Hydrogen sulfide and hemeproteins: knowledge and mysteries

Generation of Thiosulfate, Selenite, Dithiosulfite, Perthionitrite, Nitric Oxide, and Reactive Chalcogen Species by Binuclear Zinc(II)-Chalcogenolato/-Polychalcogenido Complexes

A comparative bioinspired reactivity study of new binuclear Zn(II) complexes featuring coordinated thiolate, selenolate, trisulfide and diselenide in relation with (i) the generation of reactive sulfur/selenium species (RSS/RSeS), (ii) the oxygen dependent oxidation and disproportionation of polysulfide (Sn2-) to produce sulfite (SO32-), thiosulfate (S2O32-) and sulfide (S2-) by sulfur oxygenase reductase (SOR), and (iii) the reaction of Sn2- with nitrite (NO2-) to generate thionitrite (SNO-), perthionitrite (SSNO-) and nitric oxide (NO), is presented. The binuclear Zn(II)-thiolate/selenolate complexes could react with elemental sulfur to generate RSS/RSeS while similar reactions involving elemental selenium could not generate RSeS. The dizinc(II)-S3 and the dizinc(II)-Se2 complexes could react with dioxygen (O2) to generate binuclear Zn(II) complexes featuring coordinated thiosulfate (S2O32-) and selenite (SeO32-), respectively. Finally, unlike the nonreactive nature of the dizinc(II)-Se2 complex toward NO2-, reaction of the dizinc(II)-S3 complex with NO2- produced a new binuclear Zn(II) complex featuring a coordinated dithiosulfite (S3O2-) along with the formation of perthionitrite (SSNO-), of which the latter subsequently produced nitric oxide (NO) and S42-. The present work, thus, demonstrates the comparative reactivity of a series of binuclear Zn(II)-chalcogenolato/-polychalcogenido complexes for the generation of S2O32-, SeO32-, S3O2-, SSNO-, NO and RSS/RSeS.

Lactoperoxidase catalytically oxidize hydrogen sulfide via intermediate formation of sulfheme derivatives

The biological chemistry of hydrogen sulfide (H2S) with physiologically important heme proteins is in the focus of redox biology research. In this study, we investigated the interactions of lactoperoxidase (LPO) with H2S in the presence and absence of molecular dioxygen (O2) or hydrogen peroxide (H2O2). Under anaerobic conditions, native LPO forms no heme-H2S complex upon sulfide exposure. However, under aerobic conditions or in the presence of H2O2 the formation of both ferrous and ferric sulfheme (sulfLPO) derivatives was observed based on the appearances of their characteristic optical absorptions at 638 nm and 727 nm, respectively. Interestingly, we demonstrate that LPO can catalytically oxidize H2S by H2O2 via intermediate formation of relatively short-lived ferrous and ferric sulfLPO derivatives. Pilot product analyses suggested that the turnover process generates oxidized sulfide species, which include sulfateS O 4 2 - and inorganic polysulfides (H S x - ; x = 2-5). These results indicated that H2S can serve as a non-classical LPO substrate by inducing a reversible sulfheme-like modification of the heme porphyrin ring during turnover. Furthermore, electron paramagnetic resonance data suggest that H2S can act as a scavenger of H2O2 in the presence of LPO without detectable formation of any carbon-centered protein radical species, suggesting that H2S might be capable of protecting the enzyme from radical-mediated damage. We propose possible mechanisms, which explain our results as well as contrasting observations with other heme proteins, where either no sulfheme formation was observed or the generation of sulfheme derivatives provided a dead end for enzyme functions.

Nonheme binuclear transition metal complexes with hydrosulfide and polychalcogenides

The intriguing chemistry of chalcogen (S, Se)-containing ligands and their capability to bridge multiple metal centres have resulted in a plethora of reports on transition metal complexes featuring hydrosulfide (HS-) and polychalcogenides (En2-, E = S, Se). While a large number of such molecules are strictly organometallic complexes, examples of non-organometallic complexes featuring HS- and En2- with N-/O-donor ligands are relatively rare. The general synthetic procedure for the transition metal-hydrosulfido complexes involves the reaction of the corresponding metal salts with HS-/H2S and this is prone to generate sulfido bridged oligomers in the absence of sterically demanding ligands. On the other hand, the synthetic methods for the preparation of transition metal-polychalcogenido complexes include the reaction of the corresponding metal salts with En2- or the two electron oxidation of low-valent metals with elemental chalcogen, often at an elevated temperature and/or for a long time. Recently, we have developed new synthetic methods for the preparation of two new classes of binuclear transition metal complexes featuring either HS-, or Sn2- and Sen2- ligands. The new method for the synthesis of transition metal-hydrosulfido complexes involved transition metal-mediated hydrolysis of thiolates at room temperature (RT), while the method for the synthesis of transition metal-polychalcogenido complexes involved redox reaction of coordinated thiolates and exogenous elemental chalcogens at RT. An overview of the synthetic aspects, structural properties and intriguing reactivity of these two new classes of transition metal complexes is presented.

Chemistry of Hydrogen Sulfide-Pathological and Physiological Functions in Mammalian Cells

Hydrogen sulfide (H2S) was recognized as a gaseous signaling molecule, similar to nitric oxide (-NO) and carbon monoxide (CO). The aim of this review is to provide an overview of the formation of hydrogen sulfide (H2S) in the human body. H2S is synthesized by enzymatic processes involving cysteine and several enzymes, including cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE), cysteine aminotransferase (CAT), 3-mercaptopyruvate sulfurtransferase (3MST) and D-amino acid oxidase (DAO). The physiological and pathological effects of hydrogen sulfide (H2S) on various systems in the human body have led to extensive research efforts to develop appropriate methods to deliver H2S under conditions that mimic physiological settings and respond to various stimuli. These functions span a wide spectrum, ranging from effects on the endocrine system and cellular lifespan to protection of liver and kidney function. The exact physiological and hazardous thresholds of hydrogen sulfide (H2S) in the human body are currently not well understood and need to be researched in depth. This article provides an overview of the physiological significance of H2S in the human body. It highlights the various sources of H2S production in different situations and examines existing techniques for detecting this gas.

Combination of sulfide-driven partial denitrification with anammox enhanced by zeolite powder for autotrophic nitrogen and sulfide removal from wastewater

Sulfide-driven partial denitrification and anaerobic ammonia oxidizing (anammox) (SPDA) is a high-efficiency technology to achieve simultaneous nitrogen and sulfide removal. Nitrite accumulation from sulfide-driven partial denitrification is the key to achieve SPDA. Zeolite powder was added to strengthen the competition of anammox bacteria against nitrite. The nitrogen removal rate (NRR) and partial denitrification efficiency in reactor was 5.18 kg-N m-3d-1 and 92.3% during 180 days of operation, higher than those without zeolite powder, indicating an improving contribution of zeolite powder. Metabolomics analysis revealed zeolite powder addition enhanced the metabolisms of amino acids, nicotinate and porphyrin through increasing glutamate content, and improved EPS secretion, heme c content and particle size. Besides, high ammonia enriched by zeolite powder was conducive to improve anammox activity and NRR. This study provides the metabolic insights into the mechanism of zeolite powder enhancing SPDA, which is meaningful towards overcoming the limitations in practical application of SDPA.

Hydrogen Sulfide Ligation in Hemoglobin I of Lucina pectinata─A QM/MM and Local Mode Study

In this study, we investigated the interaction between the H2S ligand and the heme pocket of hemoglobin I (HbI) of Lucina pectinata for the wild-type protein; three known mutations where distal glutamine is replaced by hydrophobic valine (Gln64Val) and hydrophilic histidine in both protonation forms (Gln64Hisϵ and Gln64Hisδ); five known mutations of the so-called phenyl cage, replacing the hydrophobic phenylalanines Phe29 and Phe43 with tyrosine (Tyr), valine (Val), or leucine (Leu); and two additional mutations, Phe68Tyr and Phe68Val, in order to complement previous studies with new insights about the binding mechanism at the molecular level. A particular focus was on the intrinsic strengths of the chemical bonds involved, utilizing local vibrational force constants based on combined quantum mechanical-molecular mechanical calculations. Wild-type protein and mutations clustered into two distinct groups: Group 1 protein systems with a proton acceptor in the distal protein pocket, close to one of the H2S bonds, and Group 2 protein systems without a hydrogen acceptor close by in the active site of the protein. According to our results, the interactions between H2S and HbI of Lucina pectinata involve two important elements, namely, binding of H2S to Fe of the heme group, followed by the proton transfer from the HS bond to the distal residue. The distal residue is additionally stabilized by a second proton transfer from the distal residue to COO- of the propionate group in heme. We could identify the FeS bond as a key player and discovered that the strength of this bond depends on two mutual factors, namely, the strength of the HS bond involved in the proton transfer and the electrostatic field of the protein pocket qualifying the FeS bond as a sensitive probe for monitoring changes in H2S ligation upon protein mutations. We hope our study will inspire and guide future experimental studies, targeting new promising mutations such as Phe68Tyr, Phe68Val, or Phe43Tyr/Phe68Val.

Selection on standing genetic variation mediates convergent evolution in extremophile fish

Hydrogen sulfide is a toxic gas that disrupts numerous biological processes, including energy production in the mitochondria, yet fish in the Poecilia mexicana species complex have independently evolved sulfide tolerance several times. Despite clear evidence for convergence at the phenotypic level in these fishes, it is unclear if the repeated evolution of hydrogen sulfide tolerance is the result of similar genomic changes. To address this gap, we used a targeted capture approach to sequence genes associated with sulfide processes and toxicity from five sulfidic and five nonsulfidic populations in the species complex. By comparing sequence variation in candidate genes to a reference set, we identified similar population structure and differentiation, suggesting that patterns of variation in most genes associated with sulfide processes and toxicity are due to demographic history and not selection. But the presence of tree discordance for a subset of genes suggests that several loci are evolving divergently between ecotypes. We identified two differentiation outlier genes that are associated with sulfide detoxification in the mitochondria that have signatures of selection in all five sulfidic populations. Further investigation into these regions identified long, shared haplotypes among sulfidic populations. Together, these results reveal that selection on standing genetic variation in putatively adaptive genes may be driving phenotypic convergence in this species complex.

Autocatalytic Mechanism in the Anaerobic Reduction of Metmyoglobin by Sulfide Species

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

Carboxyhemoglobin (COHb): Unavoidable Bystander or Protective Player?

Carbon monoxide (CO) is a cytoprotective endogenous gas that is ubiquitously produced by the stress response enzyme heme-oxygenase. Being a gas, CO rapidly diffuses through tissues and binds to hemoglobin (Hb) increasing carboxyhemoglobin (COHb) levels. COHb can be formed in erythrocytes or in plasma from cell-free Hb. Herein, it is discussed as to whether endogenous COHb is an innocuous and inevitable metabolic waste product or not, and it is hypothesized that COHb has a biological role. In the present review, literature data are presented to support this hypothesis based on two main premises: (i) there is no direct correlation between COHb levels and CO toxicity, and (ii) COHb seems to have a direct cytoprotective and antioxidant role in erythrocytes and in hemorrhagic models in vivo. Moreover, CO is also an antioxidant by generating COHb, which protects against the pro-oxidant damaging effects of cell-free Hb. Up to now, COHb has been considered as a sink for both exogenous and endogenous CO generated during CO intoxication or heme metabolism, respectively. Hallmarking COHb as an important molecule with a biological (and eventually beneficial) role is a turning point in CO biology research, namely in CO intoxication and CO cytoprotection.

Simultaneous sulfide and methane oxidation by an extremophile

Hydrogen sulfide (H₂S) and methane (CH₄) are produced in anoxic environments through sulfate reduction and organic matter decomposition. Both gases diffuse upwards into oxic zones where aerobic methanotrophs mitigate CH₄ emissions by oxidizing this potent greenhouse gas. Although methanotrophs in myriad environments encounter toxic H₂S, it is virtually unknown how they are affected. Here, through extensive chemostat culturing we show that a single microorganism can oxidize CH₄ and H₂S simultaneously at equally high rates. By oxidizing H₂S to elemental sulfur, the thermoacidophilic methanotroph Methylacidiphilum fumariolicum SolV alleviates the inhibitory effects of H₂S on methanotrophy. Strain SolV adapts to increasing H₂S by expressing a sulfide-insensitive ba₃-type terminal oxidase and grows as chemolithoautotroph using H₂S as sole energy source. Genomic surveys revealed putative sulfide-oxidizing enzymes in numerous methanotrophs, suggesting that H₂S oxidation is much more widespread in methanotrophs than previously assumed, enabling them to connect carbon and sulfur cycles in novel ways.

Polysulfide metabolizing enzymes influence SqrR-mediated sulfide-induced transcription by impacting intracellular polysulfide dynamics

Sulfide plays essential roles in controlling various physiological activities in almost all organisms. Although recent evidence has demonstrated that sulfide is endogenously generated and metabolized into polysulfides inside the cells, the relationship between polysulfide metabolism and polysulfide-sensing mechanisms is not well understood. To better define this interplay between polysulfide metabolism and sensing in cells, we investigated the role of polysulfide-metabolizing enzymes such as sulfide:quinone oxidoreductase (SQR) on the temporal dynamics of cellular polysulfide speciation and on the transcriptional regulation by the persulfide-responsive transcription factor SqrR in Rhodobacter capsulatus. We show that disruption of the sqr gene resulted in the loss of SqrR repression by exogenous sulfide at longer culture times, which impacts the speciation of intracellular polysulfides of Δsqr vs. wild-type strains. Both the attenuated response of SqrR and the change in polysulfide dynamics of the Δsqr strain is fully reversed by the addition to cells of cystine-derived polysulfides, but not by glutathione disulfide (GSSG)-derived polysulfides. Furthermore, cysteine persulfide (CysSSH) yields a higher rate of oxidation of SqrR relative to glutathione persulfide (GSSH), which leads to DNA dissociation in vitro. The oxidation of SqrR was confirmed by a mass spectrometry-based kinetic profiling strategy that showed distinct polysulfide-crosslinked products obtained with CysSSH vs. GSSH. Taken together, these results establish a novel association between the metabolism of polysulfides and the mechanisms for polysulfide sensing inside the cells.

The Pleiotropic Regulator AdpA Regulates the Removal of Excessive Sulfane Sulfur in Streptomyces coelicolor

Reactive sulfane sulfur (RSS), including persulfide, polysulfide, and elemental sulfur (S₈), has important physiological functions, such as resisting antibiotics in Pseudomonas aeruginosa and Escherichia coli and regulating secondary metabolites production in Streptomyces spp. However, at excessive levels it is toxic. Streptomyces cells may use known enzymes to remove extra sulfane sulfur, and an unknown regulator is involved in the regulation of these enzymes. AdpA is a multi-functional transcriptional regulator universally present in Streptomyces spp. Herein, we report that AdpA was essential for Streptomyces coelicolor survival when facing external RSS stress. AdpA deletion also resulted in intracellular RSS accumulation. Thioredoxins and thioredoxin reductases were responsible for anti-RSS stress via reducing RSS to gaseous hydrogen sulfide (H₂S). AdpA directly activated the expression of these enzymes at the presence of excess RSS. Since AdpA and thioredoxin systems are widely present in Streptomyces, this finding unveiled a new mechanism of anti-RSS stress by these bacteria.

Interactions of reactive sulfur species with metalloproteins

Reactive sulfur species (RSS) entail a diverse family of sulfur derivatives that have emerged as important effector molecules in H₂S-mediated biological events. RSS (including H₂S) can exert their biological roles via widespread interactions with metalloproteins. Metalloproteins are essential components along the metabolic route of oxygen in the body, from the transport and storage of O₂, through cellular respiration, to the maintenance of redox homeostasis by elimination of reactive oxygen species (ROS). Moreover, heme peroxidases contribute to immune defense by killing pathogens using oxygen-derived H₂O₂ as a precursor for stronger oxidants. Coordination and redox reactions with metal centers are primary means of RSS to alter fundamental cellular functions. In addition to RSS-mediated metalloprotein functions, the reduction of high-valent metal centers by RSS results in radical formation and opens the way for subsequent per- and polysulfide formation, which may have implications in cellular protection against oxidative stress and in redox signaling. Furthermore, recent findings pointed out the potential role of RSS as substrates for mitochondrial energy production and their cytoprotective capacity, with the involvement of metalloproteins. The current review summarizes the interactions of RSS with protein metal centers and their biological implications with special emphasis on mechanistic aspects, sulfide-mediated signaling, and pathophysiological consequences. A deeper understanding of the biological actions of reactive sulfur species on a molecular level is primordial in H₂S-related drug development and the advancement of redox medicine.

Intelligent polymeric hydrogen sulfide delivery systems for therapeutic applications

Hydrogen sulfide (H₂S) plays an important role in regulating various pathological processes such as protecting mammalian cell from harmful injuries, promoting tissue regeneration, and regulating the process of various diseases caused by physiological disorders. Studies have revealed that the physiological effects of H₂S are highly associated with its concentrations. At relatively low concentration, H₂S shows beneficial functions. However, long-time and high-dose donation of H₂S would inhibit regular biological process, resulting in cell dysfunction and apoptosis. To regulate the dosage of H₂S delivery for precision medicine, H₂S delivery systems with intelligent characteristics were developed and a variety of biocompatibility polymers have been utilized to establish intelligent polymeric H₂S delivery systems, with the abilities to specifically target the lesions, smartly respond to pathological microenvironments, as well as real-timely monitor H₂S delivery and lesion conditions by incorporating imaging-capable moieties. In this review, we focus on the design, preparation, and therapeutic applications of intelligent polymeric H₂S delivery systems in cardiovascular therapy, inflammatory therapy, tissue regenerative therapy, cancer therapy and bacteria-associated therapy. Strategies for precise H₂S therapies especially imaging-guided H₂S theranostics are highlighted. Since H₂S donors with stimuli-responsive characters are vital components for establishing intelligent H₂S delivery systems, the development of H₂S donors is also briefly introduced.

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H₂S is an endogenous gasotransmitter that plays important role in regulating various physiological and pathological pathways.

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Controlled H₂S delivery is vital since the therapeutic effects of H₂S are highly associated with its concentrations.

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Intelligent polymeric H₂S delivery systems possess specific targeting, stimuli responsive and imaging guided capabilities, representing a strategic option for next generation of therapies.

A hormesis-like effect of FeS on heterotrophic denitrification and its mechanisms

To alleviate the insufficiency of carbon source in sewage, many sulfur-containing inorganic electron donors were added into traditional heterotrophic denitrification process. However, the effects of extraneous inorganic electron donors on heterotrophic denitrification were still largely unknown. In this study, a hormesis-like effect of ferrous sulfide (FeS, a representative inorganic electron donors) on Paracoccus denitrificans was observed. Total nitrogen (TN) removal efficiency of P. denitrificans rose by 15% with the increase of FeS dosage from 0 to 0.3 g L^-1 (low level), whereas the TN removal significantly decreased to 53% as the dosage of FeS mounted up to 5.0 g L^-1 (high level). Furthermore, the impacts of FeS on glucose utilization and bacterial growth exhibited hormesis-like effects. A subsequent mechanistic study revealed that above influences were caused by its released ions (Fe²⁺, Fe³⁺, and S^2-) rather than particle size. Further study illustrated that low dosage of FeS released a small amount of Fe²⁺ and Fe³⁺, which provided sufficient electrons via promoting glucose utilization, then improved denitrification. Conversely, FeS with high dosage inhibited denitrification via its released S^2-, which suppressed the activity of key denitrifying enzymes rather than influenced glucose metabolism and electron provision. Our results provide an insight into improving denitrification efficiency of the mixotrophic process coexisting with autotrophic and heterotrophic denitrifiers.

Cysteine Biosynthesis in Campylobacter jejuni: Substrate Specificity of CysM and the Dualism of Sulfide

Campylobacter jejuni is a highly successful enteric pathogen with a small, host-adapted genome (1.64 Mbp, ~1650 coding genes). As a result, C. jejuni has limited capacity in numerous metabolic pathways, including sulfur metabolism. Unable to utilise ionic sulfur, C. jejuni relies on the uptake of exogenous cysteine and its derivatives for its supply of this essential amino acid. Cysteine can also be synthesized de novo by the sole cysteine synthase, CysM. In this study, we explored the substrate specificity of purified C. jejuni CysM and define it as an O-acetyl-L-serine sulfhydrylase with an almost absolute preference for sulfide as sulfur donor. Sulfide is produced in abundance in the intestinal niche C. jejuni colonises, yet sulfide is generally viewed as highly toxic to bacteria. We conducted a series of growth experiments in sulfur-limited media and demonstrate that sulfide is an excellent sulfur source for C. jejuni at physiologically relevant concentrations, combating the view of sulfide as a purely deleterious compound to bacteria. Nonetheless, C. jejuni is indeed inhibited by elevated concentrations of sulfide and we sought to understand the targets involved. Surprisingly, we found that inactivation of the sulfide-sensitive primary terminal oxidase, the cbb₃-type cytochrome c oxidase CcoNOPQ, did not explain the majority of growth inhibition by sulfide. Therefore, further work is required to reveal the cellular targets responsible for sulfide toxicity in C. jejuni.

Bioinformatic Characterization and Molecular Evolution of the Lucina pectinata Hemoglobins

(1) Introduction: Lucina pectinata is a clam found in sulfide-rich mud environments that has three hemoglobins believed to be responsible for the transport of hydrogen sulfide (HbILp) and oxygen (HbIILp and HbIIILp) to chemoautotrophic endosymbionts. The physiological roles and evolution of these globins in sulfide-rich environments are not well understood. (2) Methods: We performed bioinformatic and phylogenetic analyses with 32 homologous mollusk globin sequences. Phylogenetics suggests a first gene duplication resulting in sulfide binding and oxygen binding genes. A more recent gene duplication gave rise to the two oxygen-binding hemoglobins. Multidimensional scaling analysis of the sequence space shows evolutionary drift of HbIILp and HbIIILp, while HbILp was closer to the Calyptogena hemoglobins. Further corroboration is seen by conservation in the coding region of hemoglobins from L. pectinata compared to those from Calyptogena. (3) Conclusions: Presence of glutamine in position E7 in organisms living in sulfide-rich environments can be considered an adaptation to prevent loss of protein function. In HbILp a substitution of phenylalanine in position B10 is accountable for its unique reactivity towards H2S. It appears that HbILp has been changing over time, apparently not subject to functional constraints of binding oxygen, and acquired a unique function for a specialized environment.

Hydrogen sulfide poisoning in forensic pathology and toxicology: mechanism and metabolites quantification analysis

Historically, hydrogen sulfide (H₂S) poisoning has extremely high and irreparable mortality. Currently, the identification of H₂S poisoning needs to combine with the case scene analysis in forensic medicine. The anatomy of the deceased seldom had obvious features. There are also a few reports about H₂S poisoning in detail. As a result, we give a comprehensive analysis of the related knowledge on the forensic aspect of H₂S poisoning. Furthermore, we provide the analytical methods of H₂S and its metabolite-which may assist in H₂S poisoning identification.

Hypertension and renal disease programming: focus on the early postnatal period

The developmental origin of hypertension and renal disease is a concept highly supported by strong evidence coming from both human and animal studies. During development there are periods in which the organs are more vulnerable to stressors. Such periods of susceptibility are also called 'sensitive windows of exposure'. It was shown that as earlier an adverse event occurs; the greater are the consequences for health impairment. However, evidence show that the postnatal period is also quite important for hypertension and renal disease programming, especially in rodents because they complete nephrogenesis postnatally, and it is also important during preterm human birth. Considering that the developing kidney is vulnerable to early-life stressors, renal programming is a key element in the developmental programming of hypertension and renal disease. The purpose of this review is to highlight the great number of studies, most of them performed in animal models, showing the broad range of stressors involved in hypertension and renal disease programming, with a particular focus on the stressors that occur during the early postnatal period. These stressors mainly include undernutrition or specific nutritional deficits, chronic behavioral stress, exposure to environmental chemicals, and pharmacological treatments that affect some important factors involved in renal physiology. We also discuss the common molecular mechanisms that are activated by the mentioned stressors and that promote the appearance of these adult diseases, with a brief description on some reprogramming strategies, which is a relatively new and promising field to treat or to prevent these diseases.

Recent Development of the Molecular and Cellular Mechanisms of Hydrogen Sulfide Gasotransmitter

Hydrogen sulfide has been recently identified as the third biological gasotransmitter, along with the more well studied nitric oxide (NO) and carbon monoxide (CO). Intensive studies on its potential as a therapeutic agent for cardiovascular, inflammatory, infectious and neuropathological diseases have been undertaken. Here we review the possible direct targets of H₂S in mammals. H₂S directly interacts with reactive oxygen/nitrogen species and is involved in redox signaling. H₂S also reacts with hemeproteins and modulates metal-containing complexes. Once being oxidized, H₂S can persulfidate proteins by adding -SSH to the amino acid cysteine. These direct modifications by H₂S have significant impact on cell structure and many cellular functions, such as tight junctions, autophagy, apoptosis, vesicle trafficking, cell signaling, epigenetics and inflammasomes. Therefore, we conclude that H₂S is involved in many important cellular and physiological processes. Compounds that donate H₂S to biological systems can be developed as therapeutics for different diseases.

The greening reaction of skipjack tuna (Katsuwonus pelamis) metmyoglobin promoted by free cysteine during thermal treatment

Background

Tuna muscle greening is a problem that occurs after heating. A hypothesis has been postulated to address this problem, involving a conserved Cys residue at position 10 (Cys-10) present on tuna myoglobin (Mb) that is exposed during the thermic treatment, forming a disulfide bond with free cysteine (Cys) in the presence of trimethylamine oxide (TMAO), resulting in the greening of the tuna Mb.

Methods

We present a study using skipjack tuna (Katsuwonus pelamis) metmyoglobin (MbFe(III)-H₂O) where the effect of free Cys (1-6 mM), TMAO (1.33 mM), and catalase on the greening reaction (GR) was monitored by UV-vis spectrometry during thermal treatment at 60 °C for 30 min. Moreover, the participation of Cys-10 on the GR was evaluated after its blocking with N-ethymaleimide.

Results

The GR occurred in tuna MbFe(III)-H₂O after heat treatment with free Cys, forming sulfmyoglobin (MbFe(II)-S) as the responsible pigment for the tuna greening. However, the rate constants of MbFe(II)-S production depended on Cys concentration (up to 4 mM) and occurred regardless of the TMAO presence. We postulate that two consecutive reactions involve an intermediate ferrylmyoglobin (promoted by H₂O₂) species with a subsequent MbFe(II)-S formation since the presence of catalase fosters the reduction of the rate reaction. Moreover, GR occurred even with blocked Cys-10 residues in tuna Mb and horse Mb (without Cys in its sequence).

Discussion

We found that GR is not exclusive to tuna Mb´s, and it can be promoted in other muscle systems. Moreover, Cys and thermal treatment are indispensable for promoting this pigmentation anomaly.

Hydrogen sulfide decreases photodynamic therapy outcome through the modulation of the cellular redox state

Photodynamic therapy (PDT) is a non-surgical treatment that has been approved for its human medical use in many cancers. PDT involves the interaction of a photosensitizer (PS) with light. The amino acid 5- aminolevulinic acid (ALA) can be used as a pro-PS, leading to the synthesis of Protoporphyrin IX. Hydrogen sulfide (H₂S) is an endogenously produced gas that belongs to the gasotransmitter family, which can diffuse through biological membranes and have relevant physiological effects such as cardiovascular functions, vasodilatation, inflammation, cell cycle and neuro-modulation. It was also proposed to have cytoprotective effects. We aimed to study the modulatory effects of H₂S on ALAPDT in the mammary adenocarcinoma cell line LM2. Exposure of the cells to NaHS (donor of H₂S) in concentrations up to 10 mM impaired the response to ALA-PDT in a dose-dependent manner. The addition of 3 doses of NaHS showed the highest effect. This decreased response to the photodynamic treatment was correlated to an increase in the GSH levels, catalase activity, a dose dependent reduction of PpIX and increased intracellular ALA, decreased levels of oxidized proteins and a decrease of PDT-induced ROS. NaHS also reduced the levels of singlet oxygen in an in vitro assay. H₂S also protected other cells of different origins against PDT mediated by ALA and other PSs. These results suggest that H₂S has a role in the modulation of the redox state of the cells, and thus impairs the response to ALA-PDT through multifactor pathways. These findings could contribute to developing new strategies to improve the effectiveness of PDT particularly mediated by ALA or other ROS-related treatments.

Mycobacterium tuberculosis DosS binds H2S through its Fe3+ heme iron to regulate the DosR dormancy regulon

Mycobacterium tuberculosis (Mtb) senses and responds to host-derived gasotransmitters NO and CO via heme-containing sensor kinases DosS and DosT and the response regulator DosR. Hydrogen sulfide (H2S) is an important signaling molecule in mammals, but its role in Mtb physiology is unclear. We have previously shown that exogenous H2S can modulate expression of genes in the Dos dormancy regulon via an unknown mechanism(s). Here, we test the hypothesis that Mtb senses and responds to H2S via the DosS/T/R system. Using UV-Vis and EPR spectroscopy, we show that H2S binds directly to the ferric (Fe3+) heme of DosS (KDapp = 5.30 μM) but not the ferrous (Fe2+) form. No interaction with DosT(Fe2+-O2) was detected. We found that the binding of sulfide can slowly reduce the DosS heme iron to the ferrous form. Steered Molecular Dynamics simulations show that H2S, and not the charged HS- species, can enter the DosS heme pocket. We also show that H2S increases DosS autokinase activity and subsequent phosphorylation of DosR, and H2S-mediated increases in Dos regulon gene expression is lost in Mtb lacking DosS. Finally, we demonstrate that physiological levels of H2S in macrophages can induce DosR regulon genes via DosS. Overall, these data reveal a novel mechanism whereby Mtb senses and responds to a third host gasotransmitter, H2S, via DosS(Fe3+). These findings highlight the remarkable plasticity of DosS and establish a new paradigm for how bacteria can sense multiple gasotransmitters through a single heme sensor kinase.

Hydrogen sulfide and metal interaction: the pathophysiological implications

Hydrogen sulfide (H₂S), previously recognized as a toxic gas, has emerged as an important gaseous signaling molecule along with nitric oxide, carbon monoxide and also hydrogen. H₂S can be endogenously produced in the mammalian body at a very low level for various pathophysiological processes. Notably, H₂S can interact with several essential metals in the body such as iron, copper, nickel, and zinc to carry out specific functions. The interactions of H₂S with metal-binding proteins have been shown to aid in its signal transduction and cellular metabolism. In addition, H₂S is capable of providing a cytoprotective role against metal toxicity. As the research in the field of H₂S signaling in biology and medicine increases, much progresses have been developed for detecting H₂S via interaction with metals. In this review, the interaction of H₂S with metals, specifically in regard to metal-driven metabolism of H₂S, the protection against metal toxicity by H₂S and the detection of H₂S using metals will be discussed. Discovering the interactions of this gasotransmitter with metals is important for determining the mechanisms underlying the cellular functions of H₂S as well as developing novel therapeutic avenues.

Genome-Wide Analyses of Heat Shock Protein Superfamily Provide New Insights on Adaptation to Sulfide-Rich Environments in Urechis unicinctus (Annelida, Echiura)

The intertidal zone is a transitional area of the land-sea continuum, in which physical and chemical properties vary during the tidal cycle and highly toxic sulfides are rich in sediments due to the dynamic regimes. As a typical species thriving in this habitat, Urechis unicinctus presents strong sulfide tolerance and is expected to be a model species for sulfide stress research. Heat shock proteins (HSPs) consist of a large group of highly conserved molecular chaperones, which play important roles in stress responses. In this study, we systematically analyzed the composition and expression of HSPs in U. unicinctus. A total of eighty-six HSP genes from seven families were identified, in which two families, including sHSP and HSP70, showed moderate expansion, and this variation may be related to the benthic habitat of the intertidal zone. Furthermore, expression analysis revealed that almost all the HSP genes in U. unicinctus were significantly induced under sulfide stress, suggesting that they may be involved in sulfide stress response. Weighted gene co-expression network analysis (WGCNA) showed that 12 HSPs, including 5 sHSP and 4 HSP70 family genes, were highly correlated with the sulfide stress response which was distributed in steelblue and green modules. Our data indicate that HSPs, especially sHSP and HSP70 families, may play significant roles in response to sulfide stress in U. unicinctus. This systematic analysis provides valuable information for further understanding of the function of the HSP gene family for sulfide adaptation in U. unicinctus and contributes a better understanding of the species adaptation strategies of marine benthos in the intertidal zone.

Hydrogen Sulfide and the Kidney: Physiological Roles, Contribution to Pathophysiology, and Therapeutic Potential

Significance: Hydrogen sulfide (H₂S), the third member of the gasotransmitter family, has a broad spectrum of biological activities, including antioxidant and cytoprotective actions, as well as vasodilatory, anti-inflammatory and antifibrotic effects. New, significant aspects of H₂S biology in the kidney continue to emerge, underscoring the importance of this signaling molecule in kidney homeostasis, function, and disease. Recent Advances: H₂S signals via three main mechanisms, by maintaining redox balance through its antioxidant actions, by post-translational modifications of cellular proteins (S-sulfhydration), and by binding to protein metal centers. Important renal functions such as glomerular filtration, renin release, or sodium reabsorption have been shown to be regulated by H₂S, using either exogenous donors or by the endogenous-producing systems. Critical Issues: Lower H₂S levels are observed in many renal pathologies, including renal ischemia-reperfusion injury and obstructive, diabetic, or hypertensive nephropathy. Unraveling the molecular targets through which H₂S exerts its beneficial effects would be of great importance not only for understanding basic renal physiology, but also for identifying new pharmacological interventions for renal disease. Future Directions: Additional studies are needed to better understand the role of H₂S in the kidney. Mapping the expression pattern of H₂S-producing and -degrading enzymes in renal cells and generation of cell-specific knockout mice based on this information will be invaluable in the effort to unravel additional roles for H₂S in kidney (patho)physiology. With this knowledge, novel targeted more effective therapeutic strategies for renal disease can be designed. Antioxid. Redox Signal. 36, 220-243.

The Pathophysiology of H2S in Renal Glomerular Diseases

Renal glomerular diseases such as glomerulosclerosis and diabetic nephropathy often result in the loss of glomerular function and consequently end-stage renal disease. The glomerulus consists of endothelial cells, mesangial cells and glomerular epithelial cells also referred to as podocytes. A fine-tuned crosstalk between glomerular cells warrants control of growth factor synthesis and of matrix production and degradation, preserving glomerular structure and function. Hydrogen sulfide (H2S) belongs together with nitric oxide (NO) and carbon monoxide (CO) to the group of gasotransmitters. During the last three decades, these higher concentration toxic gases have been found to be produced in mammalian cells in a well-coordinated manner. Recently, it became evident that H2S and the other gasotransmitters share common targets as signalling devices that trigger mainly protective pathways. In several animal models, H2S has been demonstrated as a protective factor in the context of kidney disorders, in particular of diabetic nephropathy. Here, we focus on the synthesis and action of H2S in glomerular cells, its beneficial effects in the glomerulus and its action in the context of the other gaseous signalling molecules NO and CO.

Buffering Adaptive Immunity by Hydrogen Sulfide

T cell-mediated adaptive immunity is designed to respond to non-self antigens and pathogens through the activation and proliferation of various T cell populations. T helper 1 (Th1), Th2, Th17 and Treg cells finely orchestrate cellular responses through a plethora of paracrine and autocrine stimuli that include cytokines, autacoids, and hormones. Hydrogen sulfide (H₂S) is one of these mediators able to induce/inhibit immunological responses, playing a role in inflammatory and autoimmune diseases, neurological disorders, asthma, acute pancreatitis, and sepsis. Both endogenous and exogenous H₂S modulate numerous important cell signaling pathways. In monocytes, polymorphonuclear, and T cells H₂S impacts on activation, survival, proliferation, polarization, adhesion pathways, and modulates cytokine production and sensitivity to chemokines. Here, we offer a comprehensive review on the role of H₂S as a natural buffer able to maintain over time a functional balance between Th1, Th2, Th17 and Treg immunological responses.

H2S/Thiols, NO•, and NO-/HNO: Interactions with Iron Porphyrins

In the past decade, gasotransmitters NO^• and H₂S have been thoroughly studied in biological contexts, as their biosynthesis and physiological effects became known. Moreover, an additional intricate crosstalk reaction scheme between these compounds and related species is thought to exist as part of the cascade signaling processes in physiological conditions. In this context, heme enzymes, as modeled by iron porphyrins, play a central role in catalyzing the key interconversions involved. In this work, iron porphyrin interactions with sulfide and nitric-oxide-related species are described. The stability and reactivity of mixed ternary systems are also described, and future perspectives are discussed.

Exogenous sulphide donors modify the gene expression patterns of Atlantic salmon nasal leukocytes

Hydrogen sulphide (H₂S) is a known mediator of immunity, but the regulatory function of its exogenous form is not well understood in fish particularly in the mucosa. Here we report transcriptomic changes in the nasal leukocytes of Atlantic salmon (Salmo salar) following exposure to two forms of H₂S donors - the salt sodium hydrosulfide (NaHS) and the organic analogue morpholin-4-ium 4-methoxyphenyl (morpholino) phosphinodithioate (GYY4137). Nasal leukocytes were exposed to three concentrations (1, 10 and 100 μM) of either of the two H₂S forms for 24 h before the cells were checked for viability and collected for microarray analysis. Though cellular viability was minimally affected by the exposure to two H₂S donors, GYY4137-exposed cells exhibited reduced viability compared with the NaHS group at the highest dose. The H₂S-induced transcriptomic changes in the nasal leukocytes were concentration-dependent regardless of the sulphide forms. However, a larger number of differentially expressed genes (DEGs) were identified in the NaHS-exposed versus GYY4137-exposed groups across concentrations. In all comparisons, at least 53% of the DEGs identified were significantly upregulated. Gene ontology (GO) terms enriched in the lists of upregulated DEGs at higher concentrations included ferric iron binding. A comparison of the two H₂S forms showed a clear grouping of different GO terms relative to concentrations. Pathway enrichment analysis revealed a significant influence in VEGF ligand-receptor interactions, oxidative stress, innate and adaptive immunity, and interleukin signalling especially at higher concentrations. Congruence analysis demonstrated that there were 16 GO terms overlapping; of these, 12 were upregulated by both sulphide donors including several involving iron binding and transport. The study offers the first molecular insights into how fish nasal leukocytes respond to exogenous H₂S, and the results will be vital in resolving the regulatory function of H₂S on mucosal immunity in fish.

Oxidative Post-Translational Modifications: A Focus on Cysteine S-Sulfhydration and the Regulation of Endothelial Fitness

Significance: Changes in the oxidative balance can affect cellular physiology and adaptation through redox signaling. The endothelial cells that line blood vessels are particularly sensitive to reactive oxygen species, which can alter cell function by a number of mechanisms, including the oxidative post-translational modification (oxPTM) of proteins on critical cysteine thiols. Such modifications can act as redox-switches to alter the function of targeted proteins. Recent Advances: Mapping the cysteine oxPTM proteome and characterizing the effects of individual oxPTMs to gain insight into consequences for cellular responses has proven challenging. A recent addition to the list of reversible oxPTMs that contributes to cellular redox homeostasis is persulfidation or S-sulfhydration. Critical Issues: It has been estimated that up to 25% of proteins are S-sulfhydrated, making this modification almost as abundant as phosphorylation. In the endothelium, persulfides are generated by the trans-sulfuration pathway that catabolizes cysteine and cystathionine to generate hydrogen sulfide (H₂S) and H₂S-related sulfane sulfur compounds (H₂S_n). This pathway is of particular importance for the vascular system, as the enzyme cystathionine γ lyase (CSE) in endothelial cells accounts for a significant portion of total vascular H₂S/H₂S_n production. Future Directions: Impaired CSE activity in endothelial dysfunction has been linked with marked changes in the endothelial cell S-sulfhydrome and can contribute to the development of atherosclerosis and hypertension. It will be interesting to determine how changes in the S-sulfhydration of specific networks of proteins contribute to endothelial cell physiology and pathophysiology. Antioxid. Redox Signal. 35, 1494-1514.

Nitric oxide and hydrogen sulfide: Sibling rivalry in the family of epigenetic regulators

Nitric oxide (NO) and hydrogen sulfide (H₂S) were previously only known for their toxic properties. Now they are regarded as potent gaseous messenger molecules (gasotransmitters) that rapidly transverse cell membranes and transduce cellular signals through their chemical reactions and modifications to protein targets. Both are known to regulate numerous physiological functions including angiogenesis, vascular tone, and immune response, to name a few. NO and H₂S often work synergistically and in competition to regulate each other's synthesis, target protein activity via posttranslational modifications (PTMs), and chemical interactions. In addition to their canonical modes of action, increasing evidence has demonstrated that NO and H₂S share another signaling mechanism: epigenetic regulation. This review will compare and contrast biosynthesis and metabolism of NO and H₂S, their individual and shared interactions, and the growing body of evidence for their roles as endogenous epigenetic regulatory molecules.

Reactivity of inorganic sulfide species towards a pentacoordinated heme model system

The reactivity of inorganic sulfide towards ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate has been explored by means of visible absorption and resonance Raman spectroscopies. The reaction has been previously studied in buffered solutions at neutral pH and in the presence of excess sulfide, revealing the formation of a moderately stable hexacoordinated low spin ferric sulfide complex that yields the ferrous form in the hour's timescale. In the surfactant solution, instead, the ferrous form is rapidly formed. The spectroscopic characterization of the heme structure in the surfactant milieu revealed the stabilization of a major ferric mono-histidyl high spin heme, which may be ascribed to out of plane distortions prompting the detachment of the axially ligated water molecule, thus leading to a differential reactivity. The ferric bis(N-acetyl)- microperoxidase 11 in sodium dodecyl sulfate provides a model for pentacoordinated heme platforms with an imidazole-based ligand.

Exploration and verification of the feasibility of sulfide-driven partial denitrification coupled with anammox for wastewater treatment

Anaerobic ammonia oxidation (anammox) is a well-developed biotechnology for treating high-strength ammonium wastewaters. Recently, partial denitrification has been considered as an alternative to supply anammox with the required nitrite. In this study, a process of sulfide-driven partial denitrification and anammox (SPDA) was developed and operated continuously in an upflow anaerobic sludge blanket (UASB) reactor for 392 days. This reactor was fed with synthetic wastewater containing 100 mgN/L nitrate, 80 mgN/L ammonium and 20-80 mgS/L sulfide. After 160 days of operation, the reactor reached stable performance, and the nitrogen removal efficiency and rate were maintained at 80% and 0.29 kgN/(m³•d), respectively. The estimated nitrogen removal via anammox and sulfide-driven denitrification were 87.2% and 12.8%. Additional batch experiments were conducted to investigate the effects of sulfide on anammox and the mechanisms of nitrogen removal in the SPDA system. The following results were obtained: (1) sulfide had an inhibitory effect on the specific anammox activity with IC₅₀ of 9.7 mgS-H₂S/L. (2) The rapid oxidation of sulfide by sulfur-oxidizing bacteria (SOB) could relieve the toxic effects of sulfide on the anammox in the SPDA system. (3) Sulfide bio-oxidation was a two-step reaction with biologically produced elemental sulfur (BPS⁰) as the intermediate, and the second step using BPS⁰ as the electron donor, can efficiently produce nitrite via partial denitrification (NO₃^- → NO₂^-) as a supply for anammox. Finally, a high-throughput sequencing analysis identified Thiobacillus and Sulfurimonas as the dominant genera of SOB in the SPDA system, and Candidatus Kuenenia as the dominant anammox bacteria. Overall, this research gives the foundation for the practical application of sulfide-driven partial denitrification and anammox process in the future.

Understanding hydrogen sulfide signaling in neonatal airway disease

INTRODUCTION

Airway dysfunction leading to chronic lung disease is a common consequence of premature birth and mechanisms responsible for early and progressive airway remodeling are not completely understood. Current therapeutic options are only partially effective in reducing the burden of neonatal airway disease and premature decline of lung function. Gasotransmitter hydrogen sulfide (H2S) has been recently recognized for its therapeutic potential in lung diseases.

AREAS COVERED

Contradictory to its well-known toxicity at high concentrations, H2S has been characterized to have anti-inflammatory, antioxidant, and antiapoptotic properties at physiological concentrations. In the respiratory system, endogenous H2S production participates in late lung development and exogenous H2S administration has a protective role in a variety of diseases such as acute lung injury and chronic pulmonary hypertension and fibrosis. Literature searches performed using NCBI PubMed without publication date limitations were used to construct this review, which highlights the dichotomous role of H2S in the lung, and explores its promising beneficial effects in lung diseases.

EXPERT OPINION

The emerging role of H2S in pathways involved in chronic lung disease of prematurity along with its recent use in animal models of BPD highlight H2S as a potential novel candidate in protecting lung function following preterm birth.

Cysteine metabolic circuitries: druggable targets in cancer

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

Red Blood Cells as a "Central Hub" for Sulfide Bioactivity: Scavenging, Metabolism, Transport, and Cross-Talk with Nitric Oxide

Significance: Sulfide was revealed to be an endogenous signaling molecule regulating a plethora of cellular functions. It is involved in the regulation of fundamental processes, including blood pressure regulation, suspended animation, and metabolic activity of mitochondria, pain, and inflammation. The underlying biochemical pathways and pharmacological targets are still largely unidentified. Recent Advances: Red blood cells (RBCs) are known as oxygen transporters and were proposed to contribute to cardiovascular homeostasis by regulating nitric oxide (NO) metabolism, also via interaction of hemoglobin with nitrite and NO itself. Interestingly, recent evidence indicates that RBCs may also play a central role in systemic sulfide metabolism and homeostasis, and, potentially, in the crosstalk with NO. Heme-containing proteins such as hemoglobin were shown to be targeted by both NO and sulfide. In this article, we aim at revising and discussing the potential impact of RBCs on systemic sulfide metabolism in the cardiovascular system. Critical Issues: Although the synthetic pathways and the reactivity of hemoglobin and other heme proteins with sulfide and NO are known, the in vivo role of RBCs in sulfide metabolism, physiology, pharmacology, and its pathophysiological implications have not been characterized so far. Future Directions: To allow a better understanding of the role of RBCs in systemic sulfide metabolism and its cross-talk with NO, basic and translational science studies should be focused on dissecting the enzymatic and nonenzymatic sulfur metabolic pathways in RBCs in vivo and their impact on the cardiovascular system in animal models and clinical settings.

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

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

The Role of Host-Generated H2S in Microbial Pathogenesis: New Perspectives on Tuberculosis

For centuries, hydrogen sulfide (H₂S) was considered primarily as a poisonous gas and environmental hazard. However, with the discovery of prokaryotic and eukaryotic enzymes for H₂S production, breakdown, and utilization, H₂S has emerged as an important signaling molecule in a wide range of physiological and pathological processes. Hence, H₂S is considered a gasotransmitter along with nitric oxide (•NO) and carbon monoxide (CO). Surprisingly, despite having overlapping functions with •NO and CO, the role of host H₂S in microbial pathogenesis is understudied and represents a gap in our knowledge. Given the numerous reports that followed the discovery of •NO and CO and their respective roles in microbial pathogenesis, we anticipate a rapid increase in studies that further define the importance of H₂S in microbial pathogenesis, which may lead to new virulence paradigms. Therefore, this review provides an overview of sulfide chemistry, enzymatic production of H₂S, and the importance of H₂S in metabolism and immunity in response to microbial pathogens. We then describe our current understanding of the role of host-derived H₂S in tuberculosis (TB) disease, including its influences on host immunity and bioenergetics, and on Mycobacterium tuberculosis (Mtb) growth and survival. Finally, this review discusses the utility of H₂S-donor compounds, inhibitors of H₂S-producing enzymes, and their potential clinical significance.

The ratiometric fluorescent probes for monitoring the reactive inorganic sulfur species (RISS) signal in the living cell

RSSs (reactive sulfur species) and their metabolites, such as H₂S, S_n^2-, SO₃^2-/HSO₃^-, S₂O₄^2- and S₂O₅^2- (Reactive Inorganic Sulfur Species, RISSs), play a crucial role in the cushion against oxidative stress and the other physiological events. The molecular mechanisms how they affect cellular signaling and other physiological events remain largely unknown. To address their physiological functions, the techniques that can track their levels should be invaluable. Herein, six coumarin hemicyanine scaffolds (CH-RISSs) were synthesized and their fast and strong responses upon H₂S, S_n^2-, SO₃^2-, HSO₃^-, S₂O₄^2- and S₂O₅^2- (Reactive Inorganic Sulfur Species, RISSs) were clarified in the absorption (colorimetric) and fluorescence (ratiometric) spectra, which showed good stability in the physiological pH (7.4). Upon the analytes, the maxima absorption of CH-RISSs switched from ~580 nm to ~400 nm in the absorption spectra. The fluorescence of CH-RISSs depleted at 650-660 nm and increased at 480-505 nm upon the RISSs. Both of coumarin hemicyanine structures with C12 alkyl chain (CH-RISS-3 and CH-RISS-6) showed quick and robust ratiometric fluorescence switch in the living cell imaging. Access to the fluorescent probes for RISSs sets the stage for applying the developing technologies to probe reactive sulfur biology in living systems.

Deciphering the microbial community and functional genes response of anammox sludge to sulfide stress

Sulfide has attracted increasing attention due to its odor nuisance, toxicity and corrosion. Although variations in the nitrogen removal performance of anammox under sulfide stress have been reported previously, understanding the microorganisms at the molecular level is of greater significance. This study first deciphered the microbial community and functional gene response of anammox sludge to sulfide stress. Results showed that 20 mg L^-1 sulfide could reduce specific anammox activity by 61.7%. The protein-like substances within extracellular polymeric substances were quenched at the end of the experiment. Moreover, the relative abundance of Candidatus Kuenenia significantly decreased from 28.7% to 6.4% while Thiobacillus increased from 0 to 7.2% due to sulfide stress. Furthermore, the abundances of functional genes (hzsA, hdh, nirK and nirS) significantly decreased when the sulfide concentration reached 20 mg L^-1. These findings provide a further theoretical basis for the anammox process for nitrogen removal from wastewater containing sulfide.

A detailed analysis of the spin-crossover reaction of H2S binding to heme and the six-coordinated FeP(Im)-HS- porphyrin complex

The potential energy surfaces of the H₂S binding to iron-porphyrin (FeP) with the imidazole (Im) ligand via intersystem crossings are investigated by using density functional theory. The minimum energy intersystem crossing point (MEISCP) between the quintet and triplet states (MEISCP_TQ) for the Fe(II)P(Im)-H₂S complex is located at a Fe-S distance of 3.39 Å with only 1.1 kcal/mol above the quintet state minimum. The second spin-crossover point, where a change from the triplet to the singlet state occurs, comes at a much shorter Fe-S distance of 2.79 Å, and the MEISCP_ST is located at 3.7 kcal/mol above the triplet state minimum. The nature of the chemical bonding along the Fe-S reaction coordinate from the ground state singlet to the quintet state along the path to the separated species is analyzed. An inspection of the vibrational modes reveals that the largest contribution to the triplet-quintet transition around the quintet and triplet state minimum comes from the symmetric shrinking of the pyrrole units of the porphyrin ring, indicating that the related reaction coordinate plays a main role in the intersystem crossing. The fully optimized structures of the Fe(II)P(Im)-HS^- complex corresponding to three different spin multiplicities (M = 1, 3, 5) are characterized by a bent Fe-H-S conformation. The binding of the hydrosulfide anion to Fe(II)P(Im) in the quintet state induces a 0.2 Å displacement of the Fe atom out of the nitrogen porphyrin (N_pyr) plane. The fully optimized structure of the ground state of Fe(II)P(Im)-HS^- agrees well with experimental data for the corresponding heme models.

An Update on Thiol Signaling: S-Nitrosothiols, Hydrogen Sulfide and a Putative Role for Thionitrous Acid

Long considered vital to antioxidant defenses, thiol chemistry has more recently been recognized to be of fundamental importance to cell signaling. S-nitrosothiols—such as S-nitrosoglutathione (GSNO)—and hydrogen sulfide (H₂S) are physiologic signaling thiols that are regulated enzymatically. Current evidence suggests that they modify target protein function primarily through post-translational modifications. GSNO is made by NOS and other metalloproteins; H₂S by metabolism of cysteine, homocysteine and cystathionine precursors. GSNO generally acts independently of NO generation and has a variety of gene regulatory, immune modulator, vascular, respiratory and neuronal effects. Some of this physiology is shared with H₂S, though the mechanisms differ. Recent evidence also suggests that molecules resulting from reactions between GSNO and H₂S, such as thionitrous acid (HSNO), could also have a role in physiology. Taken together, these data suggest important new potential targets for thiol-based drug development.

Hemeproteins as Targets for Sulfide Species

Significance: Sulfides are endogenous and ubiquitous signaling species that share the hemeproteins as biochemical targets with O₂, nitric oxide, and carbon monoxide. The description of the binding mechanisms is mandatory to anticipate the biochemical relevance of the interaction. Recent Advances: The binding of sulfide to ferric hemeproteins has been described in more than 40 systems, including native proteins, mutants, and model systems. Mechanisms of sulfide binding to ferric hemeproteins have been examined by a combination of kinetic and computational experiments. The distal control of the association process, dissected into the migration of the ligand to the active site and the binding event, reveals that neutral hydrogen sulfide (H₂S) reaches the active site and is the predominant binding ligand, while the HS^- is excluded by the protein matrix. Experiments with model compounds, devoid of a protein scaffold, reveal that both H₂S and HS^- can bind the ferric heme if accessing the site. A critical role of the proximal ligand in the prevention of the metal-centered reduction has been experimentally assessed. For metmyoglobin and methemoglobin, the coordination of sulfide leads to noncanonical functions: sulfide storage and its oxidative detoxification have been evidenced under physiological and excess sulfide concentrations, respectively. Critical Issues: The bound species is suggested to predominate in the monoprotonated form, although spectroscopic evidence is pending. Future Directions: A description of the role of hemeproteins as biochemical targets for inorganic sulfide requires understanding the reactivity of bound sulfide, for example: the metal-centered reduction, the reaction with excess sulfide, oxidants, or other gasotransmitters, among other biomolecules.

A Novel Possible Role for Met Hemoglobin as Carrier of Hydrogen Sulfide in the Blood

Significance: Along with other gasotransmitters nitric oxide (NO) and carbon monoxide, hydrogen sulfide (H₂S) has recently emerged as an important signaling molecule with a particularly complex metabolism. Endogenous H₂S reacts with multiple cellular targets, including protein ferric heme groups, to elicit physiological responses, such as regulation of local blood flow. Recent Advances: Recent in vitro evidence suggests that H₂S at low physiological concentrations is carried in the blood as bound to the small fraction of oxidized ferric hemoglobin (metHb). A relatively stable metHb-sulfide complex forms when H₂S and purified metHb react in vitro, with an affinity within the in vivo physiological range of sulfide in the blood. Formation and subsequent redox metabolism of metHb-sulfide complex have also been confirmed in isolated intact red blood cells (RBCs) containing enhanced metHb levels. Thus, H₂S may function as an endocrine signaling molecule and elicit responses at sites away from the site of production. In addition, metHb, considered as an inert or pathological hemoglobin derivative, may have a novel potential physiological role in the transport of H₂S in the blood. Critical Issues: The transport of H₂S in the blood mediated by metHb would represent an O₂-independent pH-dependent mechanism for the blood-mediated control of blood flow and as such it is critical to understand the in vivo significance of this transport. Future Directions: Major challenges must be resolved to understand how metHb may carry H₂S in the RBCs, in particular determination of metHb-sulfide levels in the blood and identification of targets in the vasculature.

Hydrogen sulfide stimulates Mycobacterium tuberculosis respiration, growth and pathogenesis

Hydrogen sulfide (H₂S) is involved in numerous pathophysiological processes and shares overlapping functions with CO and •NO. However, the importance of host-derived H₂S in microbial pathogenesis is unknown. Here we show that Mtb-infected mice deficient in the H₂S-producing enzyme cystathionine β-synthase (CBS) survive longer with reduced organ burden, and that pharmacological inhibition of CBS reduces Mtb bacillary load in mice. High-resolution respirometry, transcriptomics and mass spectrometry establish that H₂S stimulates Mtb respiration and bioenergetics predominantly via cytochrome bd oxidase, and that H₂S reverses •NO-mediated inhibition of Mtb respiration. Further, exposure of Mtb to H₂S regulates genes involved in sulfur and copper metabolism and the Dos regulon. Our results indicate that Mtb exploits host-derived H₂S to promote growth and disease, and suggest that host-directed therapies targeting H₂S production may be potentially useful for the management of tuberculosis and other microbial infections.

The importance of host-produced hydrogen sulfide (H₂S) in microbial pathogenesis is poorly understood. Here, Saini et al. show that H₂S alters Mycobacterium tuberculosis (Mtb) central metabolism, stimulates respiration to promote growth and TB disease, and upregulates the Dos regulon.