Glycylglycyl-L-cysteine as a substrate for renal sulfhydryl oxidase (glutathione oxidase).

Covalent chromatographically isolated bovine kidney sulfhydryl oxidase was found to catalyze the oxidation of cysteine and cysteine-containing substrates as determined by assaying with 5,5'-dithiobis(2-nitrobenzoate). Monitoring the time-course of substrate disappearance and product formation by means of high-pressure liquid chromatography revealed that such partially purified renal sulfhydryl oxidase preparations catalyze the direct oxidation of glycylglycyl-L-cysteine to its disulfide form with no other detectable metabolic products. Accordingly, Gly-Gly-Cys appears to be better suited for routine assays of sulfhydryl oxidase activity than is the traditionally employed substrate, glutathione, whose oxidation can be initiated by gamma-glutamyltransferase-catalyzed cleavage of the gamma-peptide bond, leading to falsely 'positive' assays in the absence of sulfhydryl oxidase per se.