Enzymatic catalysis of the reversible sulfitolysis of glutathione disulfide and the biological reduction of thiosulfate esters

Abstract

Rat liver supernatants were shown to contain an enzymatic activity catalyzing in both forward and reverse directions the reversible sulfitolysis of glutathione disulfide. The enzymatic sulfitolysis has maximal activity at pH 7. S-Sulfoglutathione, which is a product of the sulfitolysis, was isolated by passage through an ion-exchange column. Three different assays were applied to determine S-sulfoglutathione, viz., methods based on the ninhydrin reaction, the formation of a thiazoline derivative in strong acid, and the use of radioactively labeled glutathione. The reversal of the sulfitolysis, i.e., the reaction of S-sulfoglutathione with glutathione, was studied directly by determination of sulfite with radioactive N-ethylmaleimide, or indirectly by coupling to the NADPH- and glutathione reductase-linked reduction of glutathione disulfide. Chromatographic analysis of rat liver supernatants demonstrated that all fractions catalyzing the reversible sulfitolysis did also catalyze the previously studied thiol-disulfide interchange of glutathione and the mixed disulfide of cysteine and glutathione. The reduction of thiosulfate esters, such as S-sulfocysteine and trimethylammonium-ethylthiosulfate, with glutathione was also catalyzed by the enzyme active in the sulfitolysis, which indicates an important biosynthetic role of the enzyme in microorganisms synthesizing cysteine via S-sulfocysteine. The enzyme is also capable of participating in the formation of the naturally occurring S-sulfoglutathione.