Selenium is a trace element that is essential in small amounts, but can be toxic in larger amounts. It is an important constituent of the enzyme glutathione peroxidase, which is responsible for destroying lipid-damaging peroxides. In this role, selenium complements the anti-oxidizing function of vitamin E. The enzyme glutathione peroxidase is contained within white blood cells and blood platelets, and has importance in the immune system and blood clotting mechanisms of the body. It may also be important in the regulation of serum cholesterol levels, although this effect has not been conclusively proven experimentally. It serves to prevent the incidence of mercury poisoning by affecting the body's metabolism of the potential toxin.