present at only 5–10% the levels of α-antithrombin, due to its increased heparin affinity, it is thought that β-antithrombin is more important than α-antithrombin in controlling thrombogenic events resulting from tissue injury. indeed, thrombin inhibition after injury to the aorta has been attributed solely to β-antithrombin. deficiencies further information: antithrombin iii deficiency evidence for the important role antithrombin plays in regulating normal blood coagulation is demonstrated by the correlation between inherited or acquired antithrombin deficiencies and an increased risk of any affected individual developing thrombotic disease. antithrombin deficiency generally comes to light when a patient suffers recurrent venous thrombosis and pulmonary embolism. acquired antithrombin deficiency acquired antithrombin deficiency occurs as a result of three distinctly different mechanisms. the first mechanism is increased excretion which may occur with renal failure associated with proteinuria nephrotic syndrome. the second mechanism results from decreased production as seen in liver failure or cirrhosis or an immature liver secondary to premature birth. the third mechanism results from accelerated consumption which is most pronounced as consequence of severe injury trauma but also may be seen on a lesser scale as a result of interventions such as major surgery or cardiopulmonary bypass. inherited antithrombin deficiency the incidence of inherited antithrombin deficiency has been estimated at between 1:2000 and 1:5000 in the normal population, with the first family suffering from inherited antithrombin deficiency being described in 1965. subsequently, it was proposed that the classification of inherited antithrombin deficiency be designated as either type i or type ii, based upon functional and immunochemical antithrombin analyses. maintenance of an adequate level of antithrombin activity, which is at least 70% that of a normal functional level, is essential to ensure effective inhibition of blood coagulation proteases. typically as a result of type i or type ii antithrombin deficiency, functional antithrombin levels are reduced to below 50% of normal. type i antithrombin deficiency type i antithrombin deficiency is characterized by a decrease in both antithrombin activity and antithrombin concentration in the blood of affected individuals. type i deficiency was originally further divided into two subgroups, ia and ib, based upon heparin affinity. the antithrombin of subgroup ia individuals showed a normal affinity for heparin while the antithrombin of subgroup ib individuals showed a reduced affinity for heparin. subsequent functional analysis of a group of 1b cases found them not only to have reduced heparin affinity but multiple or 'pleiotrophic' abnormalities affecting the reactive site, the heparin binding site and antithrombin blood concentration. in a revised system of classification adopted by the scientific and standardization committee of the international society on thrombosis and haemostasis, type ib cases are now designated as type ii pe, pleiotrophic effect. most cases of type i deficiency are due to point mutations, deletions or minor insertions within the antithrombin gene. these genetic mutations result in type i deficiency through a variety of mechanisms: mutations may produce unstable antithrombins that either may be not exported into the blood correctly upon completion biosynthesis or exist in the blood for a shortened period of time, e.g., the deletion of 6 base pairs in codons 106–108. mutations may affect mrna processing of the antithrombin gene. minor insertions or deletions may lead to frame shift mutations and premature termination of the antithrombin gene. point mutations may also result in the premature generation of a termination or stop codon e.g. the mutation of codon 129, cga→tga (uga after transcription), replaces a normal codon for arginine with a termination codon. type ii antithrombin deficiency type ii antithrombin deficiency is characterized by normal antithrombin levels but reduced antithrombin activity in the blood of affected individuals. it was originally proposed that type ii deficiency be further divided into three subgroups (iia, iib, and iic) depending on which antithrombin functional activity is reduced or retained. subgroup iia - decreased thrombin inactivation, decreased factor xa inactivation and decreased heparin affinity. subgroup iib - decreased thrombin inactivation and normal heparin affinity. subgroup iic - normal thrombin inactivation, normal factor xa inactivation and decreased heparin affinity. in the revised system of classification again adopted by the scientific and standardization committee of the international society on thrombosis and haemostasis, type ii antithrombin deficiency remains subdivided into three subgroups: the already mentioned type ii pe, along with type ii rs, where mutations effect the reactive site and type ii hbs, where mutations effect the antithrombin heparin binding site. for the purposes of an antithrombin mutational database compiled by members of the plasma coagulation inhibitors subcommittee of the scientific and standardization committee of the international society on thrombosis and haemostasis, type iia cases are now classified as type ii pe, type iib cases as type ii rs and type iic cases as type ii hbs. toponyms presently it is relatively easy to characterise a specific antithrombin genetic mutation. however prior to the use of modern characterisation techniques investigators named mutations for the town or city where the individual suffering from the deficiency resided i.e. the antithrombin mutation was designated a toponym. modern mutational characterisation has since shown that many individual antithrombin toponyms are actually the result of the same genetic mutation, for example antithrombin-toyama, is equivalent to antithrombin-kumamoto, -amien, -tours, -paris-1, -paris-2, -alger, -padua-2 and -barcelona. medical uses antithrombin is used as a protein therapeutic that can be purified from human plasma or produced recombinantly (for example atryn, which is produced in the milk of genetically modified goats). it is approved by the fda as an anticoagulant for the prevention of clots before, during, or after surgery or birthing in patients with hereditary antithrombin deficiency. it has been studied in sepsis to reduce diffuse intravascular coagulation and other outcomes. it has not been found to confer any benefit in critically ill people with sepsis. cleaved and latent antithrombin figure 6. latent antithrombin cleavage at the reactive site results in entrapment of the thrombin protease, with movement of the cleaved reactive site loop together with the bound protease, such that the loop forms an extra sixth strand in the middle of beta sheet a. this movement of the reactive site loop can also be induced without cleavage, with the resulting crystallographic structure being identical to that of the physiologically latent conformation of plasminogen activator inhibitor-1 (pai-1). for this reason the conformation of antithrombin in which the reactive site loop is incorporated uncleaved into the main body of the protein is referred to as latent antithrombin. in contrast to pai-1 the transition for antithrombin from a normal or native conformation to a latent conformation is irreversible. native antithrombin can be converted to latent antithrombin (l-antithrombin) by heating alone or heating in the presence of citrate. however, without extreme heating and at 37 °c (body temperature) 10% of all antithrombin circulating in the blood is converted to the l-antithrombin over a 24-hour period. the structure of l-antithrombin is shown in figure 6. the 3-dimensional structure of native antithrombin was first determined in 1994. unexpectedly the protein crystallized as a heterodimer composed of one molecule of native antithrombin and one molecule of latent antithrombin. latent antithrombin on formation immediately links to a molecule of native antithrombin to form the heterodimer, and it is not until the concentration of latent antithrombin exceeds 50% of the total antithrombin that it can be detected analytically. not only is the latent form of antithrombin inactive against its target coagulation proteases, but its dimerisation with an otherwise active native antithrombin molecule also results in the native molecules inactivation. the physiological impact of the loss of antithrombin activity either through latent antithrombin formation or through subsequent dimer formation is exacerbated by the preference for dimerisation to occur between heparin activated β-antithrombin and latent antithrombin as opposed to α-antithrombin. a form of antithrombin that is an intermediate in the conversion between native and latent forms of antithrombin has also been isolated and this has been termed prelatent antithrombin. antiangiogenic antithrombin angiogenesis is a physiological process involving the growth of new blood vessels from pre-existing vessels. under normal physiological conditions angiogenesis is tightly regulated and is controlled by a balance of angiogenic stimulators and angiogenic inhibitors. tumor growth is dependent upon angiogenesis and during tumor development a sustained production of angiogenic stimulatory factors is required along with a reduction in the quantity of angiogenic inhibitory factors tumor cells produce. the cleaved and latent form of antithrombin potently inhibit angiogenesis and tumor growth in animal models. the prelatent form of antithrombin has been shown to inhibit angiogenesis in-vitro but to date has not been tested in experimental animal models.