Superoxide Dismutase (SOD): The dismutation of superoxide by SOD to hydrogen peroxide is generally considered to be the primarily antioxidant defense of the body because this enzyme prevents the further generation of free radicals. SOD exists in virtually every oxygen-respiring organism and its major function is to catalyze the dismulative reaction.

The superoxide dismutases are divided into three distinct classes depending on the metal ion content, namely, Cu/ZnSOD, MnSOD, and FeSOD. Some of the SOD activity is extracellular, but the bulk of the activity is intracellular, where it is divided between the mitochondria (MnDOS) and the cytosolic compartments (Cu/ZnSOD). The SOD level varies according to the part of the body. The highest levels of SOD are found in the liver, adrenal gland, kidney, and spleen.

Catalase. As one of the major primary antioxidant defense mechanisms, catalase works primarily to catalyze the decomposition of hydrogen peroxide to water, a function shared with glutathione peroxidase. Both enzymes detoxify oxygen-reactive radicals by catalyzing the formation of hydrogen peroxide derived from superoxide. The liver, kidney and red blood cells possess high levels of catalase.

Glutathione Peroxidase. The enzyme glutathione peroxidase catalyzes the reduction of hydrogen peroxide and organic hydroperoxides.

Both selenium dependent and selenium independent glutathione peroxidases protect against free radical damage by reducing peroxides. Glutathione peroxidases are intracellular, located in the mitochondria and the cytosol of the cell. Glutathione peroxidases play an important role in the inhibition of lipid peroxidation of the intracellular membranes.

Secondary Defenses

Lipolytic Enzymes: The lipolytic enzymes, a group of the phospholipases, are concerned with the maintenance of cellular membrane integrity. The peroxidation of cellular membranes stimulates the lipolytic action of phospholipase A2. The predilection of this enzyme for oxidized lipids is important in membrane repair and detoxification by providing cells with additional protection against lipid peroxidation (Yu et al., 1994).

Proteolytic Enzymes: Various proteolytic enzymes act as a secondary antioxidant defense mechanism. By preferentially degrading many oxidatively-altered proteins, they prevent an accumulation of damaged proteins in the cell. This enzymatic action is believed by some investigators to be of importance in aging (Yu, 1994). Proteasome, a multicatalytic protinase complex, may also play an important role in the degradation and elimination of oxidatively damaged proteins (Yu, 1994).