The apparent glutathione oxidase activity of gamma-glutamyl transpeptidase. Chemical mechanism.

The apparent glutathione oxidase activity of y-glutamyl transpeptidase is due to nonenzymatic oxidation and transhydrogenation reactions of cysteinylglycine, an enzymatic product formed from glutathione by hydrolysis or autotranspeptidation. Since cysteinylglycine reacts with oxygen more rapidly than does glutathione, the rate of disulfide formation is increased and either cystinyl-bis-glycine or the mixed disulfide of cysteinylglycine and glutathione forms as an intermediate product.Nonenzymatic transhydrogenation reactions of these disulfides with glutathione yield glutathione disulfide and thus account for the apparent glutathione oxidase activity of y-glutamyl transpeptidase.A sensitive assay for glutathione oxidation is described, and it is shown that covalent inhibitors of y-glutamyl transpeptidase abolish the oxidase activity of the purified enzyme and of crude homogenates of mouse and rat kidney.The oxidation of GSH to GSSG in oxygenated aqueous solutions is slow in comparison to the "spontaneous" oxidation of other biological thiols such as cysteine or cysteamine.In 1945 and 1947, Elvehjem et al. (1, 2 ) reported that kidney extracts catalyzed the oxidation of GSH and that liver extracts lacked this activity.Later reports indicated that isolated kidney tubule cells oxidize GSH present in the suspending medium; despite the high levels of y-glutamyi transpeptidase on the membranes of these cells, the oxidation was much faster than the cleavage of GSH or GSSG (3, 4).Recently, it was found that the tissue distribution of glutathione oxidase activity paralleled that of transpeptidase (5), and it was shown that homogeneous preparations of transpeptidase catalyzed the oxidation of GSH (6).We now report that GSH oxidation in the presence of transpeptidase is due to the enzyme-mediated formation of cysteinylglycine and to the subsequent nonenzymatic oxidation and transhydrogenation reactions of that product.Transpeptidase is, therefore, an initiator of GSH oxidation rather than a true catalyst of the oxidation.In the absence of transpeptidase, the addition of small amounts of cystinyl-bis-glycine, cysteine, or cystine to solutions of GSH results in the