Catalytic promiscuity and heme-dependent redox regulation of H2S synthesis

Sulfide oxidation promotes hypoxic angiogenesis and neovascularization

Angiogenic programming in the vascular endothelium is a tightly regulated process for maintaining tissue homeostasis and is activated in tissue injury and the tumor microenvironment. The metabolic basis of how gas signaling molecules regulate angiogenesis is elusive. Here, we report that hypoxic upregulation of ·NO in endothelial cells reprograms the transsulfuration pathway to increase biogenesis of hydrogen sulfide (H2S), a proangiogenic metabolite. However, decreased H2S oxidation due to sulfide quinone oxidoreductase (SQOR) deficiency synergizes with hypoxia, inducing a reductive shift and limiting endothelial proliferation that is attenuated by dissipation of the mitochondrial NADH pool. Tumor xenografts in whole-body (WBCreSqorfl/fl) and endothelial-specific (VE-cadherinCre-ERT2Sqorfl/fl) Sqor-knockout mice exhibit lower mass and angiogenesis than control mice. WBCreSqorfl/fl mice also exhibit decreased muscle angiogenesis following femoral artery ligation compared to control mice. Collectively, our data reveal the molecular intersections between H2S, O2 and ·NO metabolism and identify SQOR inhibition as a metabolic vulnerability for endothelial cell proliferation and neovascularization.

Disease-causing cystathionine β-synthase linker mutations impair allosteric regulation

Cystathionine β-synthase (CBS) catalyzes the committing step in the transsulfuration pathway, which is important for clearing homocysteine and furnishing cysteine. The transsulfuration pathway also generates H2S, a signaling molecule. CBS is a modular protein with a heme and pyridoxal phosphate-binding catalytic core, which is separated by a linker region from the C-terminal regulatory domain that binds S-adenosylmethionine (AdoMet), an allosteric activator. Recent cryo-EM structures reveal that CBS exists in a fibrillar form and undergoes a dramatic architectural rearrangement between the basal and AdoMet-bound states. CBS is the single most common locus of mutations associated with homocystinuria, and, in this study, we have characterized three clinical variants (K384E/N and M391I), which reside in the linker region. The native fibrillar form is destabilized in the variants, and differences in their limited proteolytic fingerprints also reveal conformational alterations. The crystal structure of the truncated K384N variant, lacking the regulatory domain, reveals that the overall fold of the catalytic core is unperturbed. M391I CBS exhibits a modest (1.4-fold) decrease while the K384E/N variants exhibit a significant (∼8-fold) decrease in basal activity, which is either unresponsive to or inhibited by AdoMet. Pre-steady state kinetic analyses reveal that the K384E/N substitutions exhibit pleiotropic effects and that the differences between them are expressed in the second half reaction, that is, homocysteine binding and reaction with the aminoacrylate intermediate. Together, these studies point to an important role for the linker in stabilizing the higher-order oligomeric structure of CBS and enabling AdoMet-dependent regulation.

Hypoxia increases persulfide and polysulfide formation by AMP kinase dependent cystathionine gamma lyase phosphorylation

Hydropersulfide and hydropolysulfide metabolites are increasingly important reactive sulfur species (RSS) regulating numerous cellular redox dependent functions. Intracellular production of these species is known to occur through RSS interactions or through translational mechanisms involving cysteinyl t-RNA synthetases. However, regulation of these species under cell stress conditions, such as hypoxia, that are known to modulate RSS remain poorly understood. Here we define an important mechanism of increased persulfide and polysulfide production involving cystathionine gamma lyase (CSE) phosphorylation at serine 346 and threonine 355 in a substrate specific manner, under acute hypoxic conditions. Hypoxic phosphorylation of CSE occurs in an AMP kinase dependent manner increasing enzyme activity involving unique inter- and intramolecular interactions within the tetramer. Importantly, both cellular hypoxia and tissue ischemia result in AMP Kinase dependent CSE phosphorylation that regulates blood flow in ischemic tissues. Our findings reveal hypoxia molecular signaling pathways regulating CSE dependent persulfide and polysulfide production impacting tissue and cellular response to stress.

Autocatalytic Mechanism in the Anaerobic Reduction of Metmyoglobin by Sulfide Species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by sulfide species (H2S/HS-) under an argon atmosphere has been studied by a combination of spectroscopic, kinetic, and computational methods. Asymmetric S-shaped time-traces for the formation of MbFeII at varying ratios of excess sulfide were observed at pH 5.3 < pH < 8.0 and 25 °C, suggesting an autocatalytic reaction mechanism. An increased rate at more alkaline pHs points to HS- as relevant reactive species for the reduction. The formation of the sulfanyl radical (HS•) in the slow initial phase was assessed using the spin-trap phenyl N-tert-butyl nitrone. This radical initiates the formation of S-S reactive species as disulfanuidyl/ disulfanudi-idyl radical anions and disulfide (HSSH•-/HSS•2- and HSS-, respectively). The autocatalysis has been ascribed to HSS-, formed after HSSH•-/HSS•2- disproportionation, which behaves as a fast reductant toward the intermediate complex MbFeIII(HS-). We propose a reaction mechanism for the sulfide-mediated reduction of metmyoglobin where only ferric heme iron initiates the oxidation of sulfide species. Beside the chemical interest, this insight into the MbFeIII/sulfide reaction under an argon atmosphere is relevant for the interpretation of biochemical aspects of ectopic myoglobins found on hypoxic tissues toward reactive sulfur species.

Cystathionine γ-Lyase Self-Inactivates by Polysulfidation during Cystine Metabolism

Cystathionine γ-lyase (CSE) is an enzyme responsible for the biosynthesis of cysteine from cystathionine in the final step of the transsulfuration pathway. It also has β-lyase activity toward cystine, generating cysteine persulfide (Cys-SSH). The chemical reactivity of Cys-SSH is thought to be involved in the catalytic activity of particular proteins via protein polysulfidation, the formation of -S-(S)n-H on their reactive cysteine residues. The Cys136/171 residues of CSE have been proposed to be redox-sensitive residues. Herein, we investigated whether CSE polysulfidation occurs at Cys136/171 during cystine metabolism. Transfection of wild-type CSE into COS-7 cells resulted in increased intracellular Cys-SSH production, which was significantly increased when Cys136Val or Cys136/171Val CSE mutants were transfected, instead of the wild-type enzyme. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that CSE polysulfidation occurs at Cys136 during cystine metabolism. In vitro incubation of CSE with CSE-enzymatically synthesized Cys-SSH resulted in the inhibition of Cys-SSH production. In contrast, the mutant CSEs (Cys136Val and Cys136/171Val) proved resistant to inhibition. The Cys-SSH-producing CSE activity of Cys136/171Val CSE was higher than that of the wild-type enzyme. Meanwhile, the cysteine-producing CSE activity of this mutant was equivalent to that of the wild-type enzyme. It is assumed that Cys-SSH-producing CSE activity could be auto-inactivated via the polysulfidation of the enzyme during cystine metabolism. Thus, the polysulfidation of CSE at the Cys136 residue may be an integral feature of cystine metabolism, which functions to down-regulate Cys-SSH synthesis by the enzyme.

The chemistry of hydropersulfides (RSSH) as related to possible physiological functions

Hydropersulfides (RSSH) are oxidized thiol (RSH) derivatives that have been shown to be biologically prevalent with likely important functions (along with other polysulfur compounds). The functional utility of RSSH can be gleaned from their unique chemical properties. That is, RSSH possess chemical reactivity not present in other biologically relevant sulfur species that should allow them to be used in specific ways in biology as effector/signaling molecules. For example, compared to RSH, RSSH are considered to be superior nucleophiles, reductants and metal ligands. Moreover, unlike RSH, RSSH can be either reductants/nucleophiles or oxidants/electrophiles depending on the protonated state. It has also become clear that studies related to the chemical biology and physiology of hydrogen suflide (H₂S) must also consider the effects of RSSH (and related polysulfur species) as they are biochemically linked. Herein is a discussion of the relevant chemistry of RSSH that can serve as a basis for understanding how RSSH can be used by cells to, for example, combat stresses and used in signaling. Also, discussed are some current experimental studies regarding the biological activity of RSSH that can be explained by their chemical properties.

Appraising the Role of Astrocytes as Suppliers of Neuronal Glutathione Precursors

The metabolism and intercellular transfer of glutathione or its precursors may play an important role in cellular defense against oxidative stress, a common hallmark of neurodegeneration. In the 1990s, several studies in the Neurobiology field led to the widely accepted notion that astrocytes produce large amounts of glutathione that serve to feed neurons with precursors for glutathione synthesis. This assumption has important implications for health and disease since a reduction in this supply from astrocytes could compromise the capacity of neurons to cope with oxidative stress. However, at first glance, this shuttling would imply a large energy expenditure to get to the same point in a nearby cell. Thus, are there additional underlying reasons for this expensive mechanism? Are neurons unable to import and/or synthesize the three non-essential amino acids that are the glutathione building blocks? The rather oxidizing extracellular environment favors the presence of cysteine (Cys) as cystine (Cis), less favorable for neuronal import. Therefore, it has also been proposed that astrocytic GSH efflux could induce a change in the redox status of the extracellular space nearby the neurons, locally lowering the Cis/Cys ratio. This astrocytic glutathione release would also increase their demand for precursors, stimulating Cis uptake, which these cells can import, further impacting the local decline of the Cis/Cys ratio, in turn, contributing to a more reduced extracellular environment and subsequently favoring neuronal Cys import. Here, we revisit the experimental evidence that led to the accepted hypothesis of astrocytes acting as suppliers of neuronal glutathione precursors, considering recent data from the Human Protein Atlas. In addition, we highlight some potential drawbacks of this hypothesis, mainly supported by heterogeneous cellular models. Finally, we outline additional and more cost-efficient possibilities by which astrocytes could support neuronal glutathione levels, including its shuttling in extracellular vesicles.

Sulfide oxidation promotes hypoxic angiogenesis and neovascularization

Angiogenic programming in the vascular endothelium is a tightly regulated process to maintain tissue homeostasis and is activated in tissue injury and the tumor microenvironment. The metabolic basis of how gas signaling molecules regulate angiogenesis is elusive. Herein, we report that hypoxic upregulation of NO synthesis in endothelial cells reprograms the transsulfuration pathway and increases H 2 S biogenesis. Furthermore, H 2 S oxidation by mitochondrial sulfide quinone oxidoreductase (SQOR) rather than downstream persulfides, synergizes with hypoxia to induce a reductive shift, limiting endothelial cell proliferation that is attenuated by dissipation of the mitochondrial NADH pool. Tumor xenografts in whole-body WB Cre SQOR fl/fl knockout mice exhibit lower mass and reduced angiogenesis compared to SQOR fl/fl controls. WB Cre SQOR fl/fl mice also exhibit reduced muscle angiogenesis following femoral artery ligation, compared to controls. Collectively, our data reveal the molecular intersections between H 2 S, O 2 and NO metabolism and identify SQOR inhibition as a metabolic vulnerability for endothelial cell proliferation and neovascularization.

Highlights

Hypoxic induction of •NO in endothelial cells inhibits CBS and switches CTH reaction specificity Hypoxic interruption of the canonical transsulfuration pathway promotes H 2 S synthesis Synergizing with hypoxia, SQOR deficiency induces a reductive shift in the ETC and restricts proliferationSQOR KO mice exhibit lower neovascularization in tumor xenograft and hind limb ischemia models.

Sulfide regulation of cardiovascular function in health and disease

Hydrogen sulfide (H2S) has emerged as a gaseous signalling molecule with crucial implications for cardiovascular health. H2S is involved in many biological functions, including interactions with nitric oxide, activation of molecular signalling cascades, post-translational modifications and redox regulation. Various preclinical and clinical studies have shown that H2S and its synthesizing enzymes - cystathionine γ-lyase, cystathionine β-synthase and 3-mercaptosulfotransferase - can protect against cardiovascular pathologies, including arrhythmias, atherosclerosis, heart failure, myocardial infarction and ischaemia-reperfusion injury. The bioavailability of H2S and its metabolites, such as hydropersulfides and polysulfides, is substantially reduced in cardiovascular disease and has been associated with single-nucleotide polymorphisms in H2S synthesis enzymes. In this Review, we highlight the role of H2S, its synthesizing enzymes and metabolites, their roles in the cardiovascular system, and their involvement in cardiovascular disease and associated pathologies. We also discuss the latest clinical findings from the field and outline areas for future study.

Implications of hydrogen sulfide in colorectal cancer: Mechanistic insights and diagnostic and therapeutic strategies

Hydrogen sulfide (H₂S) is an important signaling molecule in colorectal cancer (CRC). It is produced in the colon by the catalytic synthesis of the colonocytes' enzymatic systems and the release of intestinal microbes, and is oxidatively metabolized in the colonocytes' mitochondria. Both endogenous H₂S in colonic epithelial cells and exogenous H₂S in intestinal lumen contribute to the onset and progression of CRC. The up-regulation of endogenous synthetases is thought to be the cause of the elevated H₂S levels in CRC cells. Different diagnostic probes and combination therapies, as well as tumor treatment approaches through H₂S modulation, have been developed in recent years and have become active area of investigation for the diagnosis and treatment of CRC. In this review, we focus on the specific mechanisms of H₂S production and oxidative metabolism as well as the function of H₂S in the occurrence, progression, diagnosis, and treatment of CRC. We also discuss the present challenges and provide insights into the future research of this burgeoning field.

A Metabolic Paradigm for Hydrogen Sulfide Signaling via Electron Transport Chain Plasticity

Significance: A burgeoning literature has attributed varied physiological effects to hydrogen sulfide (H2S), which is a product of eukaryotic sulfur amino acid metabolism. Protein persulfidation represents a major focus of studies elucidating the mechanism underlying H2S signaling. On the contrary, the capacity of H2S to induce reductive stress by targeting the electron transport chain (ETC) and signal by reprogramming redox metabolism has only recently begun to be elucidated. Recent Advances: In contrast to the nonspecific reaction of H2S with oxidized cysteines to form protein persulfides, its inhibition of complex IV represents a specific mechanism of action. Studies on the dual impact of H2S as an ETC substrate and an inhibitor have led to the exciting discovery of ETC plasticity and the use of fumarate as a terminal electron acceptor. H2S oxidation combined with complex IV targeting generates mitochondrial reductive stress, which is signaled through the metabolic network, leading to increased aerobic glycolysis, glutamine-dependent reductive carboxylation, and lipogenesis. Critical Issues: Insights into H2S-induced metabolic reprogramming are ushering in a paradigm shift for understanding the mechanism of its cellular action. It will be critical to reevaluate the physiological effects of H2S, for example, cytoprotection against ischemia-reperfusion injury, through the framework of metabolic reprogramming and ETC remodeling by H2S. Future Directions: The metabolic ramifications of H2S in other cellular compartments, for example, the endoplasmic reticulum and the nucleus, as well as the intersections between hypoxia and H2S signaling are important future directions that merit elucidation. Antioxid. Redox Signal. 38, 57-67.

Human ultrarare genetic disorders of sulfur metabolism demonstrate redundancies in H2S homeostasis

Regulation of H₂S homeostasis in humans is poorly understood. Therefore, we assessed the importance of individual enzymes in synthesis and catabolism of H₂S by studying patients with respective genetic defects. We analyzed sulfur compounds (including bioavailable sulfide) in 37 untreated or insufficiently treated patients with seven ultrarare enzyme deficiencies and compared them to 63 controls. Surprisingly, we observed that patients with severe deficiency in cystathionine β-synthase (CBS) or cystathionine γ-lyase (CSE) - the enzymes primarily responsible for H₂S synthesis - exhibited increased and normal levels of bioavailable sulfide, respectively. However, an approximately 21-fold increase of urinary homolanthionine in CBS deficiency strongly suggests that lacking CBS activity is compensated for by an increase in CSE-dependent H₂S synthesis from accumulating homocysteine, which suggests a control of H₂S homeostasis in vivo. In deficiency of sulfide:quinone oxidoreductase - the first enzyme in mitochondrial H₂S oxidation - we found normal H₂S concentrations in a symptomatic patient and his asymptomatic sibling, and elevated levels in an asymptomatic sibling, challenging the requirement for this enzyme in catabolizing H₂S under physiological conditions. Patients with ethylmalonic encephalopathy and sulfite oxidase/molybdenum cofactor deficiencies exhibited massive accumulation of thiosulfate and sulfite with formation of large amounts of S-sulfocysteine and S-sulfohomocysteine, increased renal losses of sulfur compounds and concomitant strong reduction in plasma total cysteine. Our results demonstrate the value of a comprehensive assessment of sulfur compounds in severe disorders of homocysteine/cysteine metabolism and provide evidence for redundancy and compensatory mechanisms in the maintenance of H₂S homeostasis.

•

Cystathionine γ-lyase can compensate for decreased H₂S synthesis in cystathionine β-synthase deficiency.

•

Sulfide:quinone oxidoreductase deficiency is compatible with normal H₂S plasma levels under non-stressed conditions.

•

Persulfide dioxygenase deficiency (ethylmalonic encephalopathy) causes the largest accumulation of H₂S among disorders of sulfur metabolism.

•

Excess sulfite forms S-sulfocysteine and S-sulfohomocysteine, and interferes with vitamin B₆ metabolism.

•

S-sulfocysteine correlates directly with sulfite and is a stable biomarker of sulfite accumulation.

Progress and perspective on hydrogen sulfide donors and their biomedical applications

Following the discovery of nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂ S) has been identified as the third gasotransmitter in humans. Increasing evidence have shown that H₂ S is of preventive or therapeutic effects on diverse pathological complications. As a consequence, it is of great significance to develop suitable approaches of H₂ S-based therapeutics for biomedical applications. H₂ S-releasing agents (H₂ S donors) play important roles in exploring and understanding the physiological functions of H₂ S. More importantly, accumulating studies have validated the theranostic potential of H₂ S donors in extensive repertoires of in vitro and in vivo disease models. Thus, it is imperative to summarize and update the literatures in this field. In this review, first, the background of H₂ S on its chemical and biological aspects is concisely introduced. Second, the studies regarding the H₂ S-releasing compounds are categorized and described, and accordingly, their H₂ S-donating mechanisms, biological applications, and therapeutic values are also comprehensively delineated and discussed. Necessary comparisons between related H₂ S donors are presented, and the drawbacks of many typical H₂ S donors are analyzed and revealed. Finally, several critical challenges encountered in the development of multifunctional H₂ S donors are discussed, and the direction of their future development as well as their biomedical applications is proposed. We expect that this review will reach extensive audiences across multiple disciplines and promote the innovation of H₂ S biomedicine.

Gas regulation of complex II reversal via electron shunting to fumarate in the mammalian ETC

The electron transport chain (ETC) is a major currency converter that exchanges the chemical energy of fuel oxidation to proton motive force and, subsequently, ATP generation, using O₂ as a terminal electron acceptor. Discussed herein, two new studies reveal that the mammalian ETC is forked. Hypoxia or H₂S exposure promotes the use of fumarate as an alternate terminal electron acceptor. The fumarate/succinate and CoQH₂/CoQ redox couples are nearly iso-potential, revealing that complex II is poised for facile reverse electron transfer, which is sensitive to CoQH₂ and fumarate concentrations. The gas regulators, H₂S and ^•NO, modulate O₂ affinity and/or inhibit the electron transfer rate at complex IV. Their induction under hypoxia suggests a mechanism for how traffic at the ETC fork can be regulated.

Carbon Monoxide (CO), Nitric Oxide, and Hydrogen Sulfide Signaling During Acute CO Poisoning

Major toxic effects of acute carbon monoxide (CO) poisoning result from increases in reactive oxygen species (ROS) and reactive nitrogen species (RNS) producing oxidative stress. The importance of altered nitric oxide (NO) signaling in evoking increases in RNS during CO poisoning has been established. Although there is extensive literature describing NO and hydrogen sulfide (H2S) signaling in different types of cells under normal conditions, how CO poisoning-evoked deregulation of additional NO signaling pathways and H2S signaling pathways could result in cell injury has not been previously considered in detail. The goal of this article was to do this. The approach was to use published data to describe signaling pathways driven by CO bonding to different ferroproteins and then to collate data that describe NO and H2S signaling pathways that could interact with CO signaling pathways and be important during CO poisoning. Arteriolar smooth muscle cells—endothelial cells located in the coronary and some cerebral circulations—were used as a model to illustrate major signaling pathways driven by CO bonding to different ferroproteins. The results were consistent with the concept that multiple deregulated and interacting NO and H2S signaling pathways can be involved in producing cell injury evoked during acute CO poisoning and that these pathways interact with CO signaling pathways.

Regulation of protein function and degradation by heme, heme responsive motifs, and CO

Heme is an essential biomolecule and cofactor involved in a myriad of biological processes. In this review, we focus on how heme binding to heme regulatory motifs (HRMs), catalytic sites, and gas signaling molecules as well as how changes in the heme redox state regulate protein structure, function, and degradation. We also relate these heme-dependent changes to the affected metabolic processes. We center our discussion on two HRM-containing proteins: human heme oxygenase-2, a protein that binds and degrades heme (releasing Fe²⁺ and CO) in its catalytic core and binds Fe³⁺-heme at HRMs located within an unstructured region of the enzyme, and the transcriptional regulator Rev-erbβ, a protein that binds Fe³⁺-heme at an HRM and is involved in CO sensing. We will discuss these and other proteins as they relate to cellular heme composition, homeostasis, and trafficking. In addition, we will discuss the HRM-containing family of proteins and how the stability and activity of these proteins are regulated in a dependent manner through the HRMs. Then, after reviewing CO-mediated protein regulation of heme proteins, we turn our attention to the involvement of heme, HRMs, and CO in circadian rhythms. In sum, we stress the importance of understanding the various roles of heme and the distribution of the different heme pools as they relate to the heme redox state, CO, and heme binding affinities.

Hydrogen Sulfide: An Emerging Precision Strategy for Gas Therapy

Advances in nanotechnology have enabled the rapid development of stimuli-responsive therapeutic nanomaterials for precision gas therapy. Hydrogen sulfide (H₂ S) is a significant gaseous signaling molecule with intrinsic biochemical properties, which exerts its various physiological effects under both normal and pathological conditions. Various nanomaterials with H₂ S-responsive properties, as new-generation therapeutic agents, are explored to guide therapeutic behaviors in biological milieu. The cross disciplinary of H₂ S is an emerging scientific hotspot that studies the chemical properties, biological mechanisms, and therapeutic effects of H₂ S. This review summarizes the state-of-art research on H₂ S-related nanomedicines. In particular, recent advances in H₂ S therapeutics for cancer, such as H₂ S-mediated gas therapy and H₂ S-related synergistic therapies (combined with chemotherapy, photodynamic therapy, photothermal therapy, and chemodynamic therapy) are highlighted. Versatile imaging techniques for real-time monitoring H₂ S during biological diagnosis are reviewed. Finally, the biosafety issues, current challenges, and potential possibilities in the evolution of H₂ S-based therapy that facilitate clinical translation to patients are discussed.

Hydropersulfides (RSSH) and Nitric Oxide (NO) Signaling: Possible Effects on S-Nitrosothiols (RS-NO)

S-Nitrosothiol (RS-NO) formation in proteins and peptides have been implicated as factors in the etiology of many diseases and as possible regulators of thiol protein function. They have also been proposed as possible storage forms of nitric oxide (NO). However, despite their proposed functions/roles, there appears to be little consensus regarding the physiological mechanisms of RS-NO formation and degradation. Hydropersulfides (RSSH) have recently been discovered as endogenously generated species with unique reactivity. One important reaction of RSSH is with RS-NO, which leads to the degradation of RS-NO as well as the release of NO. Thus, it can be speculated that RSSH can be a factor in the regulation of steady-state RS-NO levels, and therefore may be important in RS-NO (patho)physiology. Moreover, RSSH-mediated NO release from RS-NO may be a possible mechanism allowing RS-NO to serve as a storage form of NO.

Modulation of Human Hydrogen Sulfide Metabolism by Micronutrients, Preliminary Data

Background:

Hydrogen sulfide (H₂S) is a pivotal gasotransmitter networking with nitric oxide (NO) and carbon monoxide (CO) to regulate basic homeostatic functions. It is released by the alternative pathways of transulfuration by the enzymes Cystathionine Beta Synthase (CBS) and Cystathionine Gamma Lyase (CSE), and by Cysteine AminoTransferase (CAT)/ 3-Mercaptopyruvate Sulfur Transferase (3MPST). A non-enzymatic, intravascular release is also in place. We retrospectively investigated the possibility to modulate the endogenous H₂S release and signaling in humans by a dietary manipulation with supplemented micronutrients (L-cystine, Taurine and pyridoxal 5-phopsphate/P5P).

Methods:

Patients referring for antiaging purposes underwent a 10-day supplementation. Blood was collected at baseline and after treatment and the metabolome was investigated by mass spectrometry to monitor the changes in the metabolites reporting on H₂S metabolism and related pathways.

Results:

Data were available from 6 middle aged subjects (2 women). Micronutrients increased 3-mercaptopyruvate (P = .03), reporting on the activity of CAT that provides the substrate for H₂S release within mitochondria by 3MPST, decreased lanthionine (P = .024), reporting the release of H₂S from CBS, and had no significant effect of H₂S release from CSE. This is compatible with a homeostatic balancing. We also recorded a strong increase of reporters of H₂S-induced pathways including 5-MethylTHF (P = .001) and SAME (P = .022), reporting on methylation capacity, and of BH4 (P = .021) and BH2 (P = .028) reporting on nitric oxide metabolism. These activations may be explained by the concomitant induction of non-enzymatic release of H₂S.

Conclusions:

Although the current evidences are weak and will need to be confirmed, the effect of micronutrients was compatible with an increase of the H₂S endogenous release and signaling within the control of homeostatic mechanisms, further endorsing the role of feeding in health and disease. These effects might result in a H₂S boosting effect in case of defective activity of pathologic origin, which should be checked in duly designed clinical trials.

A Case for Hydrogen Sulfide Metabolism as an Oxygen Sensing Mechanism

The ability to detect oxygen availability is a ubiquitous attribute of aerobic organisms. However, the mechanism(s) that transduce oxygen concentration or availability into appropriate physiological responses is less clear and often controversial. This review will make the case for oxygen-dependent metabolism of hydrogen sulfide (H₂S) and polysulfides, collectively referred to as reactive sulfur species (RSS) as a physiologically relevant O₂ sensing mechanism. This hypothesis is based on observations that H₂S and RSS metabolism is inversely correlated with O₂ tension, exogenous H₂S elicits physiological responses identical to those produced by hypoxia, factors that affect H₂S production or catabolism also affect tissue responses to hypoxia, and that RSS efficiently regulate downstream effectors of the hypoxic response in a manner consistent with a decrease in O₂. H₂S-mediated O₂ sensing is then compared to the more generally accepted reactive oxygen species (ROS) mediated O₂ sensing mechanism and a number of reasons are offered to resolve some of the confusion between the two.

Human Cystathionine γ-Lyase Is Inhibited by s-Nitrosation: A New Crosstalk Mechanism between NO and H2S

The ‘gasotransmitters’ hydrogen sulfide (H₂S), nitric oxide (NO), and carbon monoxide (CO) act as second messengers in human physiology, mediating signal transduction via interaction with or chemical modification of protein targets, thereby regulating processes such as neurotransmission, blood flow, immunomodulation, or energy metabolism. Due to their broad reactivity and potential toxicity, the biosynthesis and breakdown of H₂S, NO, and CO are tightly regulated. Growing evidence highlights the active role of gasotransmitters in their mutual cross-regulation. In human physiology, the transsulfuration enzymes cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are prominent H₂S enzymatic sources. While CBS is known to be inhibited by NO and CO, little is known about CSE regulation by gasotransmitters. Herein, we investigated the effect of s-nitrosation on CSE catalytic activity. H₂S production by recombinant human CSE was found to be inhibited by the physiological nitrosating agent s-nitrosoglutathione (GSNO), while reduced glutathione had no effect. GSNO-induced inhibition was partially reverted by ascorbate and accompanied by the disappearance of one solvent accessible protein thiol. By combining differential derivatization procedures and mass spectrometry-based analysis with functional assays, seven out of the ten protein cysteine residues, namely Cys84, Cys109, Cys137, Cys172, Cys229, Cys307, and Cys310, were identified as targets of s-nitrosation. By generating conservative Cys-to-Ser variants of the identified s-nitrosated cysteines, Cys137 was identified as most significantly contributing to the GSNO-mediated CSE inhibition. These results highlight a new mechanism of crosstalk between gasotransmitters.

Severe homocysteinemia in two givosiran-treated porphyria patients: is free heme deficiency the culprit?

Givosiran is a novel approach to treat patients with acute intermittent porphyrias (AIP) by silencing of ∂-ALA-synthase 1, the first enzyme of heme biosynthesis in the liver. We included two patients in the Envision study who responded clinically well to this treatment. However, in both patients, therapy had to be discontinued because of severe adverse effects: One patient (A) developed local injection reactions which continued to spread all over her body with increasing number of injections and eventually caused a severe systemic allergic reaction. Patient B was hospitalized because of a fulminant pancreatitis. Searching for possible causes, we also measured the patients plasma homocysteine (Hcy) levels in fluoride-containing collection tubes: by LC-MS/MS unexpectedly, plasma Hcy levels were 100 and 200 in patient A and between 100 and 400 μmol/l in patient B. Searching for germline mutations in 10 genes that are relevant for homocysteine metabolism only revealed hetero- and homozygous polymorphisms in the MTHFR gene. Alternatively, an acquired inhibition of cystathionine-beta-synthase which is important for homocysteine metabolism could explain the plasma homocysteine increase. This enzyme is heme-dependent: when we gave heme arginate to our patients, Hcy levels rapidly dropped. Hence, we conclude that inhibition of ∂-ALA-synthase 1 by givosiran causes a drop of free heme in the hepatocyte and therefore the excessive increase of plasma homocysteine. Hyperhomocysteinemia may contribute to the adverse effects seen in givosiran-treated patients which may be due to protein-N-homocysteinylation.

Structural perspectives on H2S homeostasis

The enzymes involved in H₂S homeostasis regulate its production from sulfur-containing amino acids and its oxidation to thiosulfate and sulfate. Two gatekeepers in this homeostatic circuit are cystathionine beta-synthase, which commits homocysteine to cysteine, and sulfide quinone oxidoreductase, which commits H₂S to oxidation via a mitochondrial pathway. Inborn errors at either locus affect sulfur metabolism, increasing homocysteine-derived H₂S synthesis in the case of CBS deficiency and reducing complex IV activity in the case of SQOR deficiency. In this review, we focus on structural perspectives on the reaction mechanisms and regulation of these two enzymes, which are key to understanding H₂S homeostasis in health and its dysregulation and potential targeting in disease.

The mitochondrial NADH pool is involved in hydrogen sulfide signaling and stimulation of aerobic glycolysis

Hydrogen sulfide is synthesized by enzymes involved in sulfur metabolism and oxidized via a dedicated mitochondrial pathway that intersects with the electron transport chain at the level of complex III. Studies with H₂S are challenging since it is volatile and also reacts with oxidized thiols in the culture medium, forming sulfane sulfur species. The half-life of exogenously added H₂S to cultured cells is unknown. In this study, we first examined the half-life of exogenously added H₂S to human colonic epithelial cells. In plate cultures, H₂S disappeared with a t¹/₂ of 3 to 4 min at 37 °C with a small fraction being trapped as sulfane sulfur species. In suspension cultures, the rate of abiotic loss of H₂S was slower, and we demonstrated that sulfide stimulated aerobic glycolysis, which was sensitive to the mitochondrial but not the cytoplasmic NADH pool. Oxidation of mitochondrial NADH using the genetically encoded mito-LbNOX tool blunted the cellular sensitivity to sulfide-stimulated aerobic glycolysis and enhanced its oxidation to thiosulfate. In contrast, sulfide did not affect flux through the oxidative pentose phosphate pathway or the TCA cycle. Knockdown of sulfide quinone oxidoreductase, which commits H₂S to oxidation, sensitized cells to sulfide-stimulated aerobic glycolysis. Finally, we observed that sulfide decreased ATP levels in cells. The dual potential of H₂S to activate oxidative phosphorylation at low concentrations, but inhibit it at high concentrations, suggests that it might play a role in tuning electron flux and, therefore, cellular energy metabolism, particularly during cell proliferation.

Regulation of the redox metabolome and thiol proteome by hydrogen sulfide

Overproduction of reactive oxygen species and compromised antioxidant defenses perturb intracellular redox homeostasis and is associated with a myriad of human diseases as well as with the natural process of aging. Hydrogen sulfide (H₂S), which is biosynthesized by organisms ranging from bacteria to man, influences a broad range of physiological functions. A highly touted molecular mechanism by which H₂S exerts its cellular effects is via post-translational modification of the thiol redox proteome, converting cysteine thiols to persulfides, in a process referred to as protein persulfidation. The physiological relevance of this modification in the context of specific signal transmission pathways remains to be rigorously established, while a general protective role for protein persulfidation against hyper-oxidation of the cysteine proteome is better supported. A second mechanism by which H₂S modulates redox homeostasis is via remodeling the redox metabolome, targeting the electron transfer chain and perturbing the major redox nodes i.e. CoQ/CoQH₂, NAD⁺/NADH and FAD/FADH₂. The metabolic changes that result from H₂S-induced redox changes fan out from the mitochondrion to other compartments. In this review, we discuss recent developments in elucidating the roles of H₂S and its oxidation products on redox homeostasis and its role in protecting the thiol proteome.

The Role of the Transsulfuration Pathway in Non-Alcoholic Fatty Liver Disease

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing and approximately 25% of the global population may have NAFLD. NAFLD is associated with obesity and metabolic syndrome, but its pathophysiology is complex and only partly understood. The transsulfuration pathway (TSP) is a metabolic pathway regulating homocysteine and cysteine metabolism and is vital in controlling sulfur balance in the organism. Precise control of this pathway is critical for maintenance of optimal cellular function. The TSP is closely linked to other pathways such as the folate and methionine cycles, hydrogen sulfide (H₂S) and glutathione (GSH) production. Impaired activity of the TSP will cause an increase in homocysteine and a decrease in cysteine levels. Homocysteine will also be increased due to impairment of the folate and methionine cycles. The key enzymes of the TSP, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), are highly expressed in the liver and deficient CBS and CSE expression causes hepatic steatosis, inflammation, and fibrosis in animal models. A causative link between the TSP and NAFLD has not been established. However, dysfunctions in the TSP and related pathways, in terms of enzyme expression and the plasma levels of the metabolites (e.g., homocysteine, cystathionine, and cysteine), have been reported in NAFLD and liver cirrhosis in both animal models and humans. Further investigation of the TSP in relation to NAFLD may reveal mechanisms involved in the development and progression of NAFLD.

Cysteine metabolic circuitries: druggable targets in cancer

To enable survival in adverse conditions, cancer cells undergo global metabolic adaptations. The amino acid cysteine actively contributes to cancer metabolic remodelling on three different levels: first, in its free form, in redox control, as a component of the antioxidant glutathione or its involvement in protein s-cysteinylation, a reversible post-translational modification; second, as a substrate for the production of hydrogen sulphide (H2S), which feeds the mitochondrial electron transfer chain and mediates per-sulphidation of ATPase and glycolytic enzymes, thereby stimulating cellular bioenergetics; and, finally, as a carbon source for epigenetic regulation, biomass production and energy production. This review will provide a systematic portrayal of the role of cysteine in cancer biology as a source of carbon and sulphur atoms, the pivotal role of cysteine in different metabolic pathways and the importance of H2S as an energetic substrate and signalling molecule. The different pools of cysteine in the cell and within the body, and their putative use as prognostic cancer markers will be also addressed. Finally, we will discuss the pharmacological means and potential of targeting cysteine metabolism for the treatment of cancer.

A comparison of the chemical biology of hydropersulfides (RSSH) with other protective biological antioxidants and nucleophiles

The hydropersulfide (RSSH) functional group has received significant recent interest due to its unique chemical properties that set it apart from other biological species. The chemistry of RSSH predicts that one possible biological role may be as a protectant against cellular oxidative and electrophilic stress. That is, RSSH has reducing and nucleophilic properties that may combat the potentially destructive biochemistry of toxicologically relevant oxidants and electrophiles. However, there are currently numerous other molecules that have established roles in this regard. For example, ascorbate and tocopherols are potent antioxidants that quench deleterious oxidative reactions and glutathione (GSH) is a well-established and highly prevalent biological protectant against electrophile toxicity. Thus, in order to begin to understand the possible role of RSSH species as protectants against oxidative/electrophilic stress, the inherent chemical properties of RSSH versus these other protectants will be discussed and contrasted.

The multifaceted roles of sulfane sulfur species in cancer-associated processes

Sulfane sulfur species comprise a variety of biologically relevant hydrogen sulfide (H₂S)-derived species, including per- and poly-sulfidated low molecular weight compounds and proteins. A growing body of evidence suggests that H₂S, currently recognized as a key signaling molecule in human physiology and pathophysiology, plays an important role in cancer biology by modulating cell bioenergetics and contributing to metabolic reprogramming. This is accomplished through functional modulation of target proteins via H₂S binding to heme iron centers or H₂S-mediated reversible per- or poly-sulfidation of specific cysteine residues. Since sulfane sulfur species are increasingly viewed not only as a major source of H₂S but also as key mediators of some of the biological effects commonly attributed to H₂S, the multifaceted role of these species in cancer biology is reviewed here with reference to H₂S, focusing on their metabolism, signaling function, impact on cell bioenergetics and anti-tumoral properties.

The role of host defences in Covid 19 and treatments thereof

Hydrogen sulfide (H₂S) is a natural defence against the infections from enveloped RNA viruses and is likely involved also in Covid 19. It was already shown to inhibit growth and pathogenic mechanisms of a variety of enveloped RNA viruses and it was now found that circulating H₂S is higher in Covid 19 survivors compared to fatal cases. H₂S release is triggered by carbon monoxide (CO) from the catabolism of heme by inducible heme oxygenase (HO-1) and heme proteins possess catalytic activity necessary for the H₂S signalling by protein persulfidation. Subjects with a long promoter for the HMOX1 gene, coding for HO-1, are predicted for lower efficiency of this mechanism. SARS-cov-2 exerts ability to attack the heme of hemoglobin and other heme-proteins thus hampering both release and signalling of H₂S. Lack of H₂S-induced persulfidation of the K_ATP channels of leucocytes causes adhesion and release of the inflammatory cytokines, lung infiltration and systemic endothelial damage with hyper-coagulability. These events largely explain the sex and age distribution, clinical manifestations and co-morbidities of Covid-19. The understanding of this mechanism may be of guidance in re-evaluating the ongoing therapeutic strategies, with special attention to the interaction with mechanical ventilation, paracetamol and chloroquine use, and in the individuation of genetic traits causing increased susceptibility to the disruption of these physiologic processes and to a critical Covid 19. Finally, an array of therapeutic interventions with the potential to clinically modulate the HO-1/CO/H₂S axis is already available or under development. These include CO donors and H₂S donors and a boost to the endogenous production of H₂S is also possible.

Oxygen battle in the gut: Hypoxia and hypoxia-inducible factors in metabolic and inflammatory responses in the intestine

The gastrointestinal tract is a highly proliferative and regenerative tissue. The intestine also harbors a large and diverse microbial population collectively called the gut microbiome (microbiota). The microbiome-intestine cross-talk includes a dynamic exchange of gaseous signaling mediators generated by bacterial and intestinal metabolisms. Moreover, the microbiome initiates and maintains the hypoxic environment of the intestine that is critical for nutrient absorption, intestinal barrier function, and innate and adaptive immune responses in the mucosal cells of the intestine. The response to hypoxia is mediated by hypoxia-inducible factors (HIFs). In hypoxic conditions, the HIF activation regulates the expression of a cohort of genes that promote adaptation to hypoxia. Physiologically, HIF-dependent genes contribute to the aforementioned maintenance of epithelial barrier function, nutrient absorption, and immune regulation. However, chronic HIF activation exacerbates disease conditions, leading to intestinal injury, inflammation, and colorectal cancer. In this review, we aim to outline the major roles of physiological and pathological hypoxic conditions in the maintenance of intestinal homeostasis and in the onset and progression of disease with a major focus on understanding the complex pathophysiology of the intestine.

Cystathionine-β-Synthase: Molecular Regulation and Pharmacological Inhibition

Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used "CBS inhibitors" (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

Hydrogen Sulfide Metabolism and Signaling in the Tumor Microenvironment

Hydrogen sulfide (H2S), while historically perceived merely as a toxicant, has progressively emerged as a key regulator of numerous processes in mammalian physiology, exerting its signaling function essentially through interaction with and/or modification of proteins, targeting mainly cysteine residues and metal centers. As a gaseous signaling molecule that freely diffuses across aqueous and hydrophobic biological milieu, it has been designated the third 'gasotransmitter' in mammalian physiology. H2S is synthesized and detoxified by specialized endogenous enzymes that operate under a tight regulation, ensuring homeostatic levels of this otherwise toxic molecule. Indeed, imbalances in H2S levels associated with dysfunctional H2S metabolism have been growingly correlated with various human pathologies, from cardiovascular and neurodegenerative diseases to cancer. Several cancer cell lines and specimens have been shown to naturally overexpress one or more of the H2S-synthesizing enzymes. The resulting increased H2S levels have been proposed to promote cancer development through the regulation of various cancer-related processes, which led to the interest in pharmacological targeting of H2S metabolism. Herein are summarized some of the key observations that place H2S metabolism and signaling pathways at the forefront of the cellular mechanisms that support the establishment and development of a tumor within its complex and challenging microenvironment. Special emphasis is given to the mechanisms whereby H2S helps shaping cancer cell bioenergetic metabolism and affords resistance and adaptive mechanisms to hypoxia.

S-3-Carboxypropyl-l-cysteine specifically inhibits cystathionine γ-lyase-dependent hydrogen sulfide synthesis

Hydrogen sulfide (H₂S) is a gaseous signaling molecule, which modulates a wide range of mammalian physiological processes. Cystathionine γ-lyase (CSE) catalyzes H₂S synthesis and is a potential target for modulating H₂S levels under pathophysiological conditions. CSE is inhibited by propargylglycine (PPG), a widely used mechanism-based inhibitor. In this study, we report that inhibition of H₂S synthesis from cysteine, but not the canonical cystathionine cleavage reaction catalyzed by CSE in vitro, is sensitive to preincubation of the enzyme with PPG. In contrast, the efficacy of S-3-carboxpropyl-l-cysteine (CPC) a new inhibitor described herein, was not dependent on the order of substrate/inhibitor addition. We observed that CPC inhibited the γ-elimination reaction of cystathionine and H₂S synthesis from cysteine by human CSE with K_i values of 50 ± 3 and 180 ± 15 μm, respectively. We noted that CPC spared the other enzymes involved either directly (cystathionine β-synthase and mercaptopyruvate sulfurtransferase) or indirectly (cysteine aminotransferase) in H₂S biogenesis. CPC also targeted CSE in cultured cells, inhibiting transsulfuration flux by 80-90%, as monitored by the transfer of radiolabel from [³⁵S]methionine to GSH. The 2.5 Å resolution crystal structure of human CSE in complex with the CPC-derived aminoacrylate intermediate provided a structural framework for the molecular basis of its inhibitory effect. In summary, our study reveals a previously unknown confounding effect of PPG, widely used to inhibit CSE-dependent H₂S synthesis, and reports on an alternative inhibitor, CPC, which could be used as a scaffold to develop more potent H₂S biogenesis inhibitors.

Balancing on the road less traveled

Sitting down to the task of writing, I found my pen drifting inexorably to a personal recollection of the metaphorical transcontinental road that I had traveled to become a scientist, instead of reviewing a facet of our scientific contributions. Factors that prepared me for my improbable journey in an era when international calls were operator-assisted and unaffordable and the internet was the stuff of science fiction were my family's love and the sheltered environment of my all-girls school and college experiences, which nurtured my self-confidence. The path of scientific inquiry is heady, and it is hard. The paucity of diversity, of women and minorities, particularly as the road steepens, helps perpetuate stereotypes and inadvertently encourages disparities. It is my hope that by sharing snippets of my journey, enriched as it has been by a diversity of mentors, mentees, colleagues, and friends, and the opportunity to express my curiosity and creativity, that a young person contemplating the scientific road will find encouragement.

Redox-Dependent Regulation of Sulfur Metabolism in Biomolecules: Implications for Cardiovascular Health

SIGNIFICANCE

Sulfur-containing amino acids are integral to the molecular mechanisms that underlie many aspects of cellular function and homeostasis, facilitated by reversible changes in the oxidation states of sulfur atoms. Sulfur-containing amino acids are metabolically linked by interacting pathways that impact the one-carbon metabolic cycle and generation of methyl groups, the folate cycle, and maintenance of the major cellular redox buffer; glutathione. Dysregulation of these pathways is associated with diverse pathologies, notably of the cardiovascular (CV) system, which are typically characterized by inappropriate plasma levels of sulfur-containing amino acids. Recent Advances: Perhaps not surprisingly, the cellular redox state has emerged as a major regulator of many enzymatic processes within these metabolic cycles. The metabolism of cysteine can also result in the production of hydrogen sulfide (H₂S), a signaling molecule whose activity is potentially linked to intracellular levels of both reactive oxygen species (ROS) and molecular oxygen.

CRITICAL ISSUES

In most cases, the endogenous physiological sources of ROS that might mediate the interlinked metabolic pathways of sulfur-containing biomolecules remain unknown. However, the family of NADPH oxidases, and Nox4 in particular, is emerging as a likely candidate.

FUTURE DIRECTIONS

This review focuses on the current knowledge of key aspects of sulfur metabolism, which are regulated by redox-based chemical reactions, and the likely intracellular oxidant sources that might mediate this regulation. This knowledge will be important to guide future targeted therapeutic interventions in diverse CV disorders.

Homeostatic impact of sulfite and hydrogen sulfide on cysteine catabolism

Cysteine is one of the two key sulfur-containing amino acids with important functions in redox homeostasis, protein functionality and metabolism. Cysteine is taken up by mammals via their diet and can also be derived from methionine via the transsulfuration pathway. The cellular concentration of cysteine is kept within a narrow range by controlling its synthesis and degradation. There are two pathways for the catabolism of cysteine leading to sulfate, taurine and thiosulfate as terminal products. The oxidative pathway produces taurine and sulfate, while the H2 S pathway involves different enzymatic reactions leading to the formation and clearance of H2 S, an important signalling molecule in mammals, resulting in thiosulfate and sulfate. Sulfite is a common intermediate in both catabolic pathways. Sulfite is considered as cytotoxic and produces neurotoxic S-sulfonates. As a result, a deficiency in the terminal steps of cysteine or H2 S catabolism leads to severe forms of encephalopathy with the accumulation of sulfite and H2 S in the body. This review links the homeostatic regulation of both cysteine catabolic pathways to sulfite and H2 S. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Screening Pyridine Derivatives against Human Hydrogen Sulfide-synthesizing Enzymes by Orthogonal Methods

Biosynthesis of hydrogen sulfide (H₂S), a key signalling molecule in human (patho)physiology, is mostly accomplished by the human enzymes cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (MST). Several lines of evidence have shown a close correlation between increased H₂S production and human diseases, such as several cancer types and amyotrophic lateral sclerosis. Identifying compounds selectively and potently inhibiting the human H₂S-synthesizing enzymes may therefore prove beneficial for pharmacological applications. Here, the human enzymes CBS, CSE and MST were expressed and purified from Escherichia coli, and thirty-one pyridine derivatives were synthesized and screened for their ability to bind and inhibit these enzymes. Using differential scanning fluorimetry (DSF), surface plasmon resonance (SPR), circular dichroism spectropolarimetry (CD), and activity assays based on fluorimetric and colorimetric H₂S detection, two compounds (C30 and C31) sharing structural similarities were found to weakly inhibit both CBS and CSE: 1 mM C30 inhibited these enzymes by approx. 50% and 40%, respectively, while 0.5 mM C31 accounted for CBS and CSE inhibition by approx. 40% and 60%, respectively. This work, while presenting a robust methodological platform for screening putative inhibitors of the human H₂S-synthesizing enzymes, highlights the importance of employing complementary methodologies in compound screenings.

Mass Spectrometry in Advancement of Redox Precision Medicine

Redox (portmanteau of reduction-oxidation) reactions involve the transfer of electrons between chemical species in biological processes fundamental to life. It is of outmost importance that cells maintain a healthy redox state by balancing the action of oxidants and antioxidants; failure to do so leads to a multitude of diseases including cancer, diabetes, fibrosis, autoimmune diseases, and cardiovascular and neurodegenerative diseases. From the perspective of precision medicine, it is therefore beneficial to interrogate the redox phenotype of the individual-similar to the use of genomic sequencing-in order to design tailored strategies for disease prevention and treatment. This chapter provides an overview of redox metabolism and focuses on how mass spectrometry (MS) can be applied to advance our knowledge in redox biology and precision medicine.

Regulators of the transsulfuration pathway

The transsulfuration pathway is a metabolic pathway where transfer of sulfur from homocysteine to cysteine occurs. The pathway leads to the generation of several sulfur metabolites, which include cysteine, GSH and the gaseous signalling molecule hydrogen sulfide (H₂ S). Precise control of this pathway is critical for maintenance of optimal cellular function and, therefore, the key enzymes of the pathway, cystathionine β-synthase and cystathionine γ-lyase, are regulated at multiple levels. Disruption of the transsulfuration pathway contributes to the pathology of several conditions such as vascular dysfunction, Huntington's disease and during ageing. Treatment with donors of hydrogen sulfide and/or stimulation of this pathway have proved beneficial in several of these disorders. In this review, we focus on the regulation of the transsulfuration pathway pertaining to cysteine and H₂ S, which could be targeted to develop novel therapeutics. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Mammalian Sulfur Amino Acid Metabolism: A Nexus Between Redox Regulation, Nutrition, Epigenetics, and Detoxification

SIGNIFICANCE

Transsulfuration allows conversion of methionine into cysteine using homocysteine (Hcy) as an intermediate. This pathway produces S-adenosylmethionine (AdoMet), a key metabolite for cell function, and provides 50% of the cysteine needed for hepatic glutathione synthesis. The route requires the intake of essential nutrients (e.g., methionine and vitamins) and is regulated by their availability. Transsulfuration presents multiple interconnections with epigenetics, adenosine triphosphate (ATP), and glutathione synthesis, polyol and pentose phosphate pathways, and detoxification that rely mostly in the exchange of substrates or products. Major hepatic diseases, rare diseases, and sensorineural disorders, among others that concur with oxidative stress, present impaired transsulfuration. Recent Advances: In contrast to the classical view, a nuclear branch of the pathway, potentiated under oxidative stress, is emerging. Several transsulfuration proteins regulate gene expression, suggesting moonlighting activities. In addition, abnormalities in Hcy metabolism link nutrition and hearing loss.

CRITICAL ISSUES

Knowledge about the crossregulation between pathways is mostly limited to the hepatic availability/removal of substrates and inhibitors. However, advances regarding protein-protein interactions involving oncogenes, identification of several post-translational modifications (PTMs), and putative moonlighting activities expand the potential impact of transsulfuration beyond methylations and Hcy.

FUTURE DIRECTIONS

Increasing the knowledge on transsulfuration outside the liver, understanding the protein-protein interaction networks involving these enzymes, the functional role of their PTMs, or the mechanisms controlling their nucleocytoplasmic shuttling may provide further insights into the pathophysiological implications of this pathway, allowing design of new therapeutic interventions. Antioxid. Redox Signal. 29, 408-452.

Hydrogen Sulfide Biochemistry and Interplay with Other Gaseous Mediators in Mammalian Physiology

Hydrogen sulfide (H₂S) has emerged as a relevant signaling molecule in physiology, taking its seat as a bona fide gasotransmitter akin to nitric oxide (NO) and carbon monoxide (CO). After being merely regarded as a toxic poisonous molecule, it is now recognized that mammalian cells are equipped with sophisticated enzymatic systems for H₂S production and breakdown. The signaling role of H₂S is mainly related to its ability to modify different protein targets, particularly by promoting persulfidation of protein cysteine residues and by interacting with metal centers, mostly hemes. H₂S has been shown to regulate a myriad of cellular processes with multiple physiological consequences. As such, dysfunctional H₂S metabolism is increasingly implicated in different pathologies, from cardiovascular and neurodegenerative diseases to cancer. As a highly diffusible reactive species, the intra- and extracellular levels of H₂S have to be kept under tight control and, accordingly, regulation of H₂S metabolism occurs at different levels. Interestingly, even though H₂S, NO, and CO have similar modes of action and parallel regulatory targets or precisely because of that, there is increasing evidence of a crosstalk between the three gasotransmitters. Herein are reviewed the biochemistry, metabolism, and signaling function of hydrogen sulfide, as well as its interplay with the other gasotransmitters, NO and CO.

Modulation of Catalytic Promiscuity during Hydrogen Sulfide Oxidation

The mitochondrial sulfide oxidation pathway prevents the toxic accumulation of hydrogen sulfide (H₂S), a signaling molecule that is maintained at low steady-state concentrations. Sulfide quinone oxidoreductase (SQR), an inner mitochondrial membrane-anchored protein, catalyzes the first and committing step in this pathway, oxidizing H₂S to persulfide. The catalytic cycle comprises sulfide addition to the active site cysteine disulfide in SQR followed by sulfur transfer to a small molecule acceptor, while a pair of electrons moves from sulfide, to FAD, to coenzyme Q. While its ability to oxidize H₂S is well characterized, SQR exhibits a remarkable degree of substrate promiscuity in vitro that could undermine its canonical enzyme activity. To assess how its promiscuity might be contained in vivo, we have used spectroscopic and kinetic analyses to characterize the reactivity of alternate substrates with SQR embedded in nanodiscs ( ndSQR) versus detergent-solubilized enzyme ( sSQR). We find that the membrane environment of ndSQR suppresses the unwanted addition of GSH but enhances sulfite addition, which might become significant under pathological conditions characterized by elevated sulfite levels. We demonstrate that methanethiol, a toxic sulfur compound produced in significant quantities by colonic and oral microbiota, can add to the SQR cysteine disulfide and also serve as a sulfur acceptor, potentially interfering with sulfide oxidation when its concentrations are elevated. These studies demonstrate that the membrane environment and substrate availability combine to minimize promiscuous reactions that would otherwise disrupt sulfide homeostasis.

A persulfidation-based mechanism controls aquaporin-8 conductance

Upon engagement of tyrosine kinase receptors, nicotinamide adenine dinucleotide phosphate (NADPH)-oxidases release H₂O₂ in the extracellular space. We reported previously that aquaporin-8 (AQP8) transports H₂O₂ across the plasma membrane and is reversibly gated during cell stress, modulating signal strength and duration. We show that AQP8 gating is mediated by persulfidation of cysteine 53 (C53). Treatment with H₂S is sufficient to block H₂O₂ entry in unstressed cells. Silencing cystathionine β-synthase (CBS) prevents closure, suggesting that this enzyme is the main source of H₂S. Molecular modeling indicates that C53 persulfidation displaces a nearby histidine located in the narrowest part of the channel. We propose that H₂O₂ molecules transported through AQP8 sulfenylate C53, making it susceptible to H₂S produced by CBS. This mechanism tunes H₂O₂ transport and may control signaling and limit oxidative stress.

A timeline of hydrogen sulfide (H2S) research: From environmental toxin to biological mediator

The history of H2S - as an environmental toxin - dates back to 1700, to the observations of the Italian physician Bernardino Ramazzini, whose book "De Morbis Artificum Diatriba" described the painful eye irritation and inflammation of "sewer gas" in sewer workers. The gas has subsequently been identified as hydrogen sulfide (H2S), and opened three centuries of research into the biological roles of H2S. The current article highlights the key discoveries in the field of H2S research, including (a) the toxicological studies, which characterized H2S as an environmental toxin, and identified some of its modes of action, including the inhibition of mitochondrial respiration; (b) work in the field of bacteriology, which, starting in the early 1900s, identified H2S as a bacterial product - with subsequently defined roles in the regulation of periodontal disease (oral bacterial flora), intestinal epithelial cell function (enteral bacterial flora) as well as in the regulation of bacterial resistance to antibiotics; and (c), work in diverse fields of mammalian biology, which, starting in the 1940s, identified H2S as an endogenous mammalian enzymatic product, the functions of which - among others, in the cardiovascular and nervous system - have become subjects of intensive investigation for the last decade. The current review not only enumerates the key discoveries related to H2S made over the last three centuries, but also compiles the most frequently cited papers in the field which have been published over the last decade and highlights some of the current 'hot topics' in the field of H2S biology.

Mutations in SELENBP1, encoding a novel human methanethiol oxidase, cause extraoral halitosis

Selenium-binding protein 1 (SELENBP1) has been associated with several cancers, although its exact role is unknown. We show that SELENBP1 is a methanethiol oxidase (MTO), related to the MTO in methylotrophic bacteria, that converts methanethiol to H₂O₂, formaldehyde, and H₂S, an activity not previously known to exist in humans. We identified mutations in SELENBP1 in five patients with cabbage-like breath odor. The malodor was attributable to high levels of methanethiol and dimethylsulfide, the main odorous compounds in their breath. Elevated urinary excretion of dimethylsulfoxide was associated with MTO deficiency. Patient fibroblasts had low SELENBP1 protein levels and were deficient in MTO enzymatic activity; these effects were reversed by lentivirus-mediated expression of wild-type SELENBP1. Selenbp1-knockout mice showed biochemical characteristics similar to those in humans. Our data reveal a potentially frequent inborn error of metabolism that results from MTO deficiency and leads to a malodor syndrome.

H2S and polysulfide metabolism: Conventional and unconventional pathways

It is now well established that hydrogen sulfide (H₂S) is an effector of a wide variety of physiological processes. It is also clear that many of the effects of H₂S are mediated through reactions with cysteine sulfur on regulatory proteins and most of these are not mediated directly by H₂S but require prior oxidation of H₂S and the formation of per- and polysulfides (H₂S_n, n = 2-8). Attendant with understanding the regulatory functions of H₂S and H₂S_n is an appreciation of the mechanisms that control, i.e., both increase and decrease, their production and catabolism. Although a number of standard "conventional" pathways have been described and well characterized, novel "unconventional" pathways are continuously being identified. This review summarizes our current knowledge of both the conventional and unconventional.