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...

A 2 orders of magnitude gas transport improvement in a medium pore ZSM-5 zeolite has been achieved upon introduction of intracrystalline mesoporosity in gradient-free crystals by desilication post-treatment in alkaline medium.

The removal of acetylene from ethylene/acetylene mixtures containing 1% acetylene is a technologically very important, but highly challenging task. Current removal approaches include the partial hydrogenation over a noble metal catalyst and the solvent extraction of cracked olefins, both of which are cost and energy consumptive. Here we report a microporous metal–organic framework in which the suitable pore/cage spaces preferentially take up much more acetylene than ethylene while the functional amine groups on the pore/cage surfaces further enforce their interactions with acetylene molecules, leading to its superior performance for this separation. The single X-ray diffraction studies, temperature dependent gas sorption isotherms, simulated and experimental column breakthrough curves and molecular simulation studies collaboratively support the claim, underlying the potential of this material for the industrial usage of the removal of acetylene from ethylene/acetylene mixtures containing 1% acetylene at room temperature through the cost- and energy-efficient adsorption separation process.