No abstract

In mammals, the two enzymes in the trans-sulfuration pathway, cystathionine ␤-synthase (CBS) and cystathionine ␥-lyase (CSE), are believed to be chiefly responsible for hydrogen sulfide (H 2 S) biogenesis. In this study, we report a detailed kinetic analysis of the human and yeast CBS-catalyzed reactions that result in H 2 S generation. CBS from both organisms shows a marked preference for H 2 S generation by ␤-replacement of cysteine by homocysteine. The alternative H 2 S-generating reactions, i.e. ␤-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 H 2 S production by CBS would account for ϳ25-70% of the total H 2 S generated via the trans-sulfuration pathway depending on the extent of allosteric activation of CBS by S-adenosylmethionine. The relative contribution of CBS to H 2 S 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 H 2 S-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 H 2 S-yielding reactions but also yields novel thioether metabolites, thus increasing the complexity of the sulfur metabolome.

Although there is a growing recognition of the significance of hydrogen sulfide (H 2 S) as a biological signaling molecule involved in vascular and nervous system functions, its biogenesis and regulation are poorly understood. It is widely assumed that desulfhydration of cysteine is the major source of H 2 S in mammals and is catalyzed by the transsulfuration pathway enzymes, cystathionine ␤-synthase and cystathionine ␥-lyase (CSE). In this study, we demonstrate that the profligacy of human CSE results in a variety of reactions that generate H 2 S from cysteine and homocysteine. The ␥-replacement reaction, which condenses two molecules of homocysteine, yields H 2 S and a novel biomarker, homolanthionine, which has been reported in urine of homocystinuric patients, whereas a ␤-replacement reaction, which condenses two molecules of cysteine, generates lanthionine. Kinetic simulations at physiologically relevant concentrations of cysteine and homocysteine, reveal that the ␣,␤-elimination of cysteine accounts for ϳ70% of H 2 S generation. However, the relative importance of homocysteinederived H 2 S increases progressively with the grade of hyperhomocysteinemia, and under conditions of severely elevated homocysteine (200 M), the ␣,␥-elimination and ␥-replacement reactions of homocysteine together are predicted to account for ϳ90% of H 2 S generation by CSE. Excessive H 2 S production in hyperhomocysteinemia may contribute to the associated cardiovascular pathology.H 2 S is the newest member of a growing list of gaseous signaling molecules that modulate physiological functions (1-3). Concentrations of H 2 S ranging from 50 to 160 M have been reported in the brain (4), where it appears to function as a neuromodulator by potentiating the activity of the N-methyl-Daspartate receptor and by altering induction of long term potentiation in the hippocampus, important for memory and learning (5). H 2 S levels in human plasma are reported to be ϳ50 M, and in vitro studies suggest that it functions as a vasodilator by opening K ATP channels in vascular smooth muscle cells (6).A recent in vivo study has demonstrated the efficacy of H 2 S in attenuating myocardial ischemia-reperfusion injury by protecting mitochondrial function (7). The role of H 2 S in inflammation is suggested by several studies (8 -11); however, the underlying mechanism is unknown. Remarkably, H 2 S can also induce a state of suspended animation in mice by decreasing the metabolic rate and the core body temperature presumably by inhibiting cytochrome c oxidase in the respiratory chain (12).Endogenous H 2 S is presumed to be generated primarily by desulfhydration of cysteine catalyzed by the two pyridoxal phosphate (PLP) 3 -dependent enzymes in the transsulfuration pathway: cystathionine ␤-synthase (CBS) and cystathionine ␥-lyase (CSE) (13,14). In fact, it is widely assumed, based on the reported absences of CSE in the brain (15) and of H 2 S in the brain of CBS knock-out mice (16), that CBS is the primary source of H 2 S in this organ, whereas CSE plays the...

The role of endogenously produced H2S in mediating varied physiological effects in mammals has spurred enormous recent interest in understanding its biology and in exploiting its pharmacological potential. In these early days in the field of H2S signaling, large gaps exist in our understanding of its biological targets, its mechanisms of action and the regulation of its biogenesis and its clearance. Two branches within the sulfur metabolic pathway contribute to H2S production: (i) the reverse transsulfuration pathway in which two pyridoxal 5′-phosphate-dependent (PLP) enzymes, cystathionine β-synthase and cystathionine γ-lyase convert homocysteine successively to cystathionine and cysteine and (ii) a branch of the cysteine catabolic pathway which converts cysteine to mercaptopyruvate via a PLP-dependent cysteine aminotransferase and subsequently, to mercaptopyruvate sulfur transferase-bound persulfide from which H2S can be liberated. In this review, we present an overview of the kinetics of the H2S-generating reactions, compare the structures of the PLP-enzymes involved in its biogenesis and discuss strategies for their regulation.