ROLE OF THE TRANSSULFURATION PATHWAY AND OF GAMMA-CYSTATHIONASE ACTIVITY IN THE FORMATION OF CYSTEINE AND SULFATE FROM METHIONINE IN RAT HEPATOCYTES

To assess the extent to which low hepatic γ-cystathionase levels affect methionine flux to cysteine in hepatocytes, the effect of inhibition of γ-cystathionase activity with propargylglycine on the metabolism of l-[35S]methionine was determined in studies with freshly isolated rat hepatocytes. γ-Cystathionase activity was inhibited 25%, 42%, 63% and 76% (maximal inhibition) by treatment with 2.5 μmol/L, 0.01 mmol/L, 0.02 mmol/L and 2 mmol/L propargylglycine, respectively. Inhibition of γ-cystathionase activity with up to 0.02 mmol/L propargylglycine had no statistically significant effect on [35S]glutathione, [35S]sulfate or [35S]cysteine formation from [35S]methionine. However, treatment of cells with 2 mmol/L propargylglycine markedly inhibited the metabolism of [35S]methionine to [35S]glutathione by 93%, to [35S]sulfate by 88% and to [35S]cysteine by 89%; [35S]cystathionine accumulation in these incubation systems was 60 times control. Hepatic γ-cystathione activity in premature infants has been reported to be about 23% of mature levels (Zlotkin and Anderson, 1982; Pediatr. Res. 16: 65-68); this level of γ-cystathionase activity may limit cysteine synthesis by the methionine transsulfuration pathway. No evidence for cysteine synthesis from serine and sulfide, which can be catalyzed by cystathionine β-synthase, or for methionine metabolism by an S-adenosylmethionine-independent pathway was obtained.