Cystine rather than cysteine is the preferred substrate for β-elimination by cystathionine γ-lyase: implications for dietary methionine restriction.

Abstract

Dietary methionine restriction(MR) increases longevity by improving health. In experimental models, MR is accompanied by decreased cystathionine β-synthase activity and increased cystathionine γ-lyase activity. These enzymes are parts ofthe transsulfuration pathway which produces cysteine and 2-oxobutanoate. Thus, the decrease in cystathionine β-synthase activity is likely to account for the loss of tissue cysteineobserved in MR animals.Despite this decrease in cysteine levels, these tissues exhibit increasedH2Sproduction which is thought to be generated by β-elimination of the thiol moiety of cysteine, as catalyzed by cystathionine β-synthase or cystathionine γ-lyase. Another possibility for this H2S production is thecystathionine γ-lyase-catalyzed β-elimination of cysteine persulfide from cystine, which upon reduction yields H2Sand cysteine. Here, we demonstrate that MR increases cystathionine γ-lyase production and activities in the liver and kidneys, and that cystine is a superior substrate for cystathionine γ-lyase catalyzed β-elimination as compared to cysteine. Moreover, cystine and cystathionine exhibit comparable Kcat/Km values (6000M-1s-1)as substrates forcystathionine γ-lyase-catalyzed β-elimination. By contrast, cysteine inhibits cystathionine γ-lyasein a non-competitive manner (Ki ~ 0.5mM), which limits its ability to function as a substrate for β-elimination by this enzyme. Cysteine inhibits the enzyme by reacting with its pyridoxal 5'-phosphate cofactor to form a thiazolidineand in so doing prevents further catalysis. These enzymological observations are consistent with the notion that duringMR cystathionine γ-lyaseis repurposed to catabolize cystine and therebyformcysteine persulfide, which upon reduction produces cysteine.