Body cells and tissues are vulnerable to free radical damage and reactive oxygen species which are produced during normal oxygen metabolism, by other chemical reactions, and by unwanted agents in the environment. Free radicals, once formed, are capable of disrupting metabolic activity and cell structure. When this occurs, additional free radicals are produced which, in turn, can alter cells and tissues, particularly the oxidation of DNA, proteins, and membrane lipids. The uncontrolled production of free radicals is thought to be a major contributing factor to oxidative damage. Superoxide dismutase (SOD) is a prime antioxidant enzyme found in two forms. One, complexed with zinc and copper, is localized in the cytosol, while the other, bound with manganese, is found in the mitochondrial matrix. Both forms of this metalloenzyme catalyze the inactivation of unwanted reactive oxygen species by converting them to hydrogen peroxide which is then transformed to water and oxygen by the enzyme catalase. Superoxide dismutase has been shown to be useful in joint, gastrointestinal, mitochondrial and respiratory health.