Proteins and Amino Acids Role of the Transsulfuration Pathway and of y-Cystathionase Activity in the Formation of Cysteine and Sulfate from Methionine in Rat Hepatocytes1 ASHA M. RAO, MARIER. DRAKE AND MARTHA H. STIPANÃœK' Division of /Vutriitona/ Sciences, Cornell University, Ithaca, NY 14653 ABSTRACT To assess the extent to which low he patic y-cystathionase levels affect methionine flux to cysteine in hepatocytes, the effect of inhibition of y-cystathionase activity with propargylglycine on the metabolism of L-[35S]methionine was determined in studies with freshly isolated rat hepatocytes. y-Cys- tathionase activity was inhibited 25%, 42%, 63% and 76% (maximal inhibition) by treatment with 2.5 umol/L, 0.01 mmol/L, 0.02 mmol/L and 2 mmol/L propargylglycine, respectively. Inhibition of y-cysta thionase activity with up to 0.02 mmol/L propar gylglycine had no statistically significant effect on pSjglutathione, [35SJsulfate or [3?S]cysteine forma tion 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 [3%Jcysteine by 89%; [35S]cystathionine accumulation in these incuba tion systems was 60 times control. Hepatic y-cys tathionase 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 Y-cystathionase activity may limit cysteine synthesis by the methionine transsulfuration pathway. No evi dence for cysteine synthesis from serine and sulfide, which can be catalyzed by cystathionine ß-synthase, or for methionine metabolism by an S-adenosyImethio- nine-independent pathway was obtained. J. Nutr. 120:837-845, 1990. INDEXING KEY WORDS: • j-cystathionase •propargylglycine •transsulfuration •methionine •rat hepatocytes Methionine sulfur metabolism is thought to occur primarily by the transsulfuration pathway, which re sults in the transfer of the sulfur of methionine to serine to form cysteine (Fig. 1). Other than diet, the only known source of cyst(e)ine for mammals is this syn thetic pathway. Because adult humans have a relatively high capacity for cysteine synthesis using the sulfur of methionine, cyst(e)ine is not strictly required in the diet. However, cysteine may supply 50% or more of the Methionine o ATP X Methylthloadenoslne S S-Adenosylmethlonine (7 j PI R-CH3 S-Adenosylhomocystelne ©j-, • AlÃ-enosme Homocystelne Serine Cystathionine a -Ketobutyrate + NH Cysteine Glutathlone \ Taurine \ SO -2 FIGURE 1 Pathways of methionine catabolism in mam malian cells. Enzymes that catalyze the numbered reactions are (1) methionine adenosyltransferase, (2) various methyl- transferases, (3) S-adenosylhomocysteine hydrolase, (4) cysta thionine ß-synthase, (5) 7-cystathionase, (6) A/5- methyltetrahydrofolate-homocysteine methyltransferase or betaine-homocysteine methyltransferase, (7) enzymes in volved in polyamine synthesis and (8)enzymes involved in the methylthioadenosine salvage pathway. 'This material is based upon work supported by the Cooperative State Research Service, U.S. Department of Agriculture under Agree ment No. 88-37200-3449 and by New York State Hatch Project No. 399-492. 2Author to whom reprint requests should be addressed. 0022-3166/90 $3.00 ©1990American Institute of Nutrition. Received 15 November 1989.Accepted 13 March 1990. 837 by guest on November 16, 2015 jn.nutrition.org Downloaded from