**3.1 Primary enzymatic system**

Aerobic organisms have developed antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and DT-diaphorase. SOD is responsible for the dismutation reaction of O2 to H2O2, which in subsequent reactions, catalyzed by catalase or by GPx, is converted into H2O and O2. SOD is the most important and most powerful detoxification enzyme in the cell. SOD is a metalloenzyme and, therefore, requires a metal as a cofactor for its activity. Depending on the type of metal ion required as a cofactor by SOD, there are several forms of the enzyme [12, 13]. CAT uses iron or manganese as a cofactor and catalyzes the degradation or reduction of hydrogen peroxide (H2O2) to produce water and molecular oxygen, thus completing the detoxification process initiated by SOD [14, 15]. CAT is highly efficient at breaking down millions of H2O2 molecules in a second. CAT is mainly found in peroxisomes, and its main function is to eliminate the H2O2 generated during the oxidation of fatty acids. GPx is an important intracellular enzyme that breaks down H2O2 in water and lipid peroxides in their corresponding alcohols; this happens mainly in the mitochondria and sometimes in the cytosol [16]. The activity of GPx depends on selenium. In humans, there are at least eight enzymes GPx, GPx1–GPx8 [17]. Among glutathione peroxidases, GPx1 is the most abundant selenoperoxidase and is present in virtually all cells. The enzyme plays an important role in inhibiting the process of lipid peroxidation and, therefore, protects cells from oxidative stress [18]. Low GPx activity leads to oxidative damage of the functional proteins and the fatty acids of the cell membrane. GPx, particularly GPx1, has been implicated in the development and prevention of many diseases, such as cancer and cardiovascular diseases [19]. DT-diaphorase catalyzes the reduction of quinone to quinol and participates in the reduction of drugs of quinone structure [20]. DNA regulates the production of these enzymes in cells.
