The selenium-containing glutathione peroxidase, when in its active reduced form, was inactivated during exposure to the xanthine oxidase reaction. Superoxide dismutase completely prevented this inactivation, whereas catalase, hydroxyl radical scavengers, or chelators did not, indicating that was the responsible agent. Conversion of GSH peroxidase to its oxidized form, by exposure to hydroperoxides, rendered it insensitive toward . The oxidized enzyme regained susceptibility toward inactivation by when reduced with GSH. The inactivation by could be reversed by GSH; however, sequential exposure to and then hydroperoxides caused irreversible inactivation. Reactivity toward CN⁻ has been used as a measure of the oxidized form of GSH peroxidase, whereas reactivity toward iodoacetate has been taken as an indicator of the reduced form. By these criteria both and hydroperoxides convert the reduced form to oxidized forms. A mechanism involving oxidation of the selenocysteine residue at the active site has been proposed to account for these observations.