H2S and its role in redox signaling

The role of a conserved membrane proximal cysteine in altering αPS2CβPS integrin diffusion

Cysteine residues (Cys) in the membrane proximal region are common post-translational modification (PTM) sites in transmembrane proteins. Herein, the effects of a highly conserved membrane proximal α-subunit Cys¹³⁶⁸ on the diffusion properties of αPS2CβPS integrins are reported. Sequence alignment shows that this cysteine is palmitoylated in human α3 and α6 integrin subunits. Replacing Cys¹³⁶⁸ in wild-type integrins with valine (Val¹³⁶⁸) putatively blocks a PTM site and alters integrins' ligand binding and diffusion characteristics. Both fluorescence recovery after photobleaching (FRAP) and single particle tracking (SPT) diffusion measurements show Val¹³⁶⁸ integrins are more mobile compared to wild-type integrins. Approximately 33% and 8% more Val¹³⁶⁸ integrins are mobile as measured by FRAP and SPT, respectively. The mobile Val¹³⁶⁸ integrins also exhibit less time-dependent diffusion, as measured by FRAP. Tandem mass spectrometry data suggest that Cys¹³⁶⁸ contains a redox or palmitoylation PTM in αPS2CβPS integrins. This membrane proximal Cys may play an important role in the diffusion of other alpha subunits that contain this conserved residue.

Metabolic Response to XD14 Treatment in Human Breast Cancer Cell Line MCF-7

XD14 is a 4-acyl pyrrole derivative, which was discovered by a high-throughput virtual screening experiment. XD14 inhibits bromodomain and extra-terminal domain (BET) proteins (BRD2, BRD3, BRD4 and BRDT) and consequently suppresses cell proliferation. In this study, metabolic profiling reveals the molecular effects in the human breast cancer cell line MCF-7 (Michigan Cancer Foundation-7) treated by XD14. A three-day time series experiment with two concentrations of XD14 was performed. Gas chromatography-mass spectrometry (GC-MS) was applied for untargeted profiling of treated and non-treated MCF-7 cells. The gained data sets were evaluated by several statistical methods: analysis of variance (ANOVA), clustering analysis, principle component analysis (PCA), and partial least squares discriminant analysis (PLS-DA). Cell proliferation was strongly inhibited by treatment with 50 µM XD14. Samples could be discriminated by time and XD14 concentration using PLS-DA. From the 117 identified metabolites, 67 were significantly altered after XD14 treatment. These metabolites include amino acids, fatty acids, Krebs cycle and glycolysis intermediates, as well as compounds of purine and pyrimidine metabolism. This massive intervention in energy metabolism and the lack of available nucleotides could explain the decreased proliferation rate of the cancer cells.

H2S concentrations in the heart after acute H2S administration: methodological and physiological considerations

In this study, we have tried to characterize the limits of the approach typically used to determine H₂S concentrations in the heart based on the amount of H₂S evaporating from heart homogenates-spontaneously, after reaction with a strong reducing agent, or in a very acidic solution. Heart homogenates were prepared from male rats in control conditions or after H₂S infusion induced a transient cardiogenic shock (CS) or cardiac asystole (CA). Using a method of determination of gaseous H₂S with a detection limit of 0.2 nmol, we found that the process of homogenization could lead to a total disappearance of free H₂S unless performed in alkaline conditions. Yet, after restoration of neutral pH, free H₂S concentration from samples processed in alkaline and nonalkaline milieus were similar and averaged ∼0.2-0.4 nmol/g in both control and CS homogenate hearts and up to 100 nmol/g in the CA group. No additional H₂S was released from control, CS, or CA hearts by using the reducing agent tris(2-carboxyethyl)phosphine or a strong acidic solution (pH < 2) to "free" H₂S from combined pools. Of note, the reducing agent DTT produced a significant sulfide artifact and was not used. These data suggest that 1) free H₂S found in heart homogenates is not a reflection of H₂S present in a "living" heart and 2) the pool of combined sulfides, released in a strong reducing or acidic milieu, does not increase in the heart in a measurable manner even after toxic exposure to sulfide.

Hydrogen Sulfide as a "Double-Faced" Compound: One with Pro- and Antioxidant Effect

Hydrogen sulfide (H₂S), like other gasotransmitters such as nitric oxide (NO^•) and carbon monoxide (CO), acts as a signaling molecule in various biological systems. It may also regulate the oxidative stress observed in several diseases sometimes associated with changes of H₂S concentration. This chapter describes the "double face" of hydrogen sulfide as both an antioxidant and a prooxidant in biological systems. One proposed mechanism by which H₂S exerts its antioxidative effects is its ability to modulate the concentration of glutathione, which is a very important physiological antioxidant. This chapter discusses the interactions of H₂S with various reactive oxygen species and reactive nitrogen species, including the superoxide radical anion [Formula: see text] , hydrogen peroxide (H₂O₂), and peroxynitrite anion (ONOO^-), which is produced in a rapid reaction between [Formula: see text] and NO^•.

Highly Selective and Sensitive One- and Two-Photon Ratiometric Fluorescent Probe for Intracellular Hydrogen Polysulfide Sensing

Hydrogen polysulfide (H2Sn) has attracted increasing attention due to the fact that it is actually the key signaling molecule rather than hydrogen sulfide (H2S). Therefore, developing a sensitive and accurate assay to investigate the biosynthetic pathways of H2Sn is of physiological and pathological significance. In this work, based on the commonly used two-photon fluorophore, 1,8-naphthalimide, a new probe, NRT-HP, has been designed and synthesized that displayed both one- and two-photon ratiometric fluorescence changes toward H2Sn via H2Sn-mediated benzodithiolone formation. NRT-HP exhibits excellent pH stability, high selectivity and low detection limit (0.1 μM) in aqueous media. Furthermore, two-photon fluorescence microscopy experiments have demonstrated that NRT-HP could be used for the H2Sn detection in live cells as well as tissue slices.

Cardioprotection by H2S Donors: Nitric Oxide-Dependent and ‑Independent Mechanisms

Hydrogen sulfide (H2S) is a signaling molecule with protective effects in the cardiovascular system. To harness the therapeutic potential of H2S, a number of donors have been developed. The present study compares the cardioprotective actions of representative H2S donors from different classes and studies their mechanisms of action in myocardial injury in vitro and in vivo. Exposure of cardiomyocytes to H2O2 led to significant cytotoxicity, which was inhibited by sodium sulfide (Na2S), thiovaline (TV), GYY4137 [morpholin-4-ium 4 methoxyphenyl(morpholino) phosphinodithioate], and AP39 [(10-oxo-10-(4-(3-thioxo-3H-1,2-dithiol5yl)phenoxy)decyl) triphenylphospho-nium bromide]. Inhibition of nitric oxide (NO) synthesis prevented the cytoprotective effects of Na2S and TV, but not GYY4137 and AP39, against H2O2-induced cardiomyocyte injury. Mice subjected to left anterior descending coronary ligation were protected from ischemia-reperfusion injury by the H2S donors tested. Inhibition of nitric oxide synthase (NOS) in vivo blocked only the beneficial effect of Na2S. Moreover, Na2S, but not AP39, administration enhanced the phosphorylation of endothelial NOS and vasodilator-associated phosphoprotein. Both Na2S and AP39 reduced infarct size in mice lacking cyclophilin-D (CypD), a modulator of the mitochondrial permeability transition pore (PTP). Nevertheless, only AP39 displayed a direct effect on mitochondria by increasing the mitochondrial Ca(2+) retention capacity, which is evidence of decreased propensity to undergo permeability transition. We conclude that although all the H2S donors we tested limited infarct size, the pathways involved were not conserved. Na2S had no direct effects on PTP opening, and its action was nitric oxide dependent. In contrast, the cardioprotection exhibited by AP39 could result from a direct inhibitory effect on PTP acting at a site different than CypD.

Pathophysiological Mechanisms in Gaseous Therapies for Severe Malaria

Over 200 million people worldwide suffer from malaria every year, a disease that causes 584,000 deaths annually. In recent years, significant improvements have been achieved on the treatment of severe malaria, with intravenous artesunate proving superior to quinine. However, mortality remains high, at 8% in children and 15% in adults in clinical trials, and even worse in the case of cerebral malaria (18% and 30%, respectively). Moreover, some individuals who do not succumb to severe malaria present long-term cognitive deficits. These observations indicate that strategies focused only on parasite killing fail to prevent neurological complications and deaths associated with severe malaria, possibly because clinical complications are associated in part with a cerebrovascular dysfunction. Consequently, different adjunctive therapies aimed at modulating malaria pathophysiological processes are currently being tested. However, none of these therapies has shown unequivocal evidence in improving patient clinical status. Recently, key studies have shown that gaseous therapies based mainly on nitric oxide (NO), carbon monoxide (CO), and hyperbaric (pressurized) oxygen (HBO) alter vascular endothelium dysfunction and modulate the host immune response to infection. Considering gaseous administration as a promising adjunctive treatment against severe malaria cases, we review here the pathophysiological mechanisms and the immunological aspects of such therapies.

Hydrogen Sulfide Delays LPS-Induced Preterm Birth in Mice via Anti-Inflammatory Pathways

A major cause of preterm labor in pregnant women is intra-amniotic infection, which is mediated by an inflammatory process. Hydrogen sulfide (H2S), a gaseous transmitter, has been implicated to be involved in inflammatory responses. We sought to investigate whether H2S affects infectious preterm birth using the mouse model of lipopolysaccharides (LPS)-induced preterm birth. Administration of LPS at 0.4 mg/kg with two injections intraperitoneally (i.p.) on gestational day 14.5 induced preterm labor. LPS significantly increased leukocyte infiltration in uterus, stimulated the expression of pro-inflammatory cytokines interleukin 1β (IL-1β), IL-6, tumor necrosis factor α (TNF-α), CCL2 and CXCL15 in myometrium. Administration of NaHS (i.p.) delayed the onset of labor induced by LPS in a dose-dependent manner. NaHS prevented leukocyte infiltration into intrauterine tissues and inhibited the production of pro-inflammatory cytokines in myometrium and decreased the levels of these cytokines in maternal circulation. H2S also decreased LPS-activated extracellular signal-regulated kinase (ERK) 1/2/ nuclear factor (NF)-κB signaling pathways in myometrium. This study provides new in vivo evidence for the roles of H2S in attenuating inflammation, and a potential novel therapeutic strategy for infection-related preterm labor.

Quantitative proteomics suggests metabolic reprogramming during ETHE1 deficiency

Deficiency of mitochondrial sulfur dioxygenase (ETHE1) causes the severe metabolic disorder ethylmalonic encephalopathy, which is characterized by early-onset encephalopathy and defective cytochrome C oxidase because of hydrogen sulfide accumulation. Although the severe systemic consequences of the disorder are becoming clear, the molecular effects are not well defined. Therefore, for further elucidating the effects of ETHE1-deficiency, we performed a large scale quantitative proteomics study on liver tissue from ETHE1-deficient mice. Our results demonstrated a clear link between ETHE1-deficiency and redox active proteins, as reflected by downregulation of several proteins related to oxidation-reduction, such as different dehydrogenases and cytochrome P450 (CYP450) members. Furthermore, the protein data indicated impact of the ETHE1-deficiency on metabolic reprogramming through upregulation of glycolytic enzymes and by altering several heterogeneous ribonucleoproteins, indicating novel link between ETHE1 and gene expression regulation. We also found increase in total protein acetylation level, pointing out the link between ETHE1 and acetylation, which is likely controlled by both redox state and cellular metabolites. These findings are relevant for understanding the complexity of the disease and may shed light on important functions influenced by ETHE1 deficiency and by the concomitant increase in the gaseous mediator hydrogen sulfide. All MS data have been deposited in the ProteomeXchange with the dataset identifiers PXD002741 (http://proteomecentral.proteomexchange.org/dataset/PXD002741) and PXD002742 (http://proteomecentral.proteomexchange.org/dataset/PXD002741).

Hydrogen Sulfide--Mechanisms of Toxicity and Development of an Antidote

Hydrogen sulfide is a highly toxic gas—second only to carbon monoxide as a cause of inhalational deaths. Its mechanism of toxicity is only partially known, and no specific therapy exists for sulfide poisoning. We show in several cell types, including human inducible pluripotent stem cell (hiPSC)-derived neurons, that sulfide inhibited complex IV of the mitochondrial respiratory chain and induced apoptosis. Sulfide increased hydroxyl radical production in isolated mouse heart mitochondria and F₂-isoprostanes in brains and hearts of mice. The vitamin B₁₂ analog cobinamide reversed the cellular toxicity of sulfide, and rescued Drosophila melanogaster and mice from lethal exposures of hydrogen sulfide gas. Cobinamide worked through two distinct mechanisms: direct reversal of complex IV inhibition and neutralization of sulfide-generated reactive oxygen species. We conclude that sulfide produces a high degree of oxidative stress in cells and tissues, and that cobinamide has promise as a first specific treatment for sulfide poisoning.

The CBS/CSE system: a potential therapeutic target in NAFLD?

Non-alcoholic fatty liver disease (NAFLD) is a broad spectrum liver disorder diagnosed in patients without a history of alcohol abuse. NAFLD is growing at alarming rates worldwide. Its pathogenesis is complex and incompletely understood. The cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE) system regulates homocysteine and cysteine metabolism and contributes to endogenous hydrogen sulfide (H2S) biosynthesis. This review summarizes our current understanding of the hepatic CBS/CSE system, and for the first time, positions this system as a potential therapeutic target in NAFLD. As will be discussed, the CBS/CSE system is highly expressed and active in the liver. Its dysregulation, presenting as alterations in circulating homocysteine and (or) H2S levels, has been reported in NAFLD patients and in NAFLD-associated co-morbidities such as obesity and type 2 diabetes. Intricate links between the CBS/CSE system and a number of metabolic and stress related molecular mediators have also emerged. Various dysfunctions in the hepatic CBS/CSE system have been reported in animal models representative of each NAFLD spectrum. It is anticipated that a newfound appreciation for the hepatic CBS/CSE system will emerge that will improve our understanding of NAFLD pathogenesis, and give rise to new prospective targets for management of this disorder.

Biological signaling by small inorganic molecules

Small redox active molecules such as reactive nitrogen and oxygen species and hydrogen sulfide have emerged as important biological mediators that are involved in various physiological and pathophysiological processes. Advancement in understanding of cellular mechanisms that tightly regulate both generation and reactivity of these molecules is central to improved management of various disease states including cancer and cardiovascular dysfunction. Imbalance in the production of redox active molecules can lead to damage of critical cellular components such as cell membranes, proteins and DNA and thus may trigger the onset of disease. These small inorganic molecules react independently as well as in a concerted manner to mediate physiological responses. This review provides a general overview of the redox biology of these key molecules, their diverse chemistry relevant to physiological processes and their interrelated nature in cellular signaling.

A flavylium-based turn-on fluorescent probe for imaging hydrogen polysulfides in living cells

A flavylium-based turn-on fluorescent probe for imaging of hydrogen polysulfides in living cells has been developed.

Biosynthesis and Reactivity of Cysteine Persulfides in Signaling

Hydrogen sulfide (H2S) elicits pleiotropic physiological effects ranging from modulation of cardiovascular to CNS functions. A dominant method for transmission of sulfide-based signals is via posttranslational modification of reactive cysteine thiols to persulfides. However, the source of the persulfide donor and whether its relationship to H2S is as a product or precursor is controversial. The transsulfuration pathway enzymes can synthesize cysteine persulfide (Cys-SSH) from cystine and H2S from cysteine and/or homocysteine. Recently, Cys-SSH was proposed as the primary product of the transsulfuration pathway with H2S representing a decomposition product of Cys-SSH. Our detailed kinetic analyses demonstrate a robust capacity for Cys-SSH production by the human transsulfuration pathway enzymes, cystathionine beta-synthase and γ-cystathionase (CSE) and for homocysteine persulfide synthesis from homocystine by CSE only. However, in the reducing cytoplasmic milieu where the concentration of reduced thiols is significantly higher than of disulfides, substrate level regulation favors the synthesis of H2S over persulfides. Mathematical modeling at physiologically relevant hepatic substrate concentrations predicts that H2S rather than Cys-SSH is the primary product of the transsulfuration enzymes with CSE being the dominant producer. The half-life of the metastable Cys-SSH product is short and decomposition leads to a mixture of polysulfides (Cys-S-(S)n-S-Cys). These in vitro data, together with the intrinsic reactivity of Cys-SSH for cysteinyl versus sulfur transfer, are consistent with the absence of an observable increase in protein persulfidation in cells in response to exogenous cystine and evidence for the formation of polysulfides under these conditions.

S-sulfhydration as a cellular redox regulation

This review is focused on formation and biological significance of hydropersulfides, i.e. S-sulfhydration process. Biogenesis and properties of reactive sulfur species and their role in redox signaling are presented. The effect of S-sulfhydration on protein function is discussed.

For many years reactive oxygen and nitrogen species (ROS and RNS) have been recognized as key messengers in the process of thiol-based redox regulation. Relatively recently, literature reports began to mention reactive sulfur species (RSS) and their role in thiol regulation. This review is focused on biogenesis and biological properties of RSS, including: hydropersulfides, polysulfides and hydrogen sulfide (H₂S). Based on the most up-to-date literature data, the paper presents biological significance of S-sulfhydration process. In this reaction, sulfane sulfur is transferred to the–SH groups forming hydropersulfides. Protein cysteine residues, called ‘redox switches’ are susceptible to such reversible modifications. In line with the most recent reports, it was emphasized that sulfane sulfur-containing compounds (mainly hydrogen persulfides and polysulfides) are real and better mediators of S-sulfhydration-based signalling than H₂S. We also overviewed proteins participating in the formation and transport of RSS and in mitochondrial H₂S oxidation. In addition, we reviewed many reports about proteins unrelated to sulfur metabolism which are modified by S-sulfhydration that influences their catalytic activity. We also addressed the problem of the regulatory function of S-sulfhydration reaction in the activation of K_ATP channels (vasorelaxant) and transcription factors (e.g. NF_κB) as well as in the mechanism of therapeutic action of garlic-derived sulfur compounds. Some aspects of comparison between RNS and RSS are also discussed in this review.

H2S Synthesizing Enzymes: Biochemistry and Molecular Aspects

Hydrogen sulfide (H2S) is a biologically active gas that is synthesized naturally by three enzymes, cystathionine γ-lyase (CSE), cystathionine β-synthetase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). These enzymes are constitutively present in a wide array of biological cells and tissues and their expression can be induced by a number of disease states. It is becoming increasingly clear that H2S is an important mediator of a wide range of cell functions in health and in disease. This review therefore provides an overview of the biochemical and molecular regulation of H2S synthesizing enzymes both in physiological conditions and their modulation in disease states with particular focus on their regulation in asthma, atherosclerosis and diabetes. The importance of small molecule inhibitors in the study of molecular pathways, the current use of common H2S synthesizing enzyme inhibitors and the relevant characteristics of mice in which these enzymes have been genetically deleted will also be summarized. With a greater understanding of the molecular regulation of these enzymes in disease states, as well as the availability of novel small molecules with high specificity targeted towards H2S producing enzymes, the potential to regulate the biological functions of this intriguing gas H2S for therapeutic effect can perhaps be brought one step closer.

Overexpression of Cystathionine γ-Lyase Suppresses Detrimental Effects of Spinocerebellar Ataxia Type 3

Spinocerebellar ataxia type 3 (SCA3) is a polyglutamine (polyQ) disorder caused by a CAG repeat expansion in the ataxin-3 (ATXN3) gene resulting in toxic protein aggregation. Inflammation and oxidative stress are considered secondary factors contributing to the progression of this neurodegenerative disease. There is no cure that halts or reverses the progressive neurodegeneration of SCA3. Here we show that overexpression of cystathionine γ-lyase, a central enzyme in cysteine metabolism, is protective in a Drosophila model for SCA3. SCA3 flies show eye degeneration, increased oxidative stress, insoluble protein aggregates, reduced levels of protein persulfidation and increased activation of the innate immune response. Overexpression of Drosophila cystathionine γ-lyase restores protein persulfidation, decreases oxidative stress, dampens the immune response and improves SCA3-associated tissue degeneration. Levels of insoluble protein aggregates are not altered; therefore, the data implicate a modifying role of cystathionine γ-lyase in ameliorating the downstream consequence of protein aggregation leading to protection against SCA3-induced tissue degeneration. The cystathionine γ-lyase expression is decreased in affected brain tissue of SCA3 patients, suggesting that enhancers of cystathionine γ-lyase expression or activity are attractive candidates for future therapies.

Signaling and stress: The redox landscape in NOS2 biology

Nitric oxide (NO) has a highly diverse range of biological functions from physiological signaling and maintenance of homeostasis to serving as an effector molecule in the immune system. However, deleterious as well as beneficial roles of NO have been reported. Many of the dichotomous effects of NO and derivative reactive nitrogen species (RNS) can be explained by invoking precise interactions with different targets as a result of concentration and temporal constraints. Endogenous concentrations of NO span five orders of magnitude, with levels near the high picomolar range typically occurring in short bursts as compared to sustained production of low micromolar levels of NO during immune response. This article provides an overview of the redox landscape as it relates to increasing NO concentrations, which incrementally govern physiological signaling, nitrosative signaling and nitrosative stress-related signaling. Physiological signaling by NO primarily occurs upon interaction with the heme protein soluble guanylyl cyclase. As NO concentrations rise, interactions with nonheme iron complexes as well as indirect modification of thiols can stimulate additional signaling processes. At the highest levels of NO, production of a broader range of RNS, which subsequently interact with more diverse targets, can lead to chemical stress. However, even under such conditions, there is evidence that stress-related signaling mechanisms are triggered to protect cells or even resolve the stress. This review therefore also addresses the fundamental reactions and kinetics that initiate signaling through NO-dependent pathways, including processes that lead to interconversion of RNS and interactions with molecular targets.

The Development of Fluorescent Probes for Visualizing Intracellular Hydrogen Polysulfides

Endogenous hydrogen polysulfides (H2Sn; n>1) have been recognized as important regulators in sulfur-related redox biology. H2Sn can activate tumor suppressors, ion channels, and transcription factors with higher potency than H2S. Although H2Sn are drawing increasing attention, their exact mechanisms of action are still poorly understood. A major hurdle in this field is the lack of reliable and convenient methods for H2Sn detection. Herein we report a H2Sn-mediated benzodithiolone formation under mild conditions. This method takes advantage of the unique dual reactivity of H2Sn as both a nucleophile and an electrophile. Based on this reaction, three fluorescent probes (PSP-1, PSP-2, and PSP-3) were synthesized and evaluated. Among the probes prepared, PSP-3 showed a desirable off/on fluorescence response to H2Sn and high specificity. The probe was successfully applied in visualizing intracellular H2Sn.

Role of cystathionine β-synthase in human breast Cancer

Cystathionine β-synthase (CBS) is an enzyme in the transulfuration pathway that can catalyze the condensation of homocysteine (Hcy) and cysteine (Cys) to hydrogen sulfide (H2S) and cystathionine (CTH). CBS-derived H2S is important in angiogenesis and drug resistance in colon and ovarian cancers, respectively. However, the mechanisms by which cancer cell-derived H2S is utilized by cancer cells as a protective agent against host-derived activated macrophages are not yet investigated. This study investigated the mechanistic role of CBS-derived H2S in the protection of human breast cancer (HBC) cells against activated macrophages. HBC patient-derived tissue arrays and immunoblot analysis of HBC cells exhibited significantly increased levels of CBS when compared with their normal counterparts. This was associated with increased levels of H2S and CTH. Silencing of CBS in HBC cells caused a significant decrease in the levels of H2S and CTH but did not affect the growth of these cells per se, in in vitro cultures. However CBS-silenced cells exhibited significantly reduced growth in the presence of activated macrophages and in xenograft models. This was associated with an increase in the steady state levels of reactive aldehyde-derived protein adducts. Exogenous addition of H2S countered the effects of CBS silencing in the presence of macrophages. Conversely overexpression of CBS in human breast epithelial (HBE) cells (which do not naturally express CBS) protected them from activated macrophages, which were otherwise susceptible to the latter.

Transient Kinetic Analysis of Hydrogen Sulfide Oxidation Catalyzed by Human Sulfide Quinone Oxidoreductase

The first step in the mitochondrial sulfide oxidation pathway is catalyzed by sulfide quinone oxidoreductase (SQR), which belongs to the family of flavoprotein disulfide oxidoreductases. During the catalytic cycle, the flavin cofactor is intermittently reduced by sulfide and oxidized by ubiquinone, linking H2S oxidation to the electron transfer chain and to energy metabolism. Human SQR can use multiple thiophilic acceptors, including sulfide, sulfite, and glutathione, to form as products, hydrodisulfide, thiosulfate, and glutathione persulfide, respectively. In this study, we have used transient kinetics to examine the mechanism of the flavin reductive half-reaction and have determined the redox potential of the bound flavin to be -123 ± 7 mV. We observe formation of an unusually intense charge-transfer (CT) complex when the enzyme is exposed to sulfide and unexpectedly, when it is exposed to sulfite. In the canonical reaction, sulfide serves as the sulfur donor and sulfite serves as the acceptor, forming thiosulfate. We show that thiosulfate is also formed when sulfide is added to the sulfite-induced CT intermediate, representing a new mechanism for thiosulfate formation. The CT complex is formed at a kinetically competent rate by reaction with sulfide but not with sulfite. Our study indicates that sulfide addition to the active site disulfide is preferred under normal turnover conditions. However, under pathological conditions when sulfite concentrations are high, sulfite could compete with sulfide for addition to the active site disulfide, leading to attenuation of SQR activity and to an alternate route for thiosulfate formation.

Catalytic enhancement of the heme-based oxygen-sensing phosphodiesterase EcDOS by hydrogen sulfide is caused by changes in heme coordination structure

EcDOS is a heme-based O2-sensing phosphodiesterase in which O2 binding to the heme iron complex in the N-terminal domain substantially enhances catalysis toward cyclic-di-GMP, which occurs in the C-terminal domain. Here, we found that hydrogen sulfide enhances the catalytic activity of full-length EcDOS, possibly owing to the admixture of 6-coordinated heme Fe(III)-SH(-) and Fe(II)-O2 complexes generated during the reaction. Alanine substitution at Met95, the axial ligand for the heme Fe(II) complex, converted the heme Fe(III) complex into the heme Fe(III)-SH(-) complex, but the addition of Na2S did not further reduce it to the heme Fe(II) complex of the Met95Ala mutant, and no subsequent formation of the heme Fe(II)-O2 complex was observed. In contrast, a Met95His mutant formed a stable heme Fe(II)-O2 complex in response to the same treatment. An Arg97Glu mutant, containing a glutamate substitution at the amino acid that interacts with O2 in the heme Fe(II)-O2 complex, formed a stable heme Fe(II) complex in response to Na2S, but this complex failed to bind O2. Interestingly, the addition of Na2S promoted formation of verdoheme (oxygen-incorporated, modified protoporphyrin IX) in an Arg97Ile mutant. Catalytic enhancement by Na2S was similar for Met95 mutants and the wild type, but significantly lower for the Arg97 mutants. Thus, this study shows the first isolation of spectrometrically separated, stable heme Fe(III)-SH(-), heme Fe(II) and heme Fe(II)-O2 complexes of full-length EcDOS with Na2S, and confirms that external-ligand-bound, 6-coordinated heme Fe(III)-SH(-) or heme Fe(II)-O2 complexes critically contribute to the Na2S-induced catalytic enhancement of EcDOS.

Chemical probes for molecular imaging and detection of hydrogen sulfide and reactive sulfur species in biological systems

Hydrogen sulfide (H2S), a gaseous species produced by both bacteria and higher eukaryotic organisms, including mammalian vertebrates, has attracted attention in recent years for its contributions to human health and disease. H2S has been proposed as a cytoprotectant and gasotransmitter in many tissue types, including mediating vascular tone in blood vessels as well as neuromodulation in the brain. The molecular mechanisms dictating how H2S affects cellular signaling and other physiological events remain insufficiently understood. Furthermore, the involvement of H2S in metal-binding interactions and formation of related RSS such as sulfane sulfur may contribute to other distinct signaling pathways. Owing to its widespread biological roles and unique chemical properties, H2S is an appealing target for chemical biology approaches to elucidate its production, trafficking, and downstream function. In this context, reaction-based fluorescent probes offer a versatile set of screening tools to visualize H2S pools in living systems. Three main strategies used in molecular probe development for H2S detection include azide and nitro group reduction, nucleophilic attack, and CuS precipitation. Each of these approaches exploits the strong nucleophilicity and reducing potency of H2S to achieve selectivity over other biothiols. In addition, a variety of methods have been developed for the detection of other reactive sulfur species (RSS), including sulfite and bisulfite, as well as sulfane sulfur species and related modifications such as S-nitrosothiols. Access to this growing chemical toolbox of new molecular probes for H2S and related RSS sets the stage for applying these developing technologies to probe reactive sulfur biology in living systems.

Staphylococcus aureus CstB Is a Novel Multidomain Persulfide Dioxygenase-Sulfurtransferase Involved in Hydrogen Sulfide Detoxification

Hydrogen sulfide (H2S) is both a lethal gas and an emerging gasotransmitter in humans, suggesting that the cellular H2S level must be tightly regulated. CstB is encoded by the cst operon of the major human pathogen Staphylococcus aureus and is under the transcriptional control of the persulfide sensor CstR and H2S. Here, we show that CstB is a multifunctional Fe(II)-containing persulfide dioxygenase (PDO), analogous to the vertebrate protein ETHE1 (ethylmalonic encephalopathy protein 1). Chromosomal deletion of ethe1 is fatal in vertebrates. In the presence of molecular oxygen (O2), hETHE1 oxidizes glutathione persulfide (GSSH) to generate sulfite and reduced glutathione. In contrast, CstB oxidizes major cellular low molecular weight (LMW) persulfide substrates from S. aureus, coenzyme A persulfide (CoASSH) and bacillithiol persulfide (BSSH), directly to generate thiosulfate (TS) and reduced thiols, thereby avoiding the cellular toxicity of sulfite. Both Cys201 in the N-terminal PDO domain (CstB(PDO)) and Cys408 in the C-terminal rhodanese domain (CstB(Rhod)) strongly enhance the TS generating activity of CstB. CstB also possesses persulfide transferase (PT; reverse rhodanese) activity, which generates TS when provided with LMW persulfides and sulfite, as well as conventional thiosulfate transferase (TST; rhodanese) activity; both of these activities require Cys408. CstB protects S. aureus against H2S toxicity, with the C201S and C408S cstB genes being unable to rescue a NaHS-induced ΔcstB growth phenotype. Induction of the cst operon by NaHS reveals that functional CstB impacts cellular TS concentrations. These data collectively suggest that CstB may have evolved to facilitate the clearance of LMW persulfides that occur upon elevation of the level of cellular H2S and hence may have an impact on bacterial viability under H2S misregulation, in concert with the other enzymes encoded by the cst operon.

Biogenesis of reactive sulfur species for signaling by hydrogen sulfide oxidation pathways

The chemical species involved in H2S signaling remain elusive despite the profound and pleiotropic physiological effects elicited by this molecule. The dominant candidate mechanism for sulfide signaling is persulfidation of target proteins. However, the relatively poor reactivity of H2S toward oxidized thiols, such as disulfides, the low concentration of disulfides in the reducing milieu of the cell and the low steady-state concentration of H2S raise questions about the plausibility of persulfide formation via reaction between an oxidized thiol and a sulfide anion or a reduced thiol and oxidized hydrogen disulfide. In contrast, sulfide oxidation pathways, considered to be primarily mechanisms for disposing of excess sulfide, generate a series of reactive sulfur species, including persulfides, polysulfides and thiosulfate, that could modify target proteins. We posit that sulfide oxidation pathways mediate sulfide signaling and that sulfurtransferases ensure target specificity.

Meningeal blood flow is controlled by H2 S-NO crosstalk activating a HNO-TRPA1-CGRP signalling pathway

BACKGROUND AND PURPOSE

Meningeal blood flow is controlled by CGRP released from trigeminal afferents and NO mainly produced in arterial endothelium. The vasodilator effect of NO may be due to the NO-derived compound, nitroxyl (HNO), generated through reaction with endogenous H2 S. We investigated the involvement of HNO in CGRP release and meningeal blood flow.

EXPERIMENTAL APPROACH

Blood flow in exposed dura mater of rats was recorded by laser Doppler flowmetry. CGRP release from the dura mater in the hemisected rat head was quantified using an elisa. NO and H2 S were localized histochemically with specific sensors.

KEY RESULTS

Topical administration of the NO donor diethylamine-NONOate increased meningeal blood flow by 30%. Pretreatment with oxamic acid, an inhibitor of H2 S synthesis, reduced this effect. Administration of Na2 S increased blood flow by 20%, an effect abolished by the CGRP receptor antagonist CGRP8-37 or the TRPA1 channel antagonist HC030031 and reduced when endogenous NO synthesis was blocked. Na2 S dose-dependently increased CGRP release two- to threefold. Co-administration of diethylamine-NONOate facilitated CGRP release, while inhibition of endogenous NO or H2 S synthesis lowered basal CGRP release. NO and H2 S were mainly localized in arterial vessels, HNO additionally in nerve fibre bundles. HNO staining was lost after treatment with L-NMMA and oxamic acid.

CONCLUSIONS AND IMPLICATIONS

NO and H2 S cooperatively increased meningeal blood flow by forming HNO, which activated TRPA1 cation channels in trigeminal fibres, inducing CGRP release. This HNO-TRPA1-CGRP signalling pathway may be relevant to the pathophysiology of headaches.

A specific nucleophilic ring-opening reaction of aziridines as a unique platform for the construction of hydrogen polysulfides sensors

A hydrogen polysulfide mediated aziridine ring-opening reaction was discovered. Based on this reaction, a novel H2S(n)-specific chemosensor (AP) was developed. AP showed high sensitivity and selectivity for H2S(n). Notably, the fluorescent turn-on product (1) exhibited excellent two-photon photophysical properties, a large Stokes shift, and high solid state luminescent efficiency.

Cysteine sulfur chemistry in transcriptional regulators at the host-bacterial pathogen interface

Hosts employ myriad weapons to combat invading microorganisms as an integral feature of the host-bacterial pathogen interface. This interface is dominated by highly reactive small molecules that collectively induce oxidative stress. Successful pathogens employ transcriptional regulatory proteins that sense these small molecules directly or indirectly via a change in the ratio of reduced to oxidized low-molecular weight (LMW) thiols that collectively comprise the redox buffer in the cytoplasm. These transcriptional regulators employ either a prosthetic group or reactive cysteine residue(s) to effect changes in the transcription of genes that encode detoxification and repair systems that is driven by regulator conformational switching between high-affinity and low-affinity DNA-binding states. Cysteine harbors a highly polarizable sulfur atom that readily undergoes changes in oxidation state in response to oxidative stress to produce a range of regulatory post-translational modifications (PTMs), including sulfenylation (S-hydroxylation), mixed disulfide bond formation with LMW thiols (S-thiolation), di- and trisulfide bond formation, S-nitrosation, and S-alkylation. Here we discuss several examples of structurally characterized cysteine thiol-specific transcriptional regulators that sense changes in cellular redox balance, focusing on the nature of the cysteine PTM itself and the interplay of small molecule oxidative stressors in mediating a specific transcriptional response.

Hydrogen Sulfide: A Therapeutic Candidate for Fibrotic Disease?

Fibrotic diseases including chronic kidney disease, liver cirrhosis, idiopathic pulmonary fibrosis, and chronic disease account for 45% mortality in the developed countries and pose a great threat to the global health. Many great targets and molecules have been reported to be involved in the initiation and/or progression of fibrosis, among which inflammation and oxidative stress are well-recognized modulation targets. Hydrogen sulfide (H₂S) is the third gasotransmitter with potent properties in inhibiting inflammation and oxidative stress in various organs. Recent evidence suggests that plasma H₂S level is decreased in various animal models of fibrotic diseases and supplement of exogenous H₂S is able to ameliorate fibrosis in the kidney, lung, liver, and heart. This leads us to propose that modulation of H₂S production may represent a promising therapeutic venue for the treatment of a variety of fibrotic diseases. Here, we summarize and discuss the current data on the role and underlying mechanisms of H₂S in fibrosis diseases related to heart, liver, kidney, and other organs.

Cytosolic thiol switches regulating basic cellular functions: GAPDH as an information hub?

Cytosolic glyceraldehyde 3-phosphate dehydrogenase (GAPDH, E.C. 1.2.1.12) is present in all organisms and catalyzes the oxidation of triose phosphate during glycolysis. GAPDH is one of the most prominent cellular targets of oxidative modifications when reactive oxygen and nitrogen species are formed during metabolism and under stress conditions. GAPDH harbors a strictly conserved catalytic cysteine, which is susceptible to a variety of thiol modifications, including S-sulfenylation, S-glutathionylation, S-nitrosylation, and S-sulfhydration. Upon reversible oxidative thiol modification of GAPDH, glycolysis is inhibited leading to a diversion of metabolic flux through the pentose-phosphate cycle to increase NADPH production. Furthermore, oxidized GAPDH may adopt new functions in different cellular compartments including the nucleus, as well as in new microcompartments associated with the cytoskeleton, mitochondria and plasma membrane. This review focuses on the recently discovered mechanism underlying the eminent reactivity between GAPDH and hydrogen peroxide and the subsequent redox-dependent moonlighting functions discriminating between the induction either of adaptive responses and adjustment of metabolism or of cell death in yeast, plants, and mammals. In light of the summarized results, cytosolic GAPDH might function as a sensor for redox signals and an information hub to transduce these signals for appropriate responses.

Hydrogen sulfide regulates inward-rectifying K+ channels in conjunction with stomatal closure

Hydrogen sulfide (H2S) is the third biological gasotransmitter, and in animals, it affects many physiological processes by modulating ion channels. H2S has been reported to protect plants from oxidative stress in diverse physiological responses. H2S closes stomata, but the underlying mechanism remains elusive. Here, we report the selective inactivation of current carried by inward-rectifying K(+) channels of tobacco (Nicotiana tabacum) guard cells and show its close parallel with stomatal closure evoked by submicromolar concentrations of H2S. Experiments to scavenge H2S suggested an effect that is separable from that of abscisic acid, which is associated with water stress. Thus, H2S seems to define a unique and unresolved signaling pathway that selectively targets inward-rectifying K(+) channels.

Sulfide oxidation by a noncanonical pathway in red blood cells generates thiosulfate and polysulfides

A cardioprotectant at low concentrations, H2S is a toxin at high concentrations and inhibits cytochrome c oxidase. A conundrum in H2S homeostasis is its fate in red blood cells (RBCs), which produce H2S but lack the canonical mitochondrial sulfide oxidation pathway for its clearance. The sheer abundance of RBCs in circulation enhances the metabolic significance of their clearance strategy for H2S, necessary to avoid systemic toxicity. In this study, we demonstrate that H2S generation by RBCs is catalyzed by mercaptopyruvate sulfurtransferase. Furthermore, we have discovered the locus of sulfide oxidation in RBCs and describe a new role for an old protein, hemoglobin, which in the ferric or methemoglobin state binds H2S and oxidizes it to a mixture of thiosulfate and hydropolysulfides. Our study reveals a previously undescribed route for the biogenesis of hydropolysulfides, which are increasingly considered important for H2S-based signaling, but their origin in mammalian cells is unknown. An NADPH/flavoprotein oxidoreductase system restores polysulfide-carrying hemoglobin derivatives to ferrous hemoglobin, thus completing the methemoglobin-dependent sulfide oxidation cycle. Methemoglobin-dependent sulfide oxidation in mammals is complex and has similarities to chemistry reported for the dissolution of iron oxides in sulfidic waters and during bioleaching of metal sulfides. The catalytic oxidation of H2S by hemoglobin explains how RBCs maintain low steady-state H2S levels in circulation, and suggests that additional hemeproteins might be involved in sulfide homeostasis in other tissues.

Trapping hydrogen sulfide (H₂S) with diselenides: the application in the design of fluorescent probes

Here we report a unique reaction between phenyl diselenide-ester substrates and H₂S to form 1,2-benzothiaselenol-3-one. This reaction proceeded rapidly under mild conditions. Thiols could also react with the diselenide substrates. However, the resulted S–Se intermediate retained high reactivity toward H₂S and eventually led to the same cyclized product 1,2-benzothiaselenol-3-one. Based on this reaction two fluorescent probes were developed and showed high selectivity and sensitivity for H₂S. The presence of thiols was found not to interfere with the detection process.

Two-photon fluorescent probe for detection of exogenous and endogenous hydrogen persulfide and polysulfide in living organisms

Hydrogen persulfide and polysulfide (H2S(n)) are newly discovered intracellular reactive species considered to have high protein S-sulfhydration efficiency. The detection of H2S(n) in living systems is essential for studying their functions but is quite challenging. In this work, we report a two-photon excited fluorescent probe, QS(n), capable of tracking H2S(n) in living organisms. QS(n) exhibited turn-on two-photon fluorescence response upon reaction with H2S(n). With a favorable photophysical property, high specificity, and low cytotoxicity, QS(n) was able to recognize exogenous H2S(n) in living cells. More importantly, it realized for the first time the visualization of endogenous H2S(n) generated in cells overexpressing cystathionine β-synthase and cystathionine γ-lyase, the enzymes responsible for producing endogenous H2S(n). Taking advantage of two-photon microscopy, the probe was also applied to achieve H2S(n) detection in zebrafish embryos and to observe H2S(n) distribution in living organisms.

S-glutathionylation enhances human cystathionine β-synthase activity under oxidative stress conditions

AIMS

Cystathionine β-synthase (CBS) catalyzes the first and rate-limiting step in the two-step trans-sulfuration pathway that converts homocysteine to cysteine. It is also one of three major enzymes responsible for the biogenesis of H2S, a signaling molecule. We have previously demonstrated that CBS is activated in cells challenged by oxidative stress, but the underlying molecular mechanism of this regulation has remained unclear.

RESULTS

Here, we demonstrate that S-glutathionylation of CBS enhances its activity ∼2-fold in vitro. Loss of this post-translational modification in the presence of dithiothreitol results in reversal to basal activity. Cys346 was identified as the site for S-glutathionylation by a combination of mass spectrometric, mutagenesis, and activity analyses. To test the physiological relevance of S-glutathionylation-dependent regulation of CBS, HEK293 cells were oxidatively challenged with peroxide, which is known to enhance the trans-sulfuration flux. Under these conditions, CBS glutathionylation levels increased and were correlated with a ∼3-fold increase in CBS activity.

INNOVATION

Collectively, our results reveal a novel post-translational modification of CBS, that is, glutathionylation, which functions as an allosteric activator under oxidative stress conditions permitting enhanced synthesis of both cysteine and H2S.

CONCLUSIONS

Our study elucidates a molecular mechanism for increased cysteine and therefore glutathione, synthesis via glutathionylation of CBS. They also demonstrate the potential for increased H2S production under oxidative stress conditions, particularly in tissues where CBS is a major source of H2S.

Assay methods for H2S biogenesis and catabolism enzymes

H2S is produced from sulfur-containing amino acids, cysteine and homocysteine, or a catabolite, 3-mercaptopyruvate, by three known enzymes: cystathionine β-synthase, γ-cystathionase, and 3-mercaptopyruvate sulfurtransferase. Of these, the first two enzymes reside in the cytoplasm and comprise the transsulfuration pathway, while the third enzyme is found both in the cytoplasm and in the mitochondrion. The following mitochondrial enzymes oxidize H2S: sulfide quinone oxidoreductase, sulfur dioxygenase, rhodanese, and sulfite oxidase. The products of the sulfide oxidation pathway are thiosulfate and sulfate. Assays for enzymes involved in the production and oxidative clearance of sulfide to thiosulfate are described in this chapter.

Inhalation exposure model of hydrogen sulfide (H₂S)-induced hypometabolism in the male Sprague-Dawley rat

Hydrogen sulfide (H2S) has been accepted as a physiologically relevant cell-signaling molecule with both toxic and beneficial effects depending on its concentration in mammalian tissues. Notably, exposure to H2S in breathable air has been shown to decrease aerobic metabolism and induce a reversible hypometabolic-like state in laboratory rodent models. Herein, we describe an experimental exposure setup that can be used to define the reversible cardiovascular and metabolic physiology of rodents (rats) during H2S-induced hypometabolism and following recovery.

Use of the "tag-switch" method for the detection of protein S-sulfhydration

Protein S-sulfhydration (i.e., converting protein cysteines -SH to persulfides -SSH) is a redox-based posttranslational modification. This reaction plays an important role in signaling pathways mediated by hydrogen sulfide or other reactive sulfane sulfur species. Recently, our laboratories developed a "tag-switch" method which can be used to selectively label and detect protein S-sulfhydrated residues. In this chapter, we provide a comprehensive summary of this method, including the design of the method, preparation of the reagents, validation on small-molecule substrates, as well as applications in protein labeling. Experimental protocols for the use of the method are described in details.

Hydrogen sulfide mediated cascade reaction forming an iminocoumarin: applications in fluorescent probe development and live-cell imaging

The study on a fluorescent probe that undergoes a H₂S mediated cascade reaction to form an iminocoumarin fluorophore is reported.

Cysteine-mediated redox signalling in the mitochondria

This review represents a novel look at the many sources, cysteine targets, and signaling processes of ROS in the mitochondria.