Abstract
In mammals, the two enzymes in the trans-sulfuration pathway, cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CSE), are believed to be chiefly responsible for hydrogen sulfide (H2S) biogenesis. In this study, we report a detailed kinetic analysis of the human and yeast CBS-catalyzed reactions that result in H2S generation. CBS from both organisms shows a marked preference for H2S generation by beta-replacement of cysteine by homocysteine. The alternative H2S-generating reactions, i.e. beta-elimination of cysteine to generate serine or condensation of 2 mol of cysteine to generate lanthionine, are quantitatively less significant. The kinetic data were employed to simulate the turnover numbers of the various CBS-catalyzed reactions at physiologically relevant substrate concentrations. At equimolar concentrations of CBS and CSE, the simulations predict that H2S production by CBS would account for approximately 25-70% of the total H2S generated via the trans-sulfuration pathway depending on the extent of allosteric activation of CBS by S-adenosylmethionine. The relative contribution of CBS to H2S genesis is expected to decrease under hyperhomocysteinemic conditions. CBS is predicted to be virtually the sole source of lanthionine, and CSE, but not CBS, efficiently cleaves lanthionine. The insensitivity of the CBS-catalyzed H2S-generating reactions to the grade of hyperhomocysteinemia is in stark contrast to the responsiveness of CSE and suggests a previously unrecognized role for CSE in intracellular homocysteine management. Finally, our studies reveal that the profligacy of the trans-sulfuration pathway results not only in a multiplicity of H2S-yielding reactions but also yields novel thioether metabolites, thus increasing the complexity of the sulfur metabolome.