Hepatocyte-catalysed detoxification of cyanide by L- and D-cysteine.
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 gamma-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 beta-mercaptopyruvate and thiocystine were proposed for L-cysteine, and beta-mercaptopyruvate was proposed for D-cysteine.
Huang
J
J
Faculty of Pharmacy, University of Toronto, Ontario, Canada.
Niknahad
H
H
Khan
S
S
O'Brien
P J
PJ
D000818:Animals; D003486:Cyanides / Q000633:toxicity*; D003545:Cysteine / Q000494:pharmacology*; D003553:Cystine / Q000031:analogs & derivatives / Q000378:metabolism; D066298:In Vitro Techniques; D008658:Inactivation, Metabolic; D008099:Liver / Q000378:metabolism*; D008297:Male; D019289:Pyruvic Acid / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013861:Thiocyanates / Q000493:pharmacokinetics*
D003486:Cyanides; D013861:Thiocyanates; C019572:thiocystine; D003553:Cystine; D019289:Pyruvic Acid; D003545:Cysteine; C031760:thiocyanate