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

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