Production of the Neuromodulator H2S by Cystathionine β-Synthase via the Condensation of Cysteine and Homocysteine

Hydrogen sulfide (H2S) has been observed in relatively high concentrations in the mammalian brain and has been shown to act as a neuromodulator. However, there is confusion in the literature regarding the actual source of H2S production. Reactions catalyzed by the cystathionine β-synthase enzyme (CBS) are one possible source for the production of H2S. Here we show that the CBS enzyme can efficiently produce H2S via a β-replacement reaction in which cysteine is condensed with homocysteine to form cystathionine and H2S. The production of H2S by this reaction is at least 50 times more efficient than that produced by hydrolysis of cysteine alone via β-elimination. Kinetic studies demonstrate that theKmandKcatfor cysteine is 3-fold higher and 2-fold lower, respectively, than that for serine. Consistent with these data,in vitroreconstitution studies show that at physiologically relevant concentrations of serine, homocysteine, and cysteine, about 5% of the cystathionine formed is from cysteine. We also show that AdoMet stimulates this H2S producing reaction but that there is no evidence for stimulation by calcium and calmodulin as reported previously. In summary, these results confirm the ability of CBS to produce H2S, but show in contrast to prior reports that the major mechanism is via β-replacement and not cysteine hydrolysis. In addition, these studies provide a biochemical explanation for the previously inexplicable homocysteine-lowering effects ofN-acetylcysteine treatments in humans.