Selenium is co-factor for

Glutathione peroxidases, also known as selenoproteins, are a family of antioxidant enzymes that speed up the reaction between glutathione and free radicals, particularly toxic hydrogen peroxide, which selenium-containing glutathione peroxidases help transform to harmless water. These enzymes act both inside and outside the cells maximizing antioxidant protection. In cases of selenium deficiency and with the resulting impaired function of glutathione peroxidases destructive hydrogen peroxide breaks down into even more dangerous hydroxyl radical which damages cell membranes and cell DNA eventually leading to serious disease. Selenium is directly involved in the preservation of cell membrane integrity and DNA integrity.