Hepatocyte-Catalysed Detoxification of Cyanide by l- and d-Cysteine

Abstract

The hepatocyte metabolic pathways involved in the detoxification of cyanide by cysteine have been investigated in vitro using hepatocytes isolated from Sprague–Dawley rats. Cyanide toxicity towards isolated hepatocytes could be prevented by the addition of l- or d-cysteine, cystine, or the cysteine metabolites thiosulfate and mercaptopyruvate, which markedly increased thiocyanate formation. Prior depletion of hepatocyte GSH markedly increased thiosulfate formation from l- or d-cysteine without affecting thiocyanate formation from l- or d-cysteine. This suggested that the major metabolic pathway for thiocyanate formation did not involve thiosulfate. Mercaptopyruvate was a more likely metabolic intermediate, as thiocyanate formation from l-cysteine but not thiosulfate was inhibited markedly by aminooxyacetate, a cysteine aminotransferase inhibitor, and propargylglycine, a γ-cystathionase inhibitor. Furthermore, propargylglycine prevented l-cysteine cytoprotection against cyanide toxicity. Thiocyanate formation from d-cysteine likely also involved mercaptopyruvate, as thiocyanate formation from d-cysteine but not l-cysteine was inhibited by benzoate, an inhibitor of d-amino acid oxidase. Furthermore, benzoate prevented d-cysteine cytoprotection against cyanide toxicity. Cystine may also be an intermediate, as hepatocyte thiocyanate formation from added l-cysteine was inhibited when l-cysteine autoxidation was prevented with the copper chelator bathocuproine disulfonate. Furthermore, thiocyanate formation by rat liver homogenates with l-cystine was far more rapid than that with l-cysteine. Hepatocyte thiocyanate metabolic intermediates of β-mercaptopyruvate and thiocystine were proposed for l-cysteine, and β-mercaptopyruvate was proposed for d-cysteine.