Hydrogen Sulfide By Ronald Steriti, ND, PhD © 2012

Hydrogen sulfide (H2S) is a well-known and pungent gas that has strong odor of rotten eggs. H2S is synthesized in mammalian and human tissues, and is used in the body as a signaling molecule. Hydrogen sulfide gas is considered a broad-spectrum poison that can poison several different systems in the body, although the nervous system is most affected. Since hydrogen sulfide occurs naturally in the body, the environment and the gut, exist in the body that are capable of detoxifying it by oxidation to (harmless) sulfate. (Ramasamy, Singh et al. 2006) Metabolism Hydrogen sulfide is formed endogenously in the human body by enzymes such as cystathionine beta-synthase (CBS) in the brain and cystathionine gamma- (CSE) in liver, vascular and non-vascular smooth muscle. (Jacob, Anwar et al. 2008) Eventually the gas is converted to sulfite in the mitochondria by thiosulfate reductase, and the sulfite is further oxidized to thiosulfate and sulfate by sulfite oxidase. The sulfates are excreted in the urine. Biomarkers The critical role of mitochondrial oxidation of sulfide is emphasized by recent identification of the ETHE1 gene as the mitochondrial sulfur dioxygenase. Mutations in the ETHE1 gene caused high concentrations of C4 and C5 acylcarnitines in blood (perhaps due to inhibition of short- chain acyl-CoA dehydrogenase by H2S), and high excretion of ethylmalonic acid in urine. (Stipanuk and Ueki 2011) Similarly, ETHE1-/− mice had elevated tissue concentrations of sulfide and thiosulfate, low levels of cytochrome c oxidase in muscle and brain, elevated levels of lactate and C4 and C5 acylcarnitines in plasma, elevated ethylmalonic acid and thiosulfate in urine, lower sulfate abundance in urine, and undetectable sulfite levels in urine. Neurodegenerative Diseases Evidence for the roles of H2S in neurodegenerative diseases. (Gong, Shi et al. 2011) Alzheimer’s disease (AD) S-adenosylmethionine is severely decreased in AD patients Total homocysteine is increased in AD patients H2S are decreased in AD patients, and its alteration may be associated with the severity of AD H2S attenuated LPS-induced cognitive deficits S-propargyl-, a novel hydrogen sulfide-modulated agent, attenuates LPS-spatial learning and memory impairment S-propargyl-cysteine attenuates Abeta-induced cognitive deficits H2S reduces mRNA and protein levels of beta-site A beta PP cleaving 1 in PC12 cells H2S attenuates A beta -induced PC12 cells damage Parkinson’s disease (PD) Plasma Hcy levels are elevated in PD patients treated with L-DOPA H2S levels in the SN and striatum are considerably reduced in both 6- OHDA and rotenone-induced PD-like rats Inhalation of H2S prevented the MPTP-induced movement disorder Vascular dementia (VaD) H2S attenuates neuronal injury induced by VaD via inhibiting apoptosis in rats Huntington’s disease (HD) CBS interacts with Huntingtin Plasma Hcy levels are elevated in patients of HD

Cysteine Only two of the twenty amino acids normally present in proteins are sulfur- containing amino acids (SAAs), namely methionine and cysteine. Methionine cannot be synthesized by the human body and must be supplied by the diet, whereas cysteine requirements can, in principle, be met by an excess of dietary methionine. However, cysteine is known as a semi-essential amino acid because humans can synthesize it from methionine to a limited extent. (Predmore, Lefer et al. 2012) In spite of the critical role of sulfur in our diet, and especially of an adequate cysteine intake, dietary consumption of cysteine is generally suboptimal. On the other hand, homeostatic regulation of cysteine and GSH pools declines with age, with the onset appearing in men at a younger age than in women. (Predmore, Lefer et al. 2012) Cysteine dioxygenase (CDO) is one of the most highly regulated metabolic enzymes known to respond to diet: hepatic or adipocyte CDO concentration increases by up to 45-fold, and catalytic efficiency of the enzyme increases by up to 10-fold with increases in Cys availability. For example, hepatic CDO activity is very low in animals fed a low-protein diet (e.g., 100 g casein per kg diet) but increases dramatically when the protein level in the diet is increased to 200 g/kg (near the requirement) and even more when it is increased to 400 g/kg (excess of the requirement), with the new steady- state levels of CDO activity being reached within hours of the diet change. (Stipanuk and Ueki 2011) CDO is remarkable among metabolic enzymes in the degree to which its activity can be regulated in response to its , suggesting that tight regulation of Cys levels is a critical physiological function. (Stipanuk and Ueki 2011) NAC Since high dietary intakes of methionine have been shown to raise plasma levels of homocysteine, despite adequate intake of B vitamins, and since free cysteine can be a pro-oxidant, cysteine supplementation is nowadays achieved by oral administration of NAC. (Predmore, Lefer et al. 2012) Pyridoxal 5'-phosphate (P5P) Transsulfuration of Hcy to Cys is catalyzed by two pyridoxal 5′-phosphate (PLP)-dependent enzymes, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE). (Stipanuk and Ueki 2011) Glutathione The availability of cysteine appears to be the rate-limiting factor for GSH biosynthesis from glutamate, glycine, and cysteine. (Predmore, Lefer et al. 2012) GSH has been shown to act as a ‘‘sulfide buffer’’ when H2S starts to build up in the cell. (Predmore, Lefer et al. 2012) Involvement of the glutathione-dependent thiosulfate reductase in sulfite production is consistent with the glutathione dependence of sulfate production by hepatocytes. Glutathione depletion markedly reduced sulfate production from Cys and resulted in accumulation of thiosulfate. When thiosulfate was used as a substrate, glutathione depletion similarly blocked its conversion to sulfate. On the other hand, thiosulfate sulfurtranserase (rhodanese) has a much higher Km for thiosulfate than for sulfite, suggesting conversion of thiosulfate to sulfite by this enzyme would be limited. (Stipanuk and Ueki 2011) Molybdenum Sulfite oxidase is a heme-containing enzyme that is a member of the cytochrome b5 family and contains a molybdopterin . Along with oxidation of sulfite to sulfate, electrons are transferred, one at a time, from the molybdenum cofactor to the b5 heme of sulfite oxidase and then to the electron carrier cytochrome c. (Stipanuk and Ueki 2011) S-adenosylmethionine (SAMe) Transsulfuration is regulated by stimulation of cystathionine β-synthase and inhibition of methylene tetrahydrofolate reductase in response to changes in the level of S-adenosylmethionine, and this promotes homocysteine degradation when methionine availability is high. (Stipanuk and Ueki 2011) CBS is a highly regulated enzyme, with S- adenosylmethionine acting as an allosteric activator that increases CBS activity as well as stabilizing the enzyme (and thus increasing its concentration) when Met is abundant Addition of S-adenosylmethionine to enhance CBS activity resulted in increased H2S production from Cys by about 50% in both liver and kidney, indicating a likely contribution of CBS to Cys desulfuration, especially under conditions in which the enzyme is allosterically activated by high S- adenosylmethionine levels. α-ketoglutarate In addition to the several possible routes of Cys desulfuration, mammals catabolize Cys by cysteinesulfinate-dependent pathways in which the sulfur is partially oxidized in the first step. Alternatively, cysteinesulfinate may be transaminated (with α-ketoglutarate) in a unidirectional reaction to form the enzyme-bound keto acid β-sulfinylpyruvate, which spontaneously dissociates to gives rise to pyruvate and sulfur dioxide (SO2,) which is hydrated to sulfite in vivo. The sulfite in turn is readily oxidized to sulfate by sulfite oxidase. (Stipanuk and Ueki 2011) This production of H2S by whole-brain homogenates was markedly increased by addition of 0.1 mM α-ketoglutarate. (Stipanuk and Ueki 2011) Tyrosine The CDO Cys-Tyr moiety seems to serve a cofactor function in CDO. Although CDO possessed appreciable catalytic activity in the absence of the Cys- Tyr cofactor, cofactor formation increased CDO catalytic efficiency by ~10-fold. (Stipanuk and Ueki 2011) Iron As with other amino-acid cofactor-containing enzymes, formation of the CDO Cys-Tyr cross-link required a transition metal cofactor - ferrous iron (Fe2+) and oxygen (O2). (Stipanuk and Ueki 2011) Garlic Garlic is rich in organosulfur compounds considered responsible for most of its pharmacological activities. (diallyl thiosulfinate), the main organosulfur compound, is produced from the amino acid alliin by action of the enzyme alliinase when garlic is crushed. Allicin, unstable in aqueous solution, rapidly decomposes mainly to diallyl sulfide (DAS), (DADS), diallyl trisulfide (DATS), and ajoene. After consumption, neither allicin nor its metabolites have been found in blood or urine, indicating that these compounds are rapidly metabolized. (Benavides, Squadrito et al. 2007) Cardioprotective effects of dietary garlic are mediated in large part via the generation of hydrogen sulfide (H2S). Garlic-derived organic polysulfides are converted by erythrocytes into hydrogen sulfide, which relaxes vascular smooth muscle, induces vasodilation of blood vessels, and significantly reduces blood pressure. (Ginter and Simko 2010)

Metabolism

References

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