The glutathione dependence of inorganic sulfate formation from L- or D-cysteine in isolated rat hepatocytes

The epigenetic signatures of opioid addiction and physical dependence are prevented by D-cysteine ethyl ester and betaine

We have reported that D,L-thiol esters, including D-cysteine ethyl ester (D-CYSee), are effective at overcoming opioid-induced respiratory depression (OIRD) in rats. Our on-going studies reveal that co-injections of D-CYSee with multi-day morphine injections markedly diminish spontaneous withdrawal that usually occurs after cessation of multiple injections of morphine in rats. Chronically administered opioids are known (1) to alter cellular redox status, thus inducing an oxidative state, and (2) for an overall decrease in DNA methylation, therefore resulting in the transcriptional activation of previously silenced long interspersed elements (LINE-1) retrotransposon genes. The first objective of the present study was to determine whether D-CYSee and the one carbon metabolism with the methyl donor, betaine, would maintain redox control and normal DNA methylation levels in human neuroblastoma cell cultures (SH-SY5Y) under overnight challenge with morphine (100 nM). The second objective was to determine whether D-CYSee and/or betaine could diminish the degree of physical dependence to morphine in male Sprague Dawley rats. Our data showed that overnight treatment with morphine reduced cellular GSH levels, induced mitochondrial damage, decreased global DNA methylation, and increased LINE-1 mRNA expression. These adverse effects by morphine, which diminished the reducing capacity and compromised the maintenance of the membrane potential of SH-SY5Y cells, was prevented by concurrent application of D-CYSee (100 µM) or betaine (300 µM). Furthermore, our data demonstrated that co-injections of D-CYSee (250 μmol/kg, IV) and to a lesser extent, betaine (250 μmol/kg, IV), markedly diminished the development of physical dependence induced by multi-day morphine injections (escalating daily doses of 10-30 mg/kg, IV), as assessed by the lesser number of withdrawal phenomena elicited by the injection of the opioid receptor antagonist, naloxone (1.5 mg/kg, IV). These findings provide evidence that D-CYSee and betaine prevent the appearance of redox alterations and epigenetic signatures commonly seen in neural cells involved in opioid physical dependence/addiction, and lessen development of physical dependence to morphine.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

D-Cysteine supplementation partially protects against ferroptosis induced by xCT dysfunction via increasing the availability of glutathione

Glutathione (GSH) is synthesized from three amino acids and the overall process is highly dependent on the availability of l-cysteine (l-Cys). GSH serves as an essential cofactor for glutathione peroxidase 4 (Gpx4), which reduces phospholipid hydroperoxides. The inactivation of Gpx4 or an insufficient supply of l-Cys results in the accumulation of lipid hydroperoxides, eventually leading to iron-dependent cell death, ferroptosis. In this study, we investigated the anti-ferroptotic properties of d-cysteine (d-Cys) under conditions of dysfunction in cystine transporter, xCT. l-Cys supplementation completely rescued ferroptosis that had been induced by the erastin-mediated inhibition of xCT in Hepa 1-6 cells. Upon d-Cys supplementation, the erastin-treated cells remained completely viable for periods of up to 24 h but eventually died after 48 h. d-Cys supplementation suppressed the production of lipid peroxides, thereby ferroptosis. The addition of d-Cys sustained intracellular Cys and GSH levels to a certain extent. When Hepa 1-6 cells were treated with a combination of buthionine sulfoximine and erastin, the anti-ferroptotic effect of d-Cys was diminished. These collective results indicate that, although d-Cys is not the direct source of GSH, d-Cys supplementation protects cells from ferroptosis in a manner that is dependent on GSH synthesis via stimulating the uptake of l-Cys.

Metabolism of Sulfur-Containing Amino Acids: How the Body Copes with Excess Methionine, Cysteine, and Sulfide

Metabolism of excess methionine (Met) to homocysteine (Hcy) by transmethylation is facilitated by the expression of methionine adenosyltransferase (MAT) I/III and glycine N-methyltransferase (GNMT) in liver, and a lack of either enzyme results in hypermethioninemia despite normal concentrations of MATII and methyltransferases other than GNMT. The further metabolism of Hcy by the transsulfuration pathway is facilitated by activation of cystathionine β-synthase (CBS) by S-adenosylmethionine (SAM) as well as the relatively high KM of CBS for Hcy. Transmethylation plus transsulfuration effects catabolism of the Met molecule along with transfer of the sulfur atom of Met to serine to synthesize cysteine (Cys). Oxidation and excretion of Met sulfur depend upon Cys catabolism and sulfur oxidation pathways. Excess Cys is oxidized by cysteine dioxygenase 1 (CDO1) and further metabolized to taurine or sulfate. Some Cys is normally metabolized by desulfhydration pathways, and the hydrogen sulfide (H2S) produced is further oxidized to sulfate. If Cys or Hcy concentrations are elevated, Cys or Hcy desulfhydration can result in excess H2S and thiosulfate production. Excess Cys or Met may also promote their limited metabolism by transamination pathways.

Impact of medication on protein and amino acid metabolism in the elderly: the sulfur amino acid and paracetamol case

The optimisation of nutritional support for the growing number of older individuals does not usually take into account medication. Paracetamol (acetaminophen; APAP) is the first intention treatment of chronic pain that is highly prevalent and persistent in the elderly. Detoxification of APAP occurs in the liver and utilises sulfate and glutathione (GSH), both of which are issued from cysteine (Cys), a conditionally indispensable amino acid. The detoxification-induced siphoning of Cys could reduce the availability of Cys for skeletal muscle. Consequently, APAP could worsen sarcopenia, an important component of the frailty syndrome leading to dependency. The present review provides the rationale for the potential pro-sarcopenic effect of APAP then recent results concerning the effect of chronic APAP treatment on muscle mass and metabolism are discussed. The principal findings are that chronic treatments with doses of APAP comparable with the maximum posology for humans can increase the requirement for sulfur amino acids (SAA), reduce Cys availability for muscle, reduce muscle protein synthesis and aggravate sarcopenia in animals. One clinical study is in favour of an enhanced SAA requirement in the older individual under chronic treatment with APAP. Few clinical studies investigated the effect of chronic treatment with APAP combined with exercise, in nutritional conditions that probably did not affect Cys and GSH homeostasis. Whether APAP can aggravate sarcopenia in older individuals with low protein intake remains to be tested. If true, nutritional strategies based on enhancing Cys supply could be of prime interest to cut down the pro-sarcopenic effect of chronic treatment with APAP.

Disease prevention and delayed aging by dietary sulfur amino acid restriction: translational implications

Sulfur amino acids (SAAs) play numerous critical roles in metabolism and overall health maintenance. Preclinical studies have demonstrated that SAA-restricted diets have many beneficial effects, including extending life span and preventing the development of a variety of diseases. Dietary sulfur amino acid restriction (SAAR) is characterized by chronic restrictions of methionine and cysteine but not calories and is associated with reductions in body weight, adiposity and oxidative stress, and metabolic changes in adipose tissue and liver resulting in enhanced insulin sensitivity and energy expenditure. SAAR-induced changes in blood biomarkers include reductions in insulin, insulin-like growth factor-1, glucose, and leptin and increases in adiponectin and fibroblast growth factor 21. On the basis of these preclinical data, SAAR may also have similar benefits in humans. While little is known of the translational significance of SAAR, its potential feasibility in humans is supported by findings of its effectiveness in rodents, even when initiated in adult animals. To date, there have been no controlled feeding studies of SAAR in humans; however, there have been numerous relevant epidemiologic and disease-based clinical investigations reported. Here, we summarize observations from these clinical investigations to provide insight into the potential effectiveness of SAAR for humans.

International Union of Basic and Clinical Pharmacology. CII: Pharmacological Modulation of H2S Levels: H2S Donors and H2S Biosynthesis Inhibitors

Over the last decade, hydrogen sulfide (H2S) has emerged as an important endogenous gasotransmitter in mammalian cells and tissues. Similar to the previously characterized gasotransmitters nitric oxide and carbon monoxide, H2S is produced by various enzymatic reactions and regulates a host of physiologic and pathophysiological processes in various cells and tissues. H2S levels are decreased in a number of conditions (e.g., diabetes mellitus, ischemia, and aging) and are increased in other states (e.g., inflammation, critical illness, and cancer). Over the last decades, multiple approaches have been identified for the therapeutic exploitation of H2S, either based on H2S donation or inhibition of H2S biosynthesis. H2S donation can be achieved through the inhalation of H2S gas and/or the parenteral or enteral administration of so-called fast-releasing H2S donors (salts of H2S such as NaHS and Na2S) or slow-releasing H2S donors (GYY4137 being the prototypical compound used in hundreds of studies in vitro and in vivo). Recent work also identifies various donors with regulated H2S release profiles, including oxidant-triggered donors, pH-dependent donors, esterase-activated donors, and organelle-targeted (e.g., mitochondrial) compounds. There are also approaches where existing, clinically approved drugs of various classes (e.g., nonsteroidal anti-inflammatories) are coupled with H2S-donating groups (the most advanced compound in clinical trials is ATB-346, an H2S-donating derivative of the non-steroidal anti-inflammatory compound naproxen). For pharmacological inhibition of H2S synthesis, there are now several small molecule compounds targeting each of the three H2S-producing enzymes cystathionine-β-synthase (CBS), cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase. Although many of these compounds have their limitations (potency, selectivity), these molecules, especially in combination with genetic approaches, can be instrumental for the delineation of the biologic processes involving endogenous H2S production. Moreover, some of these compounds (e.g., cell-permeable prodrugs of the CBS inhibitor aminooxyacetate, or benserazide, a potentially repurposable CBS inhibitor) may serve as starting points for future clinical translation. The present article overviews the currently known H2S donors and H2S biosynthesis inhibitors, delineates their mode of action, and offers examples for their biologic effects and potential therapeutic utility.

Joint effect of maternal plasma homocysteine and prepregnancy obesity on child blood pressure: a prospective birth cohort study

BACKGROUND/OBJECTIVES

Elevated homocysteine (Hcy) is a known cardiovascular risk factor. However, its role in intergenerational cardiometabolic risk is unknown. We hypothesized that maternal elevated Hcy can act alone or in combination with maternal prepregnancy obesity to increase child systolic blood pressure (SBP).

METHODS

This study included 1279 mother-child pairs who were enrolled at birth and followed prospectively up to age 9 years from 2003 to 2014 at the Boston Medical Center. Child SBP percentile was calculated according to US reference data and elevated SBP was defined as SBP⩾75th percentile.

RESULTS

A U-shaped relationship between maternal Hcy and her child SBP was observed. The risk for child elevated SBP was higher among those in the lowest quartile (Q1, odds ratio (OR): 1.27; 95% confidence interval (CI): 0.94-1.72), and highest quartile (Q4, OR: 1.34; 95% CI: 1.00-1.81) as compared with those in quartiles 2 and 3. The highest risk of child elevated SBP was found among children born to obese mothers with Hcy in Q4 (OR: 2.22; 95%CI: 1.35-3.64), compared with children of non-obese mothers with Hcy in Q2-Q3. This association was independent from maternal folate and vitamin B12 status, and was not mediated by gestational age or size at birth.

CONCLUSIONS

In this prospective birth cohort, we observed a U-shaped association between maternal Hcy levels and child elevated SBP. Maternal high Hcy (Q4) and prepregnancy obesity jointly increased the risk of child elevated SBP by more than two-fold.

Comparisons of l-cysteine and d-cysteine toxicity in 4-week repeated-dose toxicity studies of rats receiving daily oral administration

Two 4-week repeated-dose toxicity studies were conducted to evaluate the potential toxicity of l-cysteine and d-cysteine. In one study, three groups of 6 male rats were each administered l-cysteine once daily by gavage at doses of 500, 1,000, or 2,000 mg/kg/day for 28 consecutive days. The control group was administered a 0.5% methylcellulose vehicle solution. The other study followed a similar protocol except that the experimental groups received d-cysteine. Toxicological observations showed that the l-cysteine-treated groups exhibited renal injuries such as basophilic tubules with eosinophilic material in the lumen, and there were increased numbers of basophilic tubules in all treated groups. In 1,000 or 2,000 mg/kg/day-treated groups, salivation and necropsy findings indicative of focal erosion in the stomach mucosa were found. Increases in reticulocyte counts were observed in the 2,000 mg/kg/day-treated group. Toxicological findings obtained for the d-cysteine-treated groups included anemia and renal injuries such as basophilic tubules with eosinophilic material in the lumen, increased numbers of basophilic tubules, and crystal deposition in the medulla in the 2,000 mg/kg/day-treated group. Additional findings included sperm granuloma in the epididymis, necropsy findings suggestive of focal erosion in the stomach mucosa, and salivation in the 1,000 or 2,000 mg/kg/day-treated groups. One rat in the 2,000 mg/kg/day-treated group died due to renal failure. In conclusion, the no-observed-adverse-effect levels (NOAELs) were estimated to be less than 500 mg/kg/day for l-cysteine and 500 mg/kg/day for d-cysteine under our study conditions. The toxicological profiles were similar for l-cysteine and d-cysteine; however, there were slight differences in the dose responses. The mechanisms underlying these differences remain to be determined.

Use of Tissue Metabolite Analysis and Enzyme Kinetics To Discriminate between Alternate Pathways for Hydrogen Sulfide Metabolism

Hydrogen sulfide (H₂S) is an endogenously synthesized signaling molecule that is enzymatically metabolized in mitochondria. The metabolism of H₂S maintains optimal concentrations of the gasotransmitter and produces sulfane sulfur (S⁰)-containing metabolites that may be functionally important in signaling. Sulfide:quinone oxidoreductase (SQOR) catalyzes the initial two-electron oxidation of H₂S to S⁰ using coenzyme Q as the electron acceptor in a reaction that requires a third substrate to act as the acceptor of S⁰. We discovered that sulfite is a highly efficient acceptor and proposed that sulfite is the physiological acceptor in a reaction that produces thiosulfate, a known metabolic intermediate. This model has been challenged by others who assume that the intracellular concentration of sulfite is very low, a scenario postulated to favor reaction of SQOR with a considerably poorer acceptor, glutathione. In this study, we measured the intracellular concentration of sulfite and other metabolites in mammalian tissues. The values observed for sulfite in rat liver (9.2 μM) and heart (38 μM) are orders of magnitude higher than previously assumed. We discovered that the apparent kinetics of oxidation of H₂S by SQOR with glutathione as the S⁰ acceptor reflect contributions from other SQOR-catalyzed reactions, including a novel glutathione:CoQ reductase reaction. We used observed metabolite levels and steady-state kinetic parameters to simulate rates of oxidation of H₂S by SQOR at physiological concentrations of different S⁰ acceptors. The results show that the reaction with sulfite as the S⁰ acceptor is a major pathway in liver and heart and provide insight into the potential dynamics of H₂S metabolism.

Cysteine Metabolism and Oxidative Processes in the Rat Liver and Kidney after Acute and Repeated Cocaine Treatment

The role of cocaine in modulating the metabolism of sulfur-containing compounds in the peripheral tissues is poorly understood. In the present study we addressed the question about the effects of acute and repeated (5 days) cocaine (10 mg/kg i.p.) administration on the total cysteine (Cys) metabolism and on the oxidative processes in the rat liver and kidney. The whole pool of sulfane sulfur, its bound fraction and hydrogen sulfide (H₂S) were considered as markers of anaerobic Cys metabolism while the sulfate as a measure of its aerobic metabolism. The total-, non-protein- and protein- SH group levels were assayed as indicators of the redox status of thiols. Additionally, the activities of enzymes involved in H₂S formation (cystathionine γ-lyase, CSE; 3-mercaptopyruvate sulfurtransferase, 3-MST) and GSH metabolism (γ-glutamyl transpeptidase, γ-GT; glutathione S-transferase, GST) were determined. Finally, we assayed the concentrations of reactive oxygen species (ROS) and malondialdehyde (MDA) as markers of oxidative stress and lipid peroxidation, respectively. In the liver, acute cocaine treatment, did not change concentrations of the whole pool of sulfane sulfur, its bound fraction, H₂S or sulfate but markedly decreased levels of non-protein SH groups (NPSH), ROS and GST activity while γ-GT was unaffected. In the kidney, acute cocaine significantly increased concentration of the whole pool of sulfane sulfur, reduced the content of its bound fraction but H₂S, sulfate and NPSH levels were unchanged while ROS and activities of GST and γ-GT were reduced. Acute cocaine enhanced activity of the CSE and 3-MST in the liver and kidney, respectively. Repeatedly administered cocaine enhanced the whole pool of sulfane sulfur and reduced H₂S level simultaneously increasing sulfate content both in the liver and kidney. After repeated cocaine, a significant decrease in ROS was still observed in the liver while in the kidney, despite unchanged ROS content, a marked increase in MDA level was visible. The repeated cocaine decreased 3-MST and increased γ-GT activities in both organs but reduced GST in the kidney. Our results show that cocaine administered at a relatively low dose shifts Cys metabolism towards the formation of sulfane sulfur compounds which possess antioxidant and redox regulatory properties and are a source of H₂S which can support mitochondrial bioenergetics.

Anti-inflammatory and cytoprotective actions of hydrogen sulfide: translation to therapeutics

SIGNIFICANCE

There is a rapidly expanding body of evidence for important roles of hydrogen sulfide in protecting against tissue injury, reducing inflammation, and promoting repair. There is also growing evidence that H2S can be successfully exploited in drug development.

RECENT ADVANCES

H2S synthesis and degradation are regulated in circumstances of inflammation and injury so as to promote repair and re-establish homeostasis. Novel H2S-releasing drugs exhibit enhanced anti-inflammatory and pro-restorative effects, while having reduced adverse effects in many tissues.

CRITICAL ISSUES

H2S is a pleiotropic mediator, having effects on many elements in the inflammatory cascade and promoting the resolution of inflammation and injury. It also contributes significantly to mucosal defence in the gastrointestinal tract, and in host defence against infection. There is strong evidence that novel, H2S-based therapeutics are safe and effective in animal models, and several are progressing through human trials.

FUTURE DIRECTIONS

A better understanding of the physiological and pathophysiological roles of H2S continues to be restrained by the lack of simple, reliable methods for measurement of H2S synthesis, and the paucity of highly selective inhibitors of enzymes that participate in endogenous H2S synthesis. On the other hand, H2S donors show promise as therapeutics for several important indications.

Organization of the human mitochondrial hydrogen sulfide oxidation pathway

Sulfide oxidation is expected to play an important role in cellular switching between low steady-state intracellular hydrogen sulfide levels and the higher concentrations where the physiological effects are elicited. Yet despite its significance, fundamental questions regarding how the sulfide oxidation pathway is wired remain unanswered, and competing proposals exist that diverge at the very first step catalyzed by sulfide quinone oxidoreductase (SQR). We demonstrate that, in addition to sulfite, glutathione functions as a persulfide acceptor for human SQR and that rhodanese preferentially synthesizes rather than utilizes thiosulfate. The kinetic behavior of these enzymes provides compelling evidence for the flow of sulfide via SQR to glutathione persulfide, which is then partitioned to thiosulfate or sulfite. Kinetic simulations at physiologically relevant metabolite concentrations provide additional support for the organizational logic of the sulfide oxidation pathway in which glutathione persulfide is the first intermediate formed.

Primary hepatocytes from mice lacking cysteine dioxygenase show increased cysteine concentrations and higher rates of metabolism of cysteine to hydrogen sulfide and thiosulfate

The oxidation of cysteine in mammalian cells occurs by two routes: a highly regulated direct oxidation pathway in which the first step is catalyzed by cysteine dioxygenase (CDO) and by desulfhydration-oxidation pathways in which the sulfur is released in a reduced oxidation state. To assess the effect of a lack of CDO on production of hydrogen sulfide (H2S) and thiosulfate (an intermediate in the oxidation of H2S to sulfate) and to explore the roles of both cystathionine γ-lyase (CTH) and cystathionine β-synthase (CBS) in cysteine desulfhydration by liver, we investigated the metabolism of cysteine in hepatocytes isolated from Cdo1-null and wild-type mice. Hepatocytes from Cdo1-null mice produced more H2S and thiosulfate than did hepatocytes from wild-type mice. The greater flux of cysteine through the cysteine desulfhydration reactions catalyzed by CTH and CBS in hepatocytes from Cdo1-null mice appeared to be the consequence of their higher cysteine levels, which were due to the lack of CDO and hence lack of catabolism of cysteine by the cysteinesulfinate-dependent pathways. Both CBS and CTH appeared to contribute substantially to cysteine desulfhydration, with estimates of 56 % by CBS and 44 % by CTH in hepatocytes from wild-type mice, and 63 % by CBS and 37 % by CTH in hepatocytes from Cdo1-null mice.

Cysteine dioxygenase 1 is a tumor suppressor gene silenced by promoter methylation in multiple human cancers

The human cysteine dioxygenase 1 (CDO1) gene is a non-heme structured, iron-containing metalloenzyme involved in the conversion of cysteine to cysteine sulfinate, and plays a key role in taurine biosynthesis. In our search for novel methylated gene promoters, we have analyzed differential RNA expression profiles of colorectal cancer (CRC) cell lines with or without treatment of 5-aza-2′-deoxycytidine. Among the genes identified, the CDO1 promoter was found to be differentially methylated in primary CRC tissues with high frequency compared to normal colon tissues. In addition, a statistically significant difference in the frequency of CDO1 promoter methylation was observed between primary normal and tumor tissues derived from breast, esophagus, lung, bladder and stomach. Downregulation of CDO1 mRNA and protein levels were observed in cancer cell lines and tumors derived from these tissue types. Expression of CDO1 was tightly controlled by promoter methylation, suggesting that promoter methylation and silencing of CDO1 may be a common event in human carcinogenesis. Moreover, forced expression of full-length CDO1 in human cancer cells markedly decreased the tumor cell growth in an in vitro cell culture and/or an in vivo mouse model, whereas knockdown of CDO1 increased cell growth in culture. Our data implicate CDO1 as a novel tumor suppressor gene and a potentially valuable molecular marker for human cancer.

Human sulfide:quinone oxidoreductase catalyzes the first step in hydrogen sulfide metabolism and produces a sulfane sulfur metabolite

Sulfide:quinone oxidoreductase (SQOR) is a membrane-bound enzyme that catalyzes the first step in the mitochondrial metabolism of H(2)S. Human SQOR is successfully expressed at low temperature in Escherichia coli by using an optimized synthetic gene and cold-adapted chaperonins. Recombinant SQOR contains noncovalently bound FAD and catalyzes the two-electron oxidation of H(2)S to S(0) (sulfane sulfur) using CoQ(1) as an electron acceptor. The prosthetic group is reduced upon anaerobic addition of H(2)S in a reaction that proceeds via a long-wavelength-absorbing intermediate (λ(max) = 673 nm). Cyanide, sulfite, or sulfide can act as the sulfane sulfur acceptor in reactions that (i) exhibit pH optima at 8.5, 7.5, or 7.0, respectively, and (ii) produce thiocyanate, thiosulfate, or a putative sulfur analogue of hydrogen peroxide (H(2)S(2)), respectively. Importantly, thiosulfate is a known intermediate in the oxidation of H(2)S by intact animals and the major product formed in glutathione-depleted cells or mitochondria. Oxidation of H(2)S by SQOR with sulfite as the sulfane sulfur acceptor is rapid and highly efficient at physiological pH (k(cat)/K(m,H(2)S) = 2.9 × 10(7) M(-1) s(-1)). A similar efficiency is observed with cyanide, a clearly artificial acceptor, at pH 8.5, whereas a 100-fold lower value is seen with sulfide as the acceptor at pH 7.0. The latter reaction is unlikely to occur in healthy individuals but may become significant under certain pathological conditions. We propose that sulfite is the physiological acceptor of the sulfane sulfur and that the SQOR reaction is the predominant source of the thiosulfate produced during H(2)S oxidation by mammalian tissues.

Knockout of the murine cysteine dioxygenase gene results in severe impairment in ability to synthesize taurine and an increased catabolism of cysteine to hydrogen sulfide

Cysteine homeostasis is dependent on the regulation of cysteine dioxygenase (CDO) in response to changes in sulfur amino acid intake. CDO oxidizes cysteine to cysteinesulfinate, which is further metabolized to either taurine or to pyruvate plus sulfate. To gain insight into the physiological function of CDO and the consequence of a loss of CDO activity, mice carrying a null CDO allele (CDO(+/-) mice) were crossed to generate CDO(-/-), CDO(+/-), and CDO(+/+) mice. CDO(-/-) mice exhibited postnatal mortality, growth deficit, and connective tissue pathology. CDO(-/-) mice had extremely low taurine levels and somewhat elevated cysteine levels, consistent with the lack of flux through CDO-dependent catabolic pathways. However, plasma sulfate levels were slightly higher in CDO(-/-) mice than in CDO(+/-) or CDO(+/+) mice, and tissue levels of acid-labile sulfide were elevated, indicating an increase in cysteine catabolism by cysteine desulfhydration pathways. Null mice had lower hepatic cytochrome c oxidase levels, suggesting impaired electron transport capacity. Supplementation of mice with taurine improved survival of male pups but otherwise had little effect on the phenotype of the CDO(-/-) mice. H(2)S has been identified as an important gaseous signaling molecule as well as a toxicant, and pathology may be due to dysregulation of H(2)S production. Control of cysteine levels by regulation of CDO may be necessary to maintain low H(2)S/sulfane sulfur levels and facilitate the use of H(2)S as a signaling molecule.

Dealing with methionine/homocysteine sulfur: cysteine metabolism to taurine and inorganic sulfur

Synthesis of cysteine as a product of the transsulfuration pathway can be viewed as part of methionine or homocysteine degradation, with cysteine being the vehicle for sulfur conversion to end products (sulfate, taurine) that can be excreted in the urine. 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. Cysteine is catabolized by several desulfuration reactions that release sulfur in a reduced oxidation state, generating sulfane sulfur or hydrogen sulfide (H₂S), which can be further oxidized to sulfate. Cysteine desulfuration is accomplished by alternate reactions catalyzed by cystathionine β-synthase and cystathionine γ-lyase. Cysteine is also catabolized by pathways that require the initial oxidation of the cysteine thiol by cysteine dioxygenase to form cysteinesulfinate. The oxidative pathway leads to production of taurine and sulfate in a ratio of approximately 2:1. Relative metabolism of cysteine by desulfuration versus oxidative pathways is influenced by cysteine dioxygenase activity, which is low in animals fed low-protein diets and high in animals fed excess sulfur amino acids. Thus, desulfuration reactions dominate when cysteine is deficient, whereas oxidative catabolism dominates when cysteine is in excess. In rats consuming a diet with an adequate level of sulfur amino acids, about two thirds of cysteine catabolism occurs by oxidative pathways and one third by desulfuration pathways. Cysteine dioxygenase is robustly regulated in response to cysteine availability and may function to provide a pathway to siphon cysteine to less toxic metabolites than those produced by cysteine desulfuration reactions.

Hydrogen sulphide synthesis in the rat and mouse gastrointestinal tract

AIMS

Hydrogen sulphide (H2S) exerts several anti-inflammatory effects, accelerates the healing of experimental gastric ulcers, and can stimulate intestinal secretion. Little is known about H2S synthesis in the gastrointestinal tract. The aim of this study was to characterize H2S synthesis throughout the gastrointestinal tract.

METHODS

H2S synthesis in various gastrointestinal tissues of rats and mice was determined. The effects and selectivity of inhibitors of two key enzymes for H2S synthesis, cystathionine-gamma-lyase and cystathionine-beta-synthase, were examined. Cystathionine-gamma-lyase and cystathionine-beta-synthase expression was evaluated by Western blotting and immunohistochemistry. Cystathionine-gamma-lyase and cystathionine-beta-synthase expression in biopsies of human colon was also examined.

RESULTS

H2S synthesis was variable throughout the gastrointestinal tract in parallel with variations in cystathionine-gamma-lyase and cystathionine-beta-synthase expression. The efficacy of cystathionine-beta-synthase and cystathionine-gamma-lyase inhibitors to reduce H2S synthesis in these tissues was also variable. Cystathionine-beta-synthase is the predominant source of H2S synthesis in the colon of rodents. Cystathionine-gamma-lyase and cystathionine-beta-synthase were also expressed in healthy human colon biopsies.

CONCLUSIONS

The capacity for H2S synthesis varies throughout the rodent gastrointestinal tract, as does the distribution and contribution of the two key enzymes. Investigation of additional enzymatic sources of H2S and the development of more selective inhibitors are suggested.

New roles for cysteine and transsulfuration enzymes: production of H2S, a neuromodulator and smooth muscle relaxant

The enzymes of the transsulfuration pathway also have the capacity to catalyze the desulfhydration of cysteine. Recent studies demonstrate a role of the transsulfuration enzymes, cystathionine gamma-lyase and cystathionine beta-synthase, in catalyzing the desulfhydration of cysteine in brain and smooth muscle. The H2S produced from cysteine functions as a neuromodulator and smooth muscle relaxant. In glutamatergic neurons, the production of H2S by cystathionine beta-synthase enhances N-methyl-D-aspartate (NMDA) receptor-mediated currents. In smooth muscle cells, H2S produced by cystathionine gamma-lyase enhances the outward flux of potassium by opening potassium channels, leading to hyperpolarization of membrane potential and smooth muscle relaxation.

Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine

Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

Molecular cytotoxic mechanisms of anticancer hydroxychalcones

Chalcones are being considered as anticancer agents as they are natural compounds that are particularly cytotoxic towards K562 leukemia or melanoma cells. In this study, we have investigated phloretin, isoliquiritigenin, and 10 other hydroxylated chalcones for their cytotoxic mechanisms towards isolated rat hepatocytes. All hydroxychalcones partly depleted hepatocyte GSH and oxidized GSH to GSSG. These chalcones also caused a collapse of mitochondrial membrane potential and increased oxygen uptake. Furthermore, glycolytic or citric acid cycle substrates prevented cytotoxicity and mitochondrial membrane potential collapse. The highest pKa chalcones were the most effective at collapsing the mitochondrial membrane potential which suggests that the cytotoxic activity of hydroxychalcones are likely because of their ability to uncouple mitochondria.

Hepatocyte-catalysed detoxification of cyanide by L- and D-cysteine

The hepatocyte metabolic pathways involved in the detoxification of cyanide by cysteine have been investigated in vitro using hepatocytes isolated from Sprague-Dawley rats. Cyanide toxicity towards isolated hepatocytes could be prevented by the addition of L- or D-cysteine, cystine, or the cysteine metabolites thiosulfate and mercaptopyruvate, which markedly increased thiocyanate formation. Prior depletion of hepatocyte GSH markedly increased thiosulfate formation from L- or D-cysteine without affecting thiocyanate formation from L- or D-cysteine. This suggested that the major metabolic pathway for thiocyanate formation did not involve thiosulfate. Mercaptopyruvate was a more likely metabolic intermediate, as thiocyanate formation from L-cysteine but not thiosulfate was inhibited markedly by aminooxyacetate, a cysteine aminotransferase inhibitor, and propargylglycine, a gamma-cystathionase inhibitor. Furthermore, propargylglycine prevented L-cysteine cytoprotection against cyanide toxicity. Thiocyanate formation from D-cysteine likely also involved mercaptopyruvate, as thiocyanate formation from D-cysteine but not L-cysteine was inhibited by benzoate, an inhibitor of D-amino acid oxidase. Furthermore, benzoate prevented D-cysteine cytoprotection against cyanide toxicity. Cystine may also be an intermediate, as hepatocyte thiocyanate formation from added L-cysteine was inhibited when L-cysteine autoxidation was prevented with the copper chelator bathocuproine disulfonate. Furthermore, thiocyanate formation by rat liver homogenates with L-cystine was far more rapid than that with L-cysteine. Hepatocyte thiocyanate metabolic intermediates of beta-mercaptopyruvate and thiocystine were proposed for L-cysteine, and beta-mercaptopyruvate was proposed for D-cysteine.