No abstract

Both cysteinesulfinate-independent and cysteinesulfinate-dependent pathways are involved in the catabolism of cyst(e)ine by freshly isolated rat renal cortical tubules. Sulfate and thiosulfate were shown to be the major sulfur-containing products that accumulated in incubations of renal tubules with 1 mmol/L or 25 mmol/L [35S]cyst(e)ine. Thiosulfate is an intermediate in the oxidation of the sulfide produced by the cysteinesulfinate-independent catabolism of cyst(e)ine by desulfhydration pathway(s), whereas sulfate is formed both by further oxidation of thiosulfate and by oxidation of the sulfite formed by the cysteinesulfinate-transamination pathway. Incubation of renal tubules with propargylglycine inhibited gamma-cystathionase activity by 85%, and this resulted in a 46% decrease in sulfate production and a 68% decrease in thiosulfate production when the treated renal tubules were incubated with 1 mmol/L [35S]cyst(e)ine. Addition of 25 mmol/L unlabeled cysteinesulfinate to create a diluting/trapping pool for [35S]cysteinesulfinate formed from [35S]cysteine resulted in a 53% decrease in [35S]sulfate production in incubations with 1 mmol/L cysteine. Thus, some cyst(e)ine catabolism probably occurred by a cysteinesulfinate-dependent pathway. No production of taurine or hypotaurine was detected in incubations with cyst(e)ine. Thus, cysteinesulfinate formed from cysteine was further catabolized primarily to sulfate instead of to taurine and hypotaurine. Most cyst(e)ine catabolism by the epithelial cells of the renal tubule probably can be accounted for by two pathways: 1) the beta-cleavage of cystine catalyzed by lambda-cystathionase and 2) the formation and transamination of cysteinesulfinate catalyzed by cysteine dioxygenase and aspartate aminotransferase.

No abstract

The metabolism of cysteine and cysteinesulfinate was studied in freshly isolated hepatocytes from fed rats and cats. In incubations of rat hepatocytes with cysteinesulfinate, the rate of hypotaurine plus taurine production was approximately the same as the rate of conversion of the 1-carbon of cysteinesulfinate to CO2. In contrast, no significant production of hypotaurine plus taurine occurred in incubations of cat hepatocytes with cysteinesulfinate. These data are consistent with the species difference in the activity of hepatic cysteinesulfinate decarboxylase, which converts cysteinesulfinate to hypotaurine. In incubations of either rat or cat hepatocytes with cysteine, no hypotaurine plus taurine production was detected. However, the 1-carbon of cysteine was converted to CO2 and the production of urea plus ammonia nitrogen was significantly increased over the rates observed in incubations of cells without substrate. Our results suggest that most cysteine oxidation by hepatocytes occurs by pathways that do not involve formation of cysteinesulfinate.