Metabolism of Cyst(e)ine in Rat Enterocytes

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Abstract
Cyst(e)ine was metabolized by rat enterocytes to pyruvate and inorganic sulfur but not to taurine. Cystine was the major extracellular form of cyst(e)ine present during the incubation, and addition of bathocuproine disulfonate, a copper chelator that maintained 60% of the total cyst(e)ine in the sulfhydryl form, had no effect on total sulfur release from cyst(e)ine. Oxidation of cyst(e)ine to 35SO42- or 14CO2 was reduced by about 50% when unlabeled cysteinesulfinate was added to incubations of enterocytes with labeled cyst(e)ine. Thus, about one half of cyst(e)ine metabolism appeared to involve its oxidation to cysteinesulfinate and the transamination of cysteinesulfinate to the putative intermediate sulfinylpyruvate, which decomposes to yield sulfite and pyruvate. The remainder of cyst(e)ine catabolism in enterocytes appeared to involve release of sulfur from cyst(e)ine prior to its oxidation. Inhibition of γ-cystathionase by propargylglycine, although incomplete, resulted in substantial inhibition of cyst(e)ine catabolism. The accumulation of cysteinethiosulfonate, which forms nonenzymatically upon incubation of cyst(e)ine with thiosulfate, and the inhibition of cysteinethiosulfonate formation by propargylglycine demonstrated the catabolism of cyst(e)ine by β-cleavage catalyzed by γ-cystathionase. Sulfide released from cyst(e)ine in this reaction appeared to be oxidized to thiosulfate before it was further oxidized to sulfite and sulfate. In addition to being oxidized to sulfate, some of the sulfite formed by enterocytes reacted with cyst(e)ine in the incubation medium to form sulfocysteine. Activities of enzymes of cyst(e)ine catabolism in rat enterocytes corresponded with the observed metabolism of cyst(e)ine by various pathways.

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