The glutathione dependence of inorganic sulfate formation from l- or d-cysteine in isolated rat hepatocytes

Abstract

The GSH dependence of the metabolic pathways involved in the conversion of cysteine to sulfate in intact cells has been investigated. It was found that hepatocyte-catalysed sulfate formation from added l-cysteine did not occur if hepatocyte GSH was depleted beforehand, but was restored when GSH levels recovered. Furthermore, sulfate formation did not recover in GSH-depleted hepatocytes if GSH synthesis was prevented with buthionine sulfoximine. Thiosulfate formation was, however, markedly enhanced in GSH-depleted hepatocytes. These results suggest that thiosulfate is an intermediate in the formation of inorganic sulfate from l-cysteine and that GSH was required for the conversion of thiosulfate to inorganic sulfate. Much less sulfate was formed if the cysteine was replaced with cysteinesulfinate. Furthermore, sulfate formation from l-cysteine was markedly inhibited by the addition of the transaminase inhibitor dl-cycloserine or the γ-cystathionase inhibitor dl-propargylglycine. The major routes of sulfate formation from l-cysteine therefore seems to involve pathways that do not involve l-cysteinesulfinate. Similar amounts of sulfate were formed from d-cysteine as l-cysteine. Thiosulfate instead of sulfate was also formed in GSH-depleted hepatocytes. However, sulfate formation from d-cysteine differed from l-cysteine in that it was inhibited by the d-aminoacid oxidase inhibitor sodium benzoate and was not affected by transaminase or γ-cystathionase inhibitors. These results suggest that thiosulfate is an intermediate in sulfate formation from d-cysteine and involves the oxidation of d-cysteine by d-amino acid oxidase to form β-mercaptopyruvate.