According to Cooper (1988) '... a literature review uses as its database reports of primary or original scholarship, and does not report new primary scholarship itself. The primary reports used in the literature may be verbal, but in the vast majority of cases reports are written documents.
Reactive oxygen species. Reactive oxygen species (ROS) are chemically reactive chemical species containing oxygen. Examples include peroxides, superoxide, hydroxyl radical, and singlet oxygen. In a biological context, ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis.
The catalase enzyme is so critical to our health that it is found in nearly every living organism on the planet that is exposed to oxygen. This antioxidant enzyme can catalyze the conversion of hydrogen peroxide into water and oxygen. Hydrogen peroxide is a by-product of cell metabolism, which serves some useful functions including healthy immune response. Catalase has one of the highest rates of turnover when compared to all other enzymes. In other words, one catalase enzyme can change 40 million molecules of hydrogen peroxide into water and oxygen in just one second.