Hydrogen polysulfide (H2S n ) signaling along with hydrogen sulfide (H2S) and nitric oxide (NO)

Analysis and characterization of sulfane sulfur

In the late 1970s, sulfane sulfur was defined as sulfur atoms covalently bound only to sulfur atoms. However, this definition was not generally accepted, as it was slightly vague and difficult to comprehend. Thus, in the early 1990s, it was defined as "bound sulfur," which easily converts to hydrogen sulfide upon reduction with a thiol-reducing agent. H2S-related bound sulfur species include persulfides (R-SSH), polysulfides (H2Sn, n ≥ 2 or R-S(S)nS-R, n ≥ 1), and protein-bound elemental sulfur (S0). Many of the biological effects currently associated with H2S may be attributed to persulfides and polysulfides. In the 20th century, quantitative determination of "sulfane sulfur" was conventionally performed using a reaction called cyanolysis. Several methods have been developed over the past 30 years. Current methods used for the detection of H2S and polysulfides include colorimetric assays for methylene blue formation, sulfide ion-selective or polarographic electrodes, gas chromatography with flame photometric or sulfur chemiluminescence detection, high-performance liquid chromatography analysis with fluorescent derivatization of sulfides, liquid chromatography with tandem mass spectrometry, the biotin switch technique, and the use of sulfide or polysulfide-sensitive fluorescent probes. In this review, we discuss the methods reported to date for measuring sulfane sulfur and the results obtained using these methods.

Hydrogen Sulfide (H2S)/Polysulfides (H2Sn) Signalling and TRPA1 Channels Modification on Sulfur Metabolism

Hydrogen sulfide (H2S) and polysulfides (H2Sn, n ≥ 2) produced by enzymes play a role as signalling molecules regulating neurotransmission, vascular tone, cytoprotection, inflammation, oxygen sensing, and energy formation. H2Sn, which have additional sulfur atoms to H2S, and other S-sulfurated molecules such as cysteine persulfide and S-sulfurated cysteine residues of proteins, are produced by enzymes including 3-mercaptopyruvate sulfurtransferase (3MST). H2Sn are also generated by the chemical interaction of H2S with NO, or to a lesser extent with H2O2. S-sulfuration (S-sulfhydration) has been proposed as a mode of action of H2S and H2Sn to regulate the activity of target molecules. Recently, we found that H2S/H2S2 regulate the release of neurotransmitters, such as GABA, glutamate, and D-serine, a co-agonist of N-methyl-D-aspartate (NMDA) receptors. H2S facilitates the induction of hippocampal long-term potentiation, a synaptic model of memory formation, by enhancing the activity of NMDA receptors, while H2S2 achieves this by activating transient receptor potential ankyrin 1 (TRPA1) channels in astrocytes, potentially leading to the activation of nearby neurons. The recent findings show the other aspects of TRPA1 channels-that is, the regulation of the levels of sulfur-containing molecules and their metabolizing enzymes. Disturbance of the signalling by H2S/H2Sn has been demonstrated to be involved in various diseases, including cognitive and psychiatric diseases. The physiological and pathophysiological roles of these molecules will be discussed.

An ICT-FRET-based ratiometric fluorescent probe for hydrogen polysulfide based on a coumarin-naphthalimide derivative

Hydrogen polysulfide (H2Sn, n > 1), as one of the important members of reactive sulfur species (RSS), plays a vital part in the processes of both their physiology and pathology. In this work, a ratiometric fluorescent probe for H2Sn had been designed and prepared based on the combination mechanism of intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET). The probe chose a coumarin derivative as the energy donor, a naphthalimide derivative as the energy acceptor and 2-fluoro-5-nitrobenzoate as the H2Sn recognition group. When H2Sn was not present in the system, the ICT process of the naphthalimide acceptor was inhibited and the FRET process from the coumarin donor to the naphthalimide acceptor was turned off. When H2Sn was added, both ICT and FRET occurred due to the nucleophilic substitution-cyclization reactions between the probe and hydrogen polysulfide. In addition, the ratio value of the emission intensities at 550 nm and 473 nm (I550 nm/I473 nm) of this probe had a good linear relationship with H2Sn concentration in the range of 6.0 × 10-7-5.0 × 10-5 mol·L-1, and a detection limit of 1.8 × 10-7 mol·L-1 was obtained. The developed probe had high selectivity and sensitivity, as well as good biocompatibility. Additionally, the probe had been used to successfully image both indigenous and exogenous hydrogen polysulfide in A549 cells using confocal microscope.

Binding mechanism of disulfide species to ferric hemeproteins: The case of metmyoglobin

The interactions of the heme iron of hemeproteins with sulfide and disulfide compounds are of potential interest as physiological signaling processes. While the interaction with hydrogen sulfide has been described computationally and experimentally, the reaction with disulfide, and specifically the molecular mechanism for ligand binding has not been studied in detail. In this work, we study the association process for disulfane and its conjugate base disulfanide at different pH conditions. Additionally, by means of advanced sampling techniques based on multiple steered molecular dynamics, we provide free energy profiles for ligand migration for both acid/base species, showing a similar behavior to the previously reported for the related H2S/HS¯ pair. Finally, we studied the ligand interchange reaction (H2O/H2S, HS¯ and H2O/HSSH, HSS¯) by means of hybrid quantum mechanics-molecular mechanics calculations. We show that the anionic species are able to displace more efficiently the H2O bound to the iron, and that the H-bond network in the distal cavity can help the neutral species to perform the reaction. Altogether, we provide a molecular explanation for the experimental information and show that the global association process depends on a fine balance between the migration towards the active site and the ligand interchange reaction.

Smart Nanoplatform for Visualizing Hydrogen Sulfide and Amplifying Oxidative Stress to Tumor Apoptosis

Oxidative stress is involved in various signaling pathways and serves a key role in inducing cell apoptosis. Therefore, it is significant to monitor oxidative stress upon drug release for the assessment of therapeutic effects in cancer cells. Herein, a glutathione (GSH)-responsive surface-enhanced Raman scattering (SERS) nanoplatform is proposed for ultra-sensitively monitoring the substance related with oxidative stress (hydrogen sulfide, H2S), depleting reactive sulfur species and releasing anticancer drugs to amplify oxidative stress for tumor apoptosis. The Au@Raman reporter@Ag (Au@M@Ag) nanoparticles, where a 4-mercaptobenzonitrile molecule as a Raman reporter was embedded between layers of gold and silver to obtain sensitive SERS response, were coated with a covalent organic framework (COF) shell to form a core-shell structure (Au@M@Ag@COFs) as the SERS nanoplatform. The COF shell loading doxorubicin (DOX) of Au@M@Ag@COFs exhibited the GSH-responsive degradation capacity to release DOX, and its Ag layer as the sensing agent was oxidized to Ag2S by H2S to result in its prominent changes in SERS signals with a low detection limit of 0.33 nM. Moreover, the releasing DOX can inhibit the generation of H2S to promote the production of reactive oxygen species, and the depletion of reactive sulfur species (GSH and H2S) in cancer cells can further enhance the oxidative stress to induce tumor apoptosis. Overall, the SERS strategy could provide a powerful tool to monitor the dynamic changes of oxidative stress during therapeutic processes in a tumor microenvironment.

Reduction of metmyoglobin by inorganic disulfide species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by inorganic disulfide species has been studied by combined spectroscopic and kinetic analyses, under argon atmosphere. The process is kinetically characterized by biexponential time traces, for variable ratios of excess disulfide to protein, in the pH interval 6.6-8.0. Using UV-vis and resonance Raman spectroscopies, we observed that MbFeIII is converted into a low spin hexacoordinated ferric complex, tentatively assigned as MbFeIII(HSS-)/MbFeIII(SS2-), in an initial fast step. The complex is slowly converted into a pentacoordinated ferrous form, assigned as MbFeII according to the resonance Raman records. The reduction is a pH-dependent process, but independent of the initial disulfide concentration, suggesting the unimolecular decomposition of the intermediate complex following a reductive homolysis. We estimated the rate of the fast formation of the complex at pH 7.4 (kon = 3.7 × 103 M-1 s-1), and a pKa2 = 7.5 for the equilibrium MbFeIII(HSS-)/MbFeIII(SS2-). Also, we estimated the rate for the slow reduction at the same pH (kred = 10-2 s-1). A reaction mechanism compliant with the experimental results is proposed. This mechanistic study provides a differential kinetic signature for the reactions of disulfide compared to sulfide species on metmyoglobin, which may be considered in other hemeprotein systems.

Microbial metabolites in the pathogenesis of periodontal diseases: a narrative review

The purpose of this narrative review is to highlight the importance of microbial metabolites in the pathogenesis of periodontal diseases. These diseases, involving gingivitis and periodontitis are inflammatory conditions initiated and maintained by the polymicrobial dental plaque/biofilm. Gingivitis is a reversible inflammatory condition while periodontitis involves also irreversible destruction of the periodontal tissues including the alveolar bone. The inflammatory response of the host is a natural reaction to the formation of plaque and the continuous release of metabolic waste products. The microorganisms grow in a nutritious and shielded niche in the periodontal pocket, protected from natural cleaning forces such as saliva. It is a paradox that the consequences of the enhanced inflammatory reaction also enable more slow-growing, fastidious, anaerobic bacteria, with often complex metabolic pathways, to colonize and thrive. Based on complex food chains, nutrient networks and bacterial interactions, a diverse microbial community is formed and established in the gingival pocket. This microbiota is dominated by anaerobic, often motile, Gram-negatives with proteolytic metabolism. Although this alternation in bacterial composition often is considered pathologic, it is a natural development that is promoted by ecological factors and not necessarily a true "dysbiosis". Normal commensals are adapting to the gingival crevice when tooth cleaning procedures are absent. The proteolytic metabolism is highly complex and involves a number of metabolic pathways with production of a cascade of metabolites in an unspecific manner. The metabolites involve short chain fatty acids (SCFAs; formic, acetic, propionic, butyric, and valeric acid), amines (indole, scatole, cadaverine, putrescine, spermine, spermidine) and gases (NH₃, CO, NO, H₂S, H₂). A homeostatic condition is often present between the colonizers and the host response, where continuous metabolic fluctuations are balanced by the inflammatory response. While it is well established that the effect of the dental biofilm on the host response and tissue repair is mediated by microbial metabolites, the mechanisms behind the tissue destruction (loss of clinical attachment and bone) are still poorly understood. Studies addressing the functions of the microbiota, the metabolites, and how they interplay with host tissues and cells, are therefore warranted.

Reactive sulfur species disaggregate the SQSTM1/p62-based aggresome-like induced structures via the HSP70 induction and prevent parthanatos

Reactive sulfur species (RSS) have emerged as key regulators of protein quality control. However, the mechanisms by which RSS contribute to cellular processes are not fully understood. In this study, we identified a novel function of RSS in preventing parthanatos, a non-apoptotic form of cell death that is induced by poly (ADP-ribose) polymerase-1 (PARP-1) and mediated by the aggresome-like induced structures (ALIS) composed of SQSTM1/p62. We found that sodium tetrasulfide (Na₂S₄), a donor of RSS, strongly suppressed oxidative stress-dependent ALIS formation and subsequent parthanatos. On the other hand, the inhibitors of the RSS-producing enzymes, such as 3-mercaptopyruvate sulfurtransferase (3-MST) and cystathionine γ-lyase (CSE), clearly enhanced ALIS formation and parthanatos. Interestingly, we found that Na₂S₄ activated heat shock factor 1 (HSF1) by promoting its dissociation from heat shock protein 90 (HSP90), leading to accelerated transcription of HSP70. Considering that the genetic deletion of HSP70 allowed the enhanced ALIS formation, these findings suggest that RSS prevent parthanatos by specifically suppressing ALIS formation through induction of HSP70. Taken together, our results demonstrate a novel mechanism by which RSS prevent cell death, as well as a novel physiological role of RSS in contributing to protein quality control through HSP70 induction, which may lead to better understanding of the bioactivity of RSS.

A Whiff of Sulfur: One Wind a Day Keeps the Doctor Away

Reactive Sulfur Species (RSS), such as allicin from garlic or sulforaphane from broccoli, are fre-quently associated with biological activities and possible health benefits in animals and humans. Among these Organic Sulfur Compounds (OSCs) found in many plants and fungi, the Volatile Sulfur Compounds (VSCs) feature prominently, not only because of their often-pungent smell, but also because they are able to access places which solids and solutions cannot reach that easily. Indeed, inorganic RSS such as hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) can be used to lit-erally fumigate entire rooms and areas. Similarly, metabolites of garlic, such as allyl methyl sulfide (AMS), are formed metabolically in humans in lower concentrations and reach the airways from inside the body as part of one’s breath. Curiously, H₂S is also formed in the gastrointestinal tract by gut bacteria, and the question of if and for which purpose this gas then crosses the barriers and enters the body is indeed a delicate matter for equally delicate studies. In any case, nature is surprisingly rich in such VSCs, as fruits (for instance, the infamous durian) demonstrate, and therefore these VSCs represent a promising group of compounds for further studies.

H2S in Horticultural Plants: Endogenous Detection by an Electrochemical Sensor, Emission by a Gas Detector, and Its Correlation with L-Cysteine Desulfhydrase (LCD) Activity

H2S has acquired great attention in plant research because it has signaling functions under physiological and stress conditions. However, the direct detection of endogenous H2S and its potential emission is still a challenge in higher plants. In order to achieve a comparative analysis of the content of H2S among different plants with agronomical and nutritional interest including pepper fruits, broccoli, ginger, and different members of the genus Allium such as garlic, leek, Welsh and purple onion, the endogenous H2S and its emission was determined using an ion-selective microelectrode and a specific gas detector, respectively. The data show that endogenous H2S content range from pmol to μmol H2S · g-1 fresh weight whereas the H2S emission of fresh-cut vegetables was only detected in the different species of the genus Allium with a maximum of 9 ppm in garlic cloves. Additionally, the activity and isozymes of the L-cysteine desulfhydrase (LCD) were analyzed, which is one of the main enzymatic sources of H2S, where the different species of the genus Allium showed the highest activities. Using non-denaturing gel electrophoresis, the data indicated the presence of up to nine different LCD isozymes from one in ginger to four in onion, leek, and broccoli. In summary, the data indicate a correlation between higher LCD activity with the endogenous H2S content and its emission in the analyzed horticultural species. Furthermore, the high content of endogenous H2S in the Allium species supports the recognized benefits for human health, which are associated with its consumption.

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

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

Protein S-sulfhydration: Unraveling the prospective of hydrogen sulfide in the brain, vasculature and neurological manifestations

Hydrogen sulfide (H₂S) and hydrogen polysulfides (H₂S_n) are essential regulatory signaling molecules generated by the entire body, including the central nervous system. Researchers have focused on the classical H₂S signaling from the past several decades, whereas the last decade has shown the emergence of H₂S-induced protein S-sulfhydration signaling as a potential therapeutic approach. Cysteine S-persulfidation is a critical paradigm of post-translational modification in the process of H₂S signaling. Additionally, studies have shown the cross-relationship between S-sulfhydration and other cysteine-induced post-translational modifications, namely nitrosylation and carbonylation. In the central nervous system, S-sulfhydration is involved in the cytoprotection through various signaling pathways, viz. inflammatory response, oxidative stress, endoplasmic reticulum stress, atherosclerosis, thrombosis, and angiogenesis. Further, studies have demonstrated H₂S-induced S-sulfhydration in regulating different biological processes, such as mitochondrial integrity, calcium homeostasis, blood-brain permeability, cerebral blood flow, and long-term potentiation. Thus, protein S-sulfhydration becomes a crucial regulatory molecule in cerebrovascular and neurodegenerative diseases. Herein, we first described the generation of intracellular H₂S followed by the application of H₂S in the regulation of cerebral blood flow and blood-brain permeability. Further, we described the involvement of S-sulfhydration in different biological and cellular functions, such as inflammatory response, mitochondrial integrity, calcium imbalance, and oxidative stress. Moreover, we highlighted the importance of S-sulfhydration in cerebrovascular and neurodegenerative diseases.

N-Acetylcysteine and Hydrogen Sulfide in Coronavirus Disease 2019

Significance: Hydrogen sulfide (H₂S) is one of the three main gasotransmitters that are endogenously produced in humans and are protective against oxidative stress. Recent findings from studies focusing on coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), shifted our attention to a potentially modulatory role of H₂S in this viral respiratory disease. Recent Advances: H₂S levels at hospital admission may be of importance since this gasotransmitter has been shown to be protective against lung damage through its antiviral, antioxidant, and anti-inflammatory actions. Furthermore, many COVID-19 cases have been described demonstrating remarkable clinical improvement upon administration of high doses of N-acetylcysteine (NAC). NAC is a renowned pharmacological antioxidant substance acting as a source of cysteine, thereby promoting endogenous glutathione (GSH) biosynthesis as well as generation of sulfane sulfur species when desulfurated to H₂S. Critical Issues: Combining H₂S physiology and currently available knowledge of COVID-19, H₂S is hypothesized to target three main vulnerabilities of SARS-CoV-2: (i) cell entry through interfering with functional host receptors, (ii) viral replication through acting on RNA-dependent RNA polymerase (RdRp), and (iii) the escalation of inflammation to a potentially lethal hyperinflammatory cytokine storm (toll-like receptor 4 [TLR4] pathway and NLR family pyrin domain containing 3 [NLRP3] inflammasome). Future Directions: Dissecting the breakdown of NAC reveals the possibility of increasing endogenous H₂S levels, which may provide a convenient rationale for the application of H₂S-targeted therapeutics. Further randomized-controlled trials are warranted to investigate its definitive role.

Hydrogen Sulfide Enhances Plant Tolerance to Waterlogging Stress

Hydrogen sulfide (H₂S) is considered the third gas signal molecule in recent years. A large number of studies have shown that H₂S not only played an important role in animals but also participated in the regulation of plant growth and development and responses to various environmental stresses. Waterlogging, as a kind of abiotic stress, poses a serious threat to land-based waterlogging-sensitive plants, and which H₂S plays an indispensable role in response to. In this review, we summarized that H₂S improves resistance to waterlogging stress by affecting lateral root development, photosynthetic efficiency, and cell fates. Here, we reviewed the roles of H₂S in plant resistance to waterlogging stress, focusing on the mechanism of its promotion to gained hypoxia tolerance. Finally, we raised relevant issues that needed to be addressed.

H2S as a Bridge Linking Inflammation, Oxidative Stress and Endothelial Biology: A Possible Defense in the Fight against SARS-CoV-2 Infection?

The endothelium controls vascular homeostasis through a delicate balance between secretion of vasodilators and vasoconstrictors. The loss of physiological homeostasis leads to endothelial dysfunction, for which inflammatory events represent critical determinants. In this context, therapeutic approaches targeting inflammation-related vascular injury may help for the treatment of cardiovascular disease and a multitude of other conditions related to endothelium dysfunction, including COVID-19. In recent years, within the complexity of the inflammatory scenario related to loss of vessel integrity, hydrogen sulfide (H₂S) has aroused great interest due to its importance in different signaling pathways at the endothelial level. In this review, we discuss the effects of H₂S, a molecule which has been reported to demonstrate anti-inflammatory activity, in addition to many other biological functions related to endothelium and sulfur-drugs as new possible therapeutic options in diseases involving vascular pathobiology, such as in SARS-CoV-2 infection.

Polysulfide inhibits hypoxia-elicited hypoxia-inducible factor activation in a mitochondria-dependent manner

In cellular signaling, the diverse physiological actions of biological gases, including O₂, CO, NO, and H₂S, have attracted much interest. Hypoxia-inducible factors (HIFs), including HIF-1 and HIF-2, are transcription factors that respond to reduced intracellular O₂ availability. Polysulfides are substances containing varying numbers of sulfur atoms (H₂S_n) that are generated endogenously from H₂S by 3-mercaptopyruvate sulfurtransferase in the presence of O₂, and regulate ion channels, specific tumor suppressors, and protein kinases. However, the effect of polysulfides on HIF activation in hypoxic mammalian cells is largely unknown. Here, we have investigated the effect of polysulfide on cells in vitro. In established cell lines, polysulfide donors reversibly reduced cellular O₂ consumption and inhibited hypoxia-induced HIF-1α protein accumulation and the expression of genes downstream of HIFs; however, these effects were not observed in anoxia. In Von Hippel-Lindau tumor suppressor (VHL)- and mitochondria-deficient cells, polysulfides did not affect HIF-1α protein synthesis but destabilized it in a VHL- and mitochondria-dependent manner. For the first time, we show that polysulfides modulate intracellular O₂ homeostasis and regulate HIF activation and subsequent hypoxia-induced gene expression in a VHL- and mitochondria-dependent manner.

H2S releasing Sodium sulfide protects from acute stress-induced hypertension by increasing the activity of endothelial nitric oxide synthase enzyme

Acute stress is a feature of our daily events that affects cardiovascular system and predisposes to hypertension. H₂S is now considered as a vasorelaxant gasotransmitter although it was considered as a toxic agent. In present work we studied the effect of H₂S releasing Na₂S in acute stress induced hypertension and cardiac damage. Rats were divided into five groups: control, Na₂S, acute stress, half dose of Na₂S (6  mg/kg), and finally full dose of Na₂S (12  mg/kg) to acute stressed rats. BP was measured then blood samples were taken for estimation of cortisol, cardiac enzymes markers, IL-6 and H₂S. Finally, animals were sacrificed, hearts and thoracic aortae were excised for histological assessment, estimation of MDA, SOD and RNA extraction of CSE. Acute stress significantly elevated BP, cortisol, cardiac enzymes markers, IL-6, and tissue levels of MDA. It also, induced cardiac cell damage with congested B.V., extravasation of blood and decreased eNOs. Moreover, acute stress reduced H₂S levels, RNA expression of CSE and SOD in cardiac tissues. Na₂S significantly decreased BP, serum levels of cortisol, cardiac enzymes markers, IL-6, and tissue levels of MDA. Also, Na₂S elevated serum H₂S, RNA expression of CSE, SOD in cardiac tissue and increased eNOs activity.

Potential Interplay between Nrf2, TRPA1, and TRPV1 in Nutrients for the Control of COVID-19

In this article, we propose that differences in COVID-19 morbidity may be associated with transient receptor potential ankyrin 1 (TRPA1) and/or transient receptor potential vanilloid 1 (TRPV1) activation as well as desensitization. TRPA1 and TRPV1 induce inflammation and play a key role in the physiology of almost all organs. They may augment sensory or vagal nerve discharges to evoke pain and several symptoms of COVID-19, including cough, nasal obstruction, vomiting, diarrhea, and, at least partly, sudden and severe loss of smell and taste. TRPA1 can be activated by reactive oxygen species and may therefore be up-regulated in COVID-19. TRPA1 and TRPV1 channels can be activated by pungent compounds including many nuclear factor (erythroid-derived 2) (Nrf2)-interacting foods leading to channel desensitization. Interactions between Nrf2-associated nutrients and TRPA1/TRPV1 may be partly responsible for the severity of some of the COVID-19 symptoms. The regulation by Nrf2 of TRPA1/TRPV1 is still unclear, but suggested from very limited clinical evidence. In COVID-19, it is proposed that rapid desensitization of TRAP1/TRPV1 by some ingredients in foods could reduce symptom severity and provide new therapeutic strategies.

Interaction among Hydrogen Sulfide and Other Gasotransmitters in Mammalian Physiology and Pathophysiology

Hydrogen sulfide (H₂S), nitric oxide (NO), carbon monoxide (CO), and sulfur dioxide (SO₂) were previously considered as toxic gases, but now they are found to be members of mammalian gasotransmitters family. Both H₂S and SO₂ are endogenously produced in sulfur-containing amino acid metabolic pathway in vivo. The enzymes catalyzing the formation of H₂S are mainly CBS, CSE, and 3-MST, and the key enzymes for SO₂ production are AAT1 and AAT2. Endogenous NO is produced from L-arginine under catalysis of three isoforms of NOS (eNOS, iNOS, and nNOS). HO-mediated heme catabolism is the main source of endogenous CO. These four gasotransmitters play important physiological and pathophysiological roles in mammalian cardiovascular, nervous, gastrointestinal, respiratory, and immune systems. The similarity among these four gasotransmitters can be seen from the same and/or shared signals. With many studies on the biological effects of gasotransmitters on multiple systems, the interaction among H₂S and other gasotransmitters has been gradually explored. H₂S not only interacts with NO to form nitroxyl (HNO), but also regulates the HO/CO and AAT/SO₂ pathways. Here, we review the biosynthesis and metabolism of the gasotransmitters in mammals, as well as the known complicated interactions among H₂S and other gasotransmitters (NO, CO, and SO₂) and their effects on various aspects of cardiovascular physiology and pathophysiology, such as vascular tension, angiogenesis, heart contractility, and cardiac protection.

Protective Effects of Hydrogen Sulfide Against the ATP-Induced Meningeal Nociception

We previously showed that extracellular ATP and hydrogen sulfide (H₂S), a recently discovered gasotransmitter, are both triggering the nociceptive firing in trigeminal nociceptors implicated in migraine pain. ATP contributes to meningeal nociception by activating the P2X3 subunit-containing receptors whereas H₂S operates mainly via TRP receptors. However, H₂S was also proposed as a neuroprotective and anti-nociceptive agent. This study aimed to test the effect of H₂S on ATP-mediated nociceptive responses in rat meningeal afferents and trigeminal neurons and on ATP-induced degranulation of dural mast cells. Electrophysiological recording of trigeminal nerve activity in meninges was supplemented by patch-clamp and calcium imaging studies of isolated trigeminal neurons. The H₂S donor NaHS induced a mild activation of afferents and fully suppressed the subsequent ATP-induced firing of meningeal trigeminal nerve fibers. This anti-nociceptive effect of H₂S was specific as an even stronger effect of capsaicin did not abolish the action of ATP. In isolated trigeminal neurons, NaHS decreased the inward currents and calcium transients evoked by activation of ATP-gated P2X3 receptors. Moreover, NaHS prevented ATP-induced P2X7 receptor-mediated degranulation of meningeal mast cells which emerged as triggers of migraine pain. Finally, NaHS decreased the concentration of extracellular ATP in the meningeal preparation. Thus, H₂S exerted the multiple protective actions against the nociceptive effects of ATP. These data highlight the novel pathways to reduce purinergic mechanisms of migraine with pharmacological donors or by stimulation production of endogenous H₂S.

Hydrogen sulfide signalling in the CNS - Comparison with NO

Hydrogen sulfide (H₂ S) together with polysulfides (H₂ S_n , n > 2) are signalling molecules like NO with various physiological roles including regulation of neuronal transmission, vascular tone, inflammation and oxygen sensing. H₂ S and H₂ S_n diffuse to the target proteins for S-sulfurating their cysteine residues that induces the conformational changes to alter the activity. On the other hand, 3-mercaptopyruvate sulfurtransferase transfers sulfur from a substrate 3-mercaptopyruvate to the cysteine residues of acceptor proteins. A similar mechanism has also been identified in S-nitrosylation. S-sulfuration and S-nitrosylation by enzymes proceed only inside the cell, while reactions induced by H₂ S, H₂ S_n and NO even extend to the surrounding cells. Disturbance of signalling by these molecules as well as S-sulfuration and S-nitrosylation causes many nervous system diseases. This review focuses on the signalling by H₂ S and H₂ S_n with S-sulfuration comparing to that of NO with S-nitrosylation and discusses on their roles in physiology and pathophysiology.

Signalling by hydrogen sulfide and polysulfides via protein S-sulfuration

Hydrogen sulfide (H2 S) is a signalling molecule that regulates neuronal transmission, vascular tone, cytoprotection, inflammatory responses, angiogenesis, and oxygen sensing. Some of these functions have recently been ascribed to its oxidized form polysulfides (H2 Sn ), which can be produced by 3-mercaptopyruvate sulfurtransferase (MPST), also known as a H2 S-producing enzyme. H2 Sn activate ion channels, tumour suppressors, transcription factors, and protein kinases. H2 Sn S-sulfurate (S-sulfhydrate) cysteine residues of these target proteins to modify their activity by inducing conformational changes through the formation of a disulfide bridge between the two cysteine residues involved. The chemical interaction between H2 S and NO also generates H2 Sn , which may be a chemical entity that exerts the synergistic effect of H2 S and NO. MPST also produces redox regulators cysteine persulfide (CysSSH), GSH persulfide (GSSH), and persulfurated proteins. In addition to MPST, haemoproteins such as haemoglobin, myoglobin, neuroglobin, and catalase as well as SOD can produce H2 Sn , and sulfide quinone oxidoreductase and cysteinyl tRNA synthetase can make GSSH and CysSSH. This review focuses on the recent progress in the study of the production and physiological roles of these persulfurated and polysulfurated molecules. LINKED ARTICLES: This article is part of a themed section on Hydrogen Sulfide in Biology & Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.4/issuetoc.

TRP Channels in the Focus of Trigeminal Nociceptor Sensitization Contributing to Primary Headaches

Pain in trigeminal areas is driven by nociceptive trigeminal afferents. Transduction molecules, among them the nonspecific cation channels transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1), which are activated by endogenous and exogenous ligands, are expressed by a significant population of trigeminal nociceptors innervating meningeal tissues. Many of these nociceptors also contain vasoactive neuropeptides such as calcitonin gene-related peptide (CGRP) and substance P. Release of neuropeptides and other functional properties are frequently examined using the cell bodies of trigeminal neurons as models of their sensory endings. Pathophysiological conditions cause phosphorylation, increased expression and trafficking of transient receptor potential (TRP) channels, neuropeptides and other mediators, which accelerate activation of nociceptive pathways. Since nociceptor activation may be a significant pathophysiological mechanism involved in both peripheral and central sensitization of the trigeminal nociceptive pathway, its contribution to the pathophysiology of primary headaches is more than likely. Metabolic disorders and medication-induced painful states are frequently associated with TRP receptor activation and may increase the risk for primary headaches.

Pharmacological polysulfide suppresses glucose-stimulated insulin secretion in an ATP-sensitive potassium channel-dependent manner

Hydrogen sulfide (H₂S) is an endogenous gaseous transmitter synthesized in various cell types. It is well established that H₂S functions in many physiological processes, including the relaxation of vascular smooth muscle, mediation of neurotransmission, regulation of inflammation, and modulation of insulin signaling. In recent years, it has been revealed that polysulfides, substances with a varying number of sulfur atoms (H₂Sn), are generated endogenously from H₂S in the presence of oxygen. A series of studies describes that sulfane sulfur has the unique ability to bind reversibly to other sulfur atoms to form hydropersulfides and polysulfides, and that polysulfides activate ion channels and promote calcium influx. Furthermore, polysulfides regulate tumor suppressor activity, promote the activation of transcription factors targeting antioxidant genes and regulate blood pressure by vascular smooth muscle relaxation. Insulin secretion from pancreatic β cells plays a critical role in response to increased blood glucose concentration. H₂S has emerged as an important regulator of glycemic control and exhibits characteristic regulation of glucose homeostasis. However, the effects of polysulfides on glucose-stimulated insulin secretion (GSIS) are largely unknown. In this study, we demonstrated that pharmacological polysulfide salts including Na₂S₂, Na₂S₃, and Na₂S₄ considerably inhibit GSIS in mouse and rat pancreatic β-cell-derived MIN6 and INS-1 cell lines, and that the effect is dependent on the activation of ATP-sensitive potassium channels. In addition, we demonstrated that a mixture of Na₂S and diethylamine NONOate inhibits GSIS in a similar way to the pharmacological administration of polysulfide salts.

Stimulation of the endogenous hydrogen sulfide synthesis suppresses oxidative-nitrosative stress and restores endothelial-dependent vasorelaxation in old rats

Hydrogen sulfide (H₂S) is an endogenous gas transmitter with profound effects on the cardiovascular system. We hypothesized that stimulation of H₂S synthesis might alleviate age-associated changes in vascular reactivity. Pyridoxal-5-phosphate (PLP), the coenzyme of H₂S-synthesizing enzymes, was administrated to old male Wistar rats per os at a dose of 0.7 mg/kg body mass once a day for 2 weeks. H₂S content in the aortic tissue, markers of oxidative stress, inducible nitric oxide synthase (iNOS) and constitutive nitric oxide synthase (cNOS), arginase activities, and endothelium-dependent vasorelaxation of the aortic rings were studied. Our results showed that PLP restored endogenous H₂S and low molecular weight S-nitrosothiol levels in old rat aorta to the levels detected in adults. PLP significantly reduced diene conjugate content, hydrogen peroxide and peroxynitrite generation rates, and iNOS and arginase activity in the aortic tissue of old rats. PLP also greatly improved acetylcholine-induced relaxation of old rat aorta (47.7% ± 4.8% versus 18.4% ± 4.1% in old rats, P < 0.05) that was abolished by NO inhibition with N-nitro-l-arginine methyl ester hydrochloride (L-NAME) or H₂S inhibition with O-carboxymethylhydroxylamine (O-CMH). Thus, PLP might be used for stimulation of endogenous H₂S synthesis and correction of oxidative and nitrosative stress and vessel tone dysfunction in aging and age-associated diseases.

Effects of inhibiting antioxidant pathways on cellular hydrogen sulfide and polysulfide metabolism

Elaborate antioxidant pathways have evolved to minimize the threat of excessive reactive oxygen species (ROS) and to regulate ROS as signaling entities. ROS are chemically and functionally similar to reactive sulfur species (RSS) and both ROS and RSS have been shown to be metabolized by the antioxidant enzymes, superoxide dismutase and catalase. Here we use fluorophores to examine the effects of a variety of inhibitors of antioxidant pathways on metabolism of two important RSS, hydrogen sulfide (H₂S with AzMC) and polysulfides (H₂S_n, where n = 2-7, with SSP4) in HEK293 cells. Cells were exposed to inhibitors for up to 5 days in normoxia (21% O₂) and hypoxia (5% O₂), conditions also known to affect ROS production. Decreasing intracellular glutathione (GSH) with l-buthionine-sulfoximine (BSO) or diethyl maleate (DEM) decreased H₂S production for 5 days but did not affect H₂S_n. The glutathione reductase inhibitor, auranofin, initially decreased H₂S and H₂S_n but after two days H₂S_n increased over controls. Inhibition of peroxiredoxins with conoidin A decreased H₂S and increased H₂S_n, whereas the glutathione peroxidase inhibitor, tiopronin, increased H₂S. Aminoadipic acid, an inhibitor of cystine uptake did not affect either H₂S or H₂S_n. In buffer, the glutathione reductase and thioredoxin reductase inhibitor, 2-AAPA, the glutathione peroxidase mimetic, ebselen, and tiopronin variously reacted directly with AzMC and SSP4, reacted with H₂S and H₂S₂, or optically interfered with AzMC or SSP4 fluorescence. Collectively these results show that antioxidant inhibitors, generally known for their ability to increase cellular ROS, have various effects on cellular RSS. These findings suggest that the inhibitors may affect cellular sulfur metabolism pathways that are not related to ROS production and in some instances they may directly affect RSS or the methods used to measure them. They also illustrate the importance of carefully evaluating RSS metabolism when biologically or pharmacologically attempting to manipulate ROS.

Pharmacological levels of hydrogen sulfide inhibit oxidative cell injury through regulating the redox state of thioredoxin

Hydrogen sulfide (H₂S) is a gaseous mediator with multifaceted biological activities. It has anti-inflammatory and anti-oxidative effects. Currently, the mechanisms are not fully understood. Given that Trx/ASK1/P38 signaling pathway mediates many oxidative cell responses, we tested whether and how H₂S affected this pathway. Exposure of podocytes to Adriamycin (ADR), an antitumor drug, led to a P38-mediated oxidative cell injury, as evidenced by the increased protein carbonylation, oxidative activation of P38, and prevention of the cell death by antioxidants, NADPH oxidase inhibitor and P38 inhibitor. In the presence of H₂S donor NaHS, however, the podocyte injury was largely prevented. NaHS also significantly prevented cell death elicited by H₂O₂, menadione, and thioredoxin (Trx) inhibitors. These effects of H₂S were also associated with a potent inhibition of P38. Further analysis revealed that H₂S did not affect the protein level of TXNIP and Trx, two pivotal regulators of ASK1/P38 activation, but it promoted the dissociation of Trx from TXNIP. Moreover, it disrupted the H₂O₂-initiated polymerization of Trx and converted Trx from the oxidized to the reduced form. In HepG2 cells, inhibition of H₂S-producing enzyme cystathionine γ-lyase (CSE) increased Trx oxidation, promoted Trx binding to TXNIP and exaggerated cell injury caused by Trx inhibition. Collectively, our results indicate that H₂S exerted its antioxidative effects through the regulation of the redox state of Trx and interference with Trx/ASK1/P38 signaling pathway. Given the importance of the pathway in the mediation of multiple oxidative cell responses, our study thus provides novel mechanistic insight into the action of H₂S.

Sulfur Dioxide Activates Cl-/HCO3 - Exchanger via Sulphenylating AE2 to Reduce Intracellular pH in Vascular Smooth Muscle Cells

Sulfur dioxide (SO₂) is a colorless and irritating gas. Recent studies indicate that SO₂ acts as the gas signal molecule and inhibits vascular smooth muscle cell (VSMC) proliferation. Cell proliferation depends on intracellular pH (pH_i). Transmembrane cystein mutation of Na⁺- independent Cl^-/HCO₃ ^- exchanger (anion exchanger, AE) affects pH_i. However, whether SO₂ inhibits VSMC proliferation by reducing pH_i is still unknown. Here, we investigated whether SO₂ reduced pH_i to inhibit the proliferation of VSMCs and explore its molecular mechanisms. Within a range of 50-200 μM, SO₂ was found to lower the pH_i in VSMCs. Concurrently, NH₄Cl pre-perfusion showed that SO₂ significantly activated AE, whereas the AE inhibitor 4,4'-diisothiocyanatostilbene- 2,20-disulfonic acid (DIDS) significantly attenuated the effect of SO₂ on pH_i in VSMCs. While 200 μM SO₂ sulphenylated AE2, while dithiothreitol (DTT) blocked the sulphenylation of AE2 and subsequent AE activation by SO₂, thereby restoring the pH_i in VSMCs. Furthermore, DIDS pretreatment eliminated SO₂-induced inhibition of PDGF-BB-stimulated VSMC proliferation. We report for the first time that SO₂ inhibits VSMC proliferation in part by direct activation of the AE via posttranslational sulphenylation and induction of intracellular acidification.

Converting organosulfur compounds to inorganic polysulfides against resistant bacterial infections

The use of natural substance to ward off microbial infections has a long history. However, the large-scale production of natural extracts often reduces antibacterial potency, thus limiting practical applications. Here we present a strategy for converting natural organosulfur compounds into nano-iron sulfides that exhibit enhanced antibacterial activity. We show that compared to garlic-derived organosulfur compounds nano-iron sulfides exhibit an over 500-fold increase in antibacterial efficacy to kill several pathogenic and drug-resistant bacteria. Furthermore, our analysis reveals that hydrogen polysulfanes released from nano-iron sulfides possess potent bactericidal activity and the release of polysulfanes can be accelerated by the enzyme-like activity of nano-iron sulfides. Finally, we demonstrate that topical applications of nano-iron sulfides can effectively disrupt pathogenic biofilms on human teeth and accelerate infected-wound healing. Together, our approach to convert organosulfur compounds into inorganic polysulfides potentially provides an antibacterial alternative to combat bacterial infections.

[PYRIDOXAL-5-PHOSPHATE RESTORES HYDROGEN SULFIDE SYNTHES AND REDOX STATE OF HEART AND BLOOD VESSELS TISSUE IN OLD ANIMALS]

It was shown the alterations in hydrogen sulfide (H(2)S) metabolism and the development of oxidative and nitrozative stress in cardiovascular system by aging. The administration of pyridoxal-5-phosphate as cofactor of H(2)S synthesizing enzymes restored endogenous H(2)S level and redox state in the heart and aorta tissues. Under these conditions, the following indicators of oxidative stress were significantly decreased in heart and aorta tissues: superoxide generation rate (·0(2)(-)) and hydroxyl (·OH) anion radicals, compared with significantly elevated levels of these parameters in old animals. We also found the reduction of non-enzymatic (diene conjugates and malonic dialdehyde) and enzymatic (uric acid, LTC(4) and TxB(2)) lipid oxidation products levels in old rats under H(2)S synthesis stimulation that confirms the restriction of oxidative stress. An important consequence of endogenous synthesis stimulation of hydrogen sulfide during aging is a decrease of nitrozative stress, such as iNOS activity and nitrate reductase, as well as recovery of constitutive NO synthase activity, indicating the importance of this gas transmitter in cardiovascular system. Thus, stimulation of hydrogen sulfide endogenous synthesis contributed to reduced production of reactive oxygen species (oxidative stress) and nitrogen (nitrozative stress) in heart and aorta tissues with aging. The presence of a pronounced antioxidant effect and modulating influence of pyridoxal-5- phosphate in the redox state of heart tissue and blood vessels during aging suggests cardioprotective properties of the substance and prospects for future research.

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.

Hydrogen Sulfide as an O2 Sensor: A Critical Analysis

There is increasing interest in the physiological actions and therapeutic potential of the gasotransmitter hydrogen sulfide (H₂S). In addition to exerting antihypertensive, anti-inflammatory, antioxidant, and pro-angiogenic effects, H₂S has been suggested to play a central and ubiquitous role in O₂ sensing. According to this concept, because H₂S is metabolized by oxidation, its cellular concentration varies inversely with the ambient pO₂ such that hypoxia causes a rise in intracellular [H₂S]; this then acts to induce appropriate cellular responses. In particular, it has been proposed that H₂S underpins O₂ sensing in the carotid body, which triggers increases in ventilation in response to hypoxemia, and also in pulmonary arteries, which constrict in response to local alveolar hypoxia. This process, termed hypoxic pulmonary vasoconstriction (HPV), acts to divert blood to better-oxygenated regions of the lung, thereby maintaining the ventilation-perfusion ratio and minimizing hypoxia-induced falls in blood O₂ saturation. In this chapter, we present a critical review of the evidence supporting and questioning this model in both HPV and the carotid body.

A robust and versatile mass spectrometry platform for comprehensive assessment of the thiol redox metabolome

Several diseases are associated with perturbations in redox signaling and aberrant hydrogen sulfide metabolism, and numerous analytical methods exist for the measurement of the sulfur-containing species affected. However, uncertainty remains about their concentrations and speciation in cells/biofluids, perhaps in part due to differences in sample processing and detection principles. Using ultrahigh-performance liquid chromatography in combination with electrospray-ionization tandem mass spectrometry we here outline a specific and sensitive platform for the simultaneous measurement of 12 analytes, including total and free thiols, their disulfides and sulfide in complex biological matrices such as blood, saliva and urine. Total assay run time is < 10 min, enabling high-throughput analysis. Enhanced sensitivity and avoidance of artifactual thiol oxidation is achieved by taking advantage of the rapid reaction of sulfhydryl groups with N-ethylmaleimide. We optimized the analytical procedure for detection and separation conditions, linearity and precision including three stable isotope labelled standards. Its versatility for future more comprehensive coverage of the thiol redox metabolome was demonstrated by implementing additional analytes such as methanethiol, N-acetylcysteine, and coenzyme A. Apparent plasma sulfide concentrations were found to vary substantially with sample pretreatment and nature of the alkylating agent. In addition to protein binding in the form of mixed disulfides (S-thiolation) a significant fraction of aminothiols and sulfide appears to be also non-covalently associated with proteins. Methodological accuracy was tested by comparing the plasma redox status of 10 healthy human volunteers to a well-established protocol optimized for reduced/oxidized glutathione. In a proof-of-principle study a deeper analysis of the thiol redox metabolome including free reduced/oxidized as well as bound thiols and sulfide was performed. Additional determination of acid-labile sulfide/thiols was demonstrated in human blood cells, urine and saliva. Using this simplified mass spectrometry-based workflow the thiol redox metabolome can be determined in samples from clinical and translational studies, providing a novel prognostic/diagnostic platform for patient stratification, drug monitoring, and identification of new therapeutic approaches in redox diseases.

Aminooxyacetic acid improves learning and memory in a rat model of chronic alcoholism

Chronic alcoholism seriously damages the central nervous system and leads to impaired learning and memory. Cell damage in chronic alcoholism is strongly associated with elevated levels of hydrogen sulfide (H₂S) and calcium ion overload. Aminooxyacetic acid is a cystathionine-β-synthase activity inhibitor that can reduce H₂S formation in the brain. This study sought to observe the effect of aminooxyacetic acid on learning and memory in a chronic alcoholism rat model. Rats were randomly divided into three groups. Rats in the control group were given pure water for 28 days. Rats in the model group were given 6% alcohol for 28 days to establish an alcoholism rat model. Rats in the aminooxyacetic acid remedy group were also given 6% alcohol for 28 days and were also intraperitoneally injected daily with aminooxyacetic acid (5 mg/kg) from day 15 to day 28. Learning and memory was tested using the Morris water maze test. The ultrastructure of mitochondria in the hippocampus was observed by electron microscopy. H₂S levels in the hippocampus were measured indirectly by spectrophotometry, and ATPase activity was measured using a commercial kit. The expression of myelin basic protein was determined by immunohistochemistry and western blotting. Compared with the control group, latency and swimming distance were prolonged in the navigation test on days 2, 3, and 4 in the model group. In the spatial probe test on day 5, the number of platform crosses was reduced in the model group. Cristae cracks, swelling or deformation of mitochondria appeared in the hippocampus, the hippocampal H₂S level was increased, the mitochondrial ATPase activity was decreased, and the expression of myelin basic protein in the hippocampus was down-regulated in the model group compared with the control group. All the above indexes were ameliorated in the aminooxyacetic acid remedy group compared with the model group. These findings indicate that aminooxyacetic acid can improve learning and memory in a chronic alcoholism rat model, which may be associated with reduction of hippocampal H₂S level and mitochondrial ATPase activity, and up-regulation of myelin basic protein levels in the hippocampus.

Gaseous Signaling Molecules in Cardiovascular Function: From Mechanisms to Clinical Translation

Carbon monoxide (CO), hydrogen sulfide (H₂S), and nitric oxide (NO) constitute endogenous gaseous molecules produced by specific enzymes. These gases are chemically simple, but exert multiple effects and act through shared molecular targets to control both physiology and pathophysiology in the cardiovascular system (CVS). The gases act via direct and/or indirect interactions with each other in proteins such as heme-containing enzymes, the mitochondrial respiratory complex, and ion channels, among others. Studies of the major impacts of CO, H₂S, and NO on the CVS have revealed their involvement in controlling blood pressure and in reducing cardiac reperfusion injuries, although their functional roles are not limited to these conditions. In this review, the basic aspects of CO, H₂S, and NO, including their production and effects on enzymes, mitochondrial respiration and biogenesis, and ion channels are briefly addressed to provide insight into their biology with respect to the CVS. Finally, potential therapeutic applications of CO, H₂S, and NO with the CVS are addressed, based on the use of exogenous donors and different types of delivery systems.

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.

Regulation of Aqueous Humor Dynamics by Hydrogen Sulfide: Potential Role in Glaucoma Pharmacotherapy

Hydrogen sulfide (H₂S) is a gaseous transmitter with well-known biological actions in a wide variety of tissues and organs. The potential involvement of this gas in physiological and pathological processes in the eye has led to several in vitro, ex vivo, and in vivo studies to understand its pharmacological role in some mammalian species. Evidence from literature demonstrates that 4 enzymes responsible for the biosynthesis of this gas (cystathionine β-synthase, CBS; cystathionine γ-lyase, CSE; 3-mercaptopyruvate sulfurtransferase, 3MST; and d-amino acid oxidase) are present in the cornea, iris, ciliary body, lens, and retina. Studies of the pharmacological actions of H₂S (using several compounds as fast- and slow-releasing gas donors) on anterior uveal tissues reveal an effect on sympathetic neurotransmission and the ability of the gas to relax precontracted iris and ocular vascular smooth muscles, responses that were blocked by inhibitors of CSE, CBS, and K_ATP channels. In the retina, there is evidence that H₂S can inhibit excitatory amino acid neurotransmission and can also protect this tissue from a wide variety of insults. Furthermore, exogenous application of H₂S-releasing compounds was reported to increase aqueous humor outflow facility in an ex vivo model of the porcine ocular anterior segment and lowered intraocular pressure (IOP) in both normotensive and glaucomatous rabbits. Taken together, the finding that H₂S-releasing compounds can lower IOP and can serve a neuroprotective role in the retina suggests that H₂S prodrugs could be used as tools or therapeutic agents in diseases such as glaucoma.

Age-Dependent, Subunit Specific Action of Hydrogen Sulfide on GluN1/2A and GluN1/2B NMDA Receptors

Hydrogen sulfide (H₂S) is an endogenously produced neuroactive gas implicated in many key processes in the peripheral and central nervous system. Whereas the neuroprotective role of H₂S has been shown in adult brain, the action of this messenger in newborns remains unclear. One of the known targets of H₂S in the nervous system is the N-methyl-D-aspartate (NMDA) glutamate receptor which can be composed of different subunits with distinct functional properties. In the present study, using patch clamp technique, we compared the effects of the H₂S donor sodium hydrosulfide (NaHS, 100 μM) on hippocampal NMDA receptor mediated currents in rats of the first and third postnatal weeks. This was supplemented by testing effects of NaHS on recombinant GluN1/2A and GluN1/2B NMDA receptors expressed in HEK293T cells. The main finding is that NaHS action on NMDA currents is age-dependent. Currents were reduced in newborns but increased in older juvenile rats. Consistent with an age-dependent switch in NMDA receptor composition, in HEK239T cells expressing GluN1/2A receptors, NaHS increased NMDA activated currents associated with acceleration of desensitization and decrease of the deactivation rate. In contrast, in GluN1/2B NMDA receptors, which are prevalent in newborns, NaHS decreased currents and reduced receptor deactivation without effect on the desensitization rate. Adenylate cyclase inhibitor MDL-12330A (10 μM) did not prevent the age-dependent effects of NaHS on NMDA evoked currents in pyramidal neurons of hippocampus. The reducing agent dithiothreitol (DTT, 2 mM) applied on HEK293T cells prevented facilitation induced by NaHS on GluN1/2A NMDA receptors, however in GluN1/2B NMDA receptors the inhibitory effect of NaHS was still observed. Our data indicate age-dependent effect of H₂S on NMDA receptor mediated currents determined by glutamate receptor subunit composition. While the inhibitory action of H₂ on GluN1/2B receptors could limit the excessive activation in early age, the enhanced functionality of GluN1/2A receptor in the presence of this gasotransmitter can enlarge synaptic efficacy and promote synaptic plasticity in adults.

The mechanism of action and role of hydrogen sulfide in the control of vascular tone

Our knowledge about hydrogen sulfide (H₂S) significantly changed over the last two decades. Today it is considered as not only a toxic gas but also as a gasotransmitter with diverse roles in different physiological and pathophysiological processes. H₂S has pleiotropic effects and its possible mechanisms of action involve (1) a reversible protein sulfhydration which can alter the function of the modified proteins similar to nitrosylation or phosphorylation; (2) direct antioxidant effects and (3) interaction with metalloproteins. Its effects on the human cardiovascular system are especially important due to the high prevalence of hypertension and myocardial infarction. The exact molecular targets that affect the vascular tone include the K_ATP channel, the endothelial nitric oxide synthase, the phosphodiesterase of the vascular smooth muscle cell and the cytochrome c oxidase among others and the combination of all these effects lead to the final result on the vascular tone. The relative role of each effect depends immensely on the used concentration and also on the used donor molecules but several other factors and experimental conditions could alter the final effect. The aim of the current review is to give a comprehensive summary of the current understanding on the mechanism of action and role of H₂S in the regulation of vascular tone and to outline the obstacles that hinder the better understanding of its effects.

Polysulfides and products of H2S/S-nitrosoglutathione in comparison to H2S, glutathione and antioxidant Trolox are potent scavengers of superoxide anion radical and produce hydroxyl radical by decomposition of H2O2

Exogenous and endogenously produced sulfide derivatives, such as H₂S/HS^-/S^2-, polysulfides and products of the H₂S/S-nitrosoglutathione interaction (S/GSNO), affect numerous biological processes in which superoxide anion (O₂^-) and hydroxyl (OH) radicals play an important role. Their cytoprotective-antioxidant and contrasting pro-oxidant-toxic effects have been reported. Therefore, the aim of our work was to contribute to resolving this apparent inconsistency by studying sulfide derivatives/free radical interactions and their consequent biological effects compared to the antioxidants glutathione (GSH) and Trolox. Using the electron paramagnetic resonance (EPR) spin trapping technique and O₂^-, we found that a polysulfide (Na₂S₄) and S/GSNO were potent scavengers of O₂^- and cPTIO radicals compared to H₂S (Na₂S), GSH and Trolox, and S/GSNO scavenged the DEPMPO-OH radical. As detected by the EPR spectra of DEPMPO-OH, the formation of OH in physiological solution by S/GSNO was suggested. All the studied sulfide derivatives, but not Trolox or GSH, had a bell-shaped potency to decompose H₂O₂ and produced OH in the following order: S/GSNO > Na₂S₄ ≥ Na₂S > GSH = Trolox = 0, but they scavenged OH at higher concentrations. In studies of the biological consequences of these sulfide derivatives/H₂O₂ properties, we found the following: (i) S/GSNO alone and all sulfide derivatives in the presence of H₂O₂ cleaved plasmid DNA; (ii) S/GSNO interfered with viral replication and consequently decreased the infectivity of viruses; (iii) the sulfide derivatives induced apoptosis in A2780 cells but inhibited apoptosis induced by H₂O₂; and (iv) Na₂S₄ modulated intracellular calcium in A87MG cells, which depended on the order of Na₂S₄/H₂O₂ application. We suggest that the apparent inconsistency of the cytoprotective-antioxidant and contrasting pro-oxidant-toxic biological effects of sulfide derivatives results from their time- and concentration-dependent radical production/scavenging properties and their interactions with O₂^-, OH and H₂O₂. The results imply a direct involvement of sulfide derivatives in O₂^- and H₂O₂/OH free radical pathways modulating antioxidant/toxic biological processes.

3-Mercaptopyruvate sulfurtransferase produces potential redox regulators cysteine- and glutathione-persulfide (Cys-SSH and GSSH) together with signaling molecules H2S2, H2S3 and H2S

Cysteine-persulfide (Cys-SSH) is a cysteine whose sulfhydryl group is covalently bound to sulfur (sulfane sulfur). Cys-SSH and its glutathione (GSH) counterpart (GSSH) have been recognized as redox regulators, some of which were previously ascribed to cysteine and GSH. However, the production of Cys-SSH and GSSH is not well understood. Here, we show that 3-mercaptopyruvate sulfurtransferase (3MST) produces Cys-SSH and GSSH together with the potential signaling molecules hydrogen per- and tri-sulfide (H₂S₂ and H₂S₃). Cys-SSH and GSSH are produced in the brain of wild-type mice but not in those of 3MST-KO mice. The levels of total persulfurated species in the brain of 3MST-KO mice are less than 50% of that in the brain of wild-type mice. Purified recombinant 3MST and lysates of COS cells expressing 3MST showed that Cys-SSH and GSSH were produced in the presence of physiological concentrations of cysteine and glutathione, while those with longer sulfur chains, Cys-SS_nH and GSS_nH, were produced in the presence of lower than physiological concentrations of cysteine and glutathione. The present study provides new insights into the production and physiological roles of these persulfurated species as well as the therapeutic targets for diseases in which these molecules are involved.

Receptor Mechanisms Mediating the Pro-Nociceptive Action of Hydrogen Sulfide in Rat Trigeminal Neurons and Meningeal Afferents

Hydrogen sulfide (H₂S), a well-established member of the gasotransmitter family, is involved in a variety of physiological functions, including pro-nociceptive action in the sensory system. Although several reports have shown that H₂S activates sensory neurons, the molecular targets of H₂S action in trigeminal (TG) nociception, implicated in migraine, remains controversial. In this study, using suction electrode recordings, we investigate the effect of the H₂S donor, sodium hydrosulfide (NaHS), on nociceptive firing in rat meningeal TG nerve fibers. The effect of NaHS was also explored with patch-clamp and calcium imaging techniques on isolated TG neurons. NaHS dramatically increased the nociceptive firing in TG nerve fibers. This effect was abolished by the TRPV1 inhibitor capsazepine but was partially prevented by the TRPA1 blocker HC 030031. In a fraction of isolated TG neurons, NaHS transiently increased amplitude of capsaicin-induced currents. Moreover, NaHS by itself induced inward currents in sensory neurons, which were abolished by the TRPV1 inhibitor capsazepine suggesting involvement of TRPV1 receptors. In contrast, the inhibitor of TRPA1 receptors HC 030031 did not prevent the NaHS-induced currents. Imaging of a large population of TG neurons revealed that NaHS induced calcium transients in 41% of tested neurons. Interestingly, this effect of NaHS in some neurons was inhibited by the TRPV1 antagonist capsazepine whereas in others it was sensitive to the TRPA1 blocker HC 030031. Our data suggest that both TRPV1 and TRPA1 receptors play a role in the pro-nociceptive action of NaHS in peripheral TG nerve endings in meninges and in somas of TG neurons. We propose that activation of TRPV1 and TRPA1 receptors by H₂S during neuro-inflammation conditions contributes to the nociceptive firing in primary afferents underlying migraine pain.

Gasotransmitter Heterocellular Signaling

SIGNIFICANCE

The family of gasotransmitter molecules, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), has emerged as an important mediator of numerous cellular signal transduction and pathophysiological responses. As such, these molecules have been reported to influence a diverse array of biochemical, molecular, and cell biology events often impacting one another. Recent Advances: Discrete regulation of gasotransmitter molecule formation, movement, and reaction is critical to their biological function. Due to the chemical nature of these molecules, they can move rapidly throughout cells and tissues acting on targets through reactions with metal groups, reactive chemical species, and protein amino acids.

CRITICAL ISSUES

Given the breadth and complexity of gasotransmitter reactions, this field of research is expanding into exciting, yet sometimes confusing, areas of study with significant promise for understanding health and disease. The precise amounts of tissue and cellular gasotransmitter levels and where they are formed, as well as how they react with molecular targets or themselves, all remain poorly understood.

FUTURE DIRECTIONS

Elucidation of specific molecular targets, characteristics of gasotransmitter molecule heterotypic interactions, and spatiotemporal formation and metabolism are all important to better understand their true pathophysiological importance in various organ systems. Antioxid. Redox Signal. 26, 936-960.

Polysulfides (H2Sn) produced from the interaction of hydrogen sulfide (H2S) and nitric oxide (NO) activate TRPA1 channels

Hydrogen sulfide (H₂S) exerts synergistic effects with another gaseous signaling molecule nitric oxide (NO) on ion channels and vasculature. However, the mechanism of the synergy is not well understood. Here, we show that the interaction between H₂S and NO generates polysulfides (H₂S_n), which activate transient receptor potential ankyrin 1 (TRPA1) channels. High performance liquid chromatography with tandem mass spectrometry analysis, along with the imaging of intracellular Ca²⁺ and H₂S_n, showed that H₂S_n and their effects were abolished by cyanolysis and by reducing substances such as dithiothreitol (DTT), cysteine, and glutathione (GSH). However, the effects of nitroxyl or nitrosopersulfide, other potential products of H₂S and NO interaction, are not affected by cyanolysis or reducing substances. This study demonstrates that H₂S_n are products of synergy between H₂S and NO and provides a new insight into the signaling mechanisms.

Sulfur Atom in its Bound State Is a Unique Element Involved in Physiological Functions in Mammals

It was in the 1950s that the term polysulfide or persulfide was introduced in biological studies. The unfamiliar term “sulfane sulfur” sometimes appeared in papers published in the 1970s, and was defined in the review article by Westley in 1983. In the article, sulfane sulfur is described as sulfur atoms that are covalently bound only with sulfur atoms, and as this explanation was somewhat difficult to comprehend, it was not generally accepted. Thus, in the early 1990s, we redefined these sulfur species as “bound sulfur”, which easily converts to hydrogen sulfide on reduction with a thiol reducing agent. In other words, bound sulfur refers to a sulfur atom that exists in a zero to divalent form (0 to −2). The first part of this review focuses on the fluorescent derivatization HPLC method—which we developed for measurement of bound sulfur—and explains the distribution of bound sulfur and the hydrogen sulfide-producing ability of various tissues, as clarified by this method. Next, we discuss diverse physiological functions and involvement of polysulfide, a typical type of bound sulfur, in the redox regulation system. Additionally, we also address its possible physiological role in the central nervous system, based on its action of scavenging reactive carbonyl compounds.