**5. Role of selenium in the body**

Selenium, an important component of selenoprotein, plays diverse biological roles ranging from antioxidant defense to synthesis of DNA to reproduction. Various metabolites formed from selenium could also play a role in the prevention of carcinogenesis. It could also improve tolerance and recuperation thus slowing down the aging process [36, 37].

#### **5.1 Selenoprotein**

## *5.1.1 Glutathione peroxidase (GPx)*

An antioxidant, whose primary role is to counteract the effect of hydrogen peroxide and other hydroperoxides in the body. GPx exists in about eight different forms grouped according to their features. They differ by mode of action and site of action. They work alongside vitamin E to protect cells from accumulated H2O2 hence they ensure the integrity of the cell wall. The first four forms of GPx enzymatic activity are directly proportional to the intake of selenium. Hence there is a correlation between oxidative stress and lack of selenium in the body [38–40].

Glutathione peroxidase-1 (GPx-1) occurs mostly in the liver, erythrocytes, lungs and kidneys. Deficiency in selenium affects the activity of GPx-1. Glutathione peroxidase-2 (GPx-2) protects against oxidation and it occurs mostly in the gastrointestinal tissues and the liver [41]. GPx-3 is found in the plasma, heart, kidneys, liver and it covers over 20% of the plasma selenium. It reduces the level of hydroperoxides [42]. GPx-4 is located in the mitochondria, nucleus and cytosol with its highest activity in the testes [43]. In addition to its antioxidant role, it prevents occurrence of peroxidation on the membrane. It is involved in the conversion of cholesterol and its ester into non-toxic derivatives and also prevents oxidation that can lead to DNA damage. The role of GPx-5 is still unknown but it is found in the embryo, while the other GPx: 6, 7, 8 are less studied [44].
