Redox chemistry and chemical biology of H2S,hydropersulfides, and derived species: Implications of their possible biological activity and utility

Hydrogen sulfide (H₂S) is an endogenously generated and putative signaling/effector molecule. Despite its numerous reported functions, the chemistry by which it elicits its functions is not understood. Moreover, recent studies allude to the existence of other sulfur species besides H₂S that may play critical physiological roles. Herein, the basic chemical biology of H₂S as well as other related or derived species is discussed and reviewed. This review particularly focuses on the per- and polysulfides which are likely in equilibrium with free H₂S and which may be important biological effectors themselves.     

Hydrogen sulfide,a signaling molecule in plant stress responses

Gaseous molecules, such as hydrogen sulfide(H_2S)and nitric oxide(NO), are crucial players in cellular and(patho)physiological processes in biological systems. The biological functions of these gaseous molecules, which were first discovered and identified as gasotransmitters in animals, have received unprecedented attention from plant scientists in recent decades. Researchers have arrived at the consensus that H_2S is synthesized endogenously and serves as a signaling molecule throughout the plant life cycle.However, the mechanisms of H_2S action in redox biology is still largely unexplored. This review highlights what we currently know about the characteristics and biosynthesis of H_2S in plants. Additionally,we summarize the role of H_2S in plant resistance to abiotic stress. Moreover, we propose and discuss possible redox-dependent mechanisms by which H_2S regulates plant physiology.     

Diacyl disulfides as the precursors for hydrogen persulfide (H2S2)

While hydrogen polysulfides (H₂S_n, n ≥ 2) are believed to play regulatory roles in biology, their fundamental chemistry and reactivity are still poorly understood. Compounds that can produce H₂S_n are useful tools. In this work we found that H₂S₂ could be effectively produced from diacyl disulfide precursors, triggered by certain nucleophiles, in both aqueous solutions and organic solvents. This method was used to explore redox reactions of H₂S₂, such as scavenging 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) and reduction of tetrazines.     

Reaction of Hydrogen Sulfide with Disulfide and Sulfenic Acid to Form the Strongly Nucleophilic Persulfide

Hydrogen sulfide (H₂S) is increasingly recognized to modulate physiological processes in mammals through mechanisms that are currently under scrutiny. H₂S is not able to react with reduced thiols (RSH). However, H₂S, more precisely HS⁻, is able to react with oxidized thiol derivatives. We performed a systematic study of the reactivity of HS⁻ toward symmetric low molecular weight disulfides (RSSR) and mixed albumin (HSA) disulfides. Correlations with thiol acidity and computational modeling showed that the reaction occurs through a concerted mechanism. Comparison with analogous reactions of thiolates indicated that the intrinsic reactivity of HS⁻ is 1 order of magnitude lower than that of thiolates. In addition, H₂S is able to react with sulfenic acids (RSOH). The rate constant of the reaction of H₂S with the sulfenic acid formed in HSA was determined. Both reactions of H₂S with disulfides and sulfenic acids yield persulfides (RSSH), recently identified post-translational modifications. The formation of this derivative in HSA was determined, and the rate constants of its reactions with a reporter disulfide and with peroxynitrite revealed that persulfides are better nucleophiles than thiols, which is consistent with the α effect. Experiments with cells in culture showed that treatment with hydrogen peroxide enhanced the formation of persulfides. Biological implications are discussed. Our results give light on the mechanisms of persulfide formation and provide quantitative evidence for the high nucleophilicity of these novel derivatives, setting the stage for understanding the contribution of the reactions of H₂S with oxidized thiol derivatives to H₂S effector processes.     

Achieving a Quasi-Solid-State Conversion of Polysulfides via Building High Efficiency Heterostructure for Room Temperature Na–S Batteries

The practical application of room temperature sodium–sulfur (RT Na–S) batteries are prevented by the sulfur insulation, the severe shuttling effect of high-order sodium polysulfides (Na₂S_n, 4 ≤ n ≤ 8), and the sluggish reaction kinetics. Therefore, designing an ideal host material to suppress the polysulfides shuttle process and accelerate the redox reactions of soluble NaPSs to Na₂S₂/Na₂S is of paramount importance for RT Na–S batteries. Here, a quasi-solid-state transformation of NaPSs is realized by building high efficiency MoC-W₂C heterostructure in freestanding multichannel carbon nanofibers via electrospinning and calcination methods (MoC-W₂C-MCNFs). The multichannel carbon nanofibers are interlinked micro-mesoporous structures that can accommodate volume change of electrode materials and confine the entire redox process of NaPSs (restraining the polysulfides shuttle process). Meanwhile, the MoC-W₂C heterostructure with abundant heterointerfaces can facilitate electron/ion transport and accelerate conversion of NaPSs. Consequently, the S/MoC-W₂C-MCNFs cathode delivers a high capacity of 640 mAh g⁻¹ after 500 cycles at 0.2 A g⁻¹ and an excellent reversible performance of 200 mAh g⁻¹ after ultralong 3500 cycles at 4 A g⁻¹. What's more, the heterostructure catalytic mechanism (a quasi-solid-state transformation) is proposed and confirmed in carbonate electrolyte by combining experimentally and theoretically.     

Role of Polysulfides in Reduction of Elemental Sulfur by the Hyperthermophilic Archaebacterium Pyrococcus furiosus

Polysulfides formed through the breakdown of elemental sulfur or other sulfur compounds were found to be reduced to H₂S by the hyperthermophilic archaebacterium Pyrococcus furiosus during growth. Metabolism of polysulfides by the organism was dissimilatory, as no incorporation of ³⁵S-labeled elemental sulfur was detected. However, [³⁵S]cysteine and [³⁵S]methionine were incorporated into cellular protein. Contact between the organism and elemental sulfur is not necessary for metabolism. The sulfide generated from metabolic reduction of polysulfides dissociates to a strong nucleophile, HS⁻, which in turn opens up the S₈ elemental sulfur ring. In addition to H₂S, P. furiosus cultures produced methyl mercaptan in a growth-associated fashion.     

兼性厌氧细菌硫化氢的产生机理及其生理功能

硫化氢(H_2S)是继一氧化氮(NO)和一氧化碳(CO)后发现的第3种气态信号分子,但其细菌生理学研究才刚刚起步。本文根据作者对奥内达希瓦氏菌的研究,结合新近文献,就细菌的H_2S产生机理及其生理功能作了较为全面的阐述。细菌的H_2S产生途径主要有2条,一是通过降解半胱氨酸产生,二是通过厌氧呼吸产生。产生的H_2S除可为互生性微生物提供能源、供氢体和无机矿质营养外,还具有抑制竞争性微生物的生长,有效占领生态位的作用。H_2S在氧化应答中也起着重要的作用,一方面可抑制过氧化氢酶活性,增加过氧化氢对细菌的杀灭效果;另一方面可作为信号分子激活细菌的氧化应答,诱导拮抗系统的表达,保护细胞免受氧化损伤。这两种看似"矛盾"的作用与H_2S的处理时间有关:短时间处理以抑制为主,而延长处理时间则以保护为主。细菌H_2S产生机理及生理功能的阐明可为硫元素生物地球化学循环规律的揭示和感染性病原细菌的控制提供有益的参考。     

Regulation of cardiovascular cell function by hydrogen sulfide (H2S)

Since the discovery of endogenously‐produced hydrogen sulfide (H₂S) in various tissues, there has been an explosion of interest in H₂S as a biological mediator alongside other gaseous mediators, nitric oxide and carbon monoxide. The identification of enzyme‐regulated H₂S synthetic pathways in the cardiovascular system has led to a number of studies examining specific regulatory actions of H₂S. We review evidence showing that endogenously‐generated and exogenously‐administered H₂S exerts a wide range of actions in vascular and myocardial cells including vasodilator/vasoconstrictor effects via modification of the smooth muscle tone, induction of apoptosis and anti‐proliferative responses in the smooth muscle cells, angiogenic actions, effects relevant to inflammation and shock, and cytoprotection in models of myocardial ischemia‐reperfusion injury. Several molecular mechanisms of action of H₂S have been described. These include interactions of H₂S with NO, redox regulation of multiple signaling proteins and regulation of K_ATP channel opening. The gaps in our current understanding of precise mechanisms, the absence of selective pharmacological tools and the limited availability of H₂S measurement techniques for living tissues, leave many questions about physiological and pathophysiological roles of H₂S unanswered at present. Nevertheless, this area of investigation is advancing rapidly. We believe H₂S holds promise as an endogenous mediator controlling a wide range of cardiovascular cell functions and integrated responses under both physiological and pathological conditions and may be amenable to therapeutic manipulation. Copyright © 2010 John Wiley & Sons, Ltd.     

Is hydrogen sulfide a circulating “gasotransmitter” in vertebrate blood?

Hydrogen sulfide (H₂S) is gaining acceptance as a signaling molecule and has been shown to elicit a variety of biological effects at concentrations between 10 and 1000 μmol/l. Dissolved H₂S is a weak acid in equilibrium with HS⁻ and S²⁻ and under physiological conditions these species, collectively referred to as sulfide, exist in the approximate ratio of 20% H₂S, 80% HS⁻ and 0% S²⁻. Numerous analyses over the past 8 years have reported plasma or blood sulfide concentrations also in this range, typically between 30 and 300 μmol/l, thus supporting the biological studies. However, there is some question whether or not these concentrations are physiological. First, many of these values have been obtained from indirect methods using relatively harsh chemical conditions. Second, most studies conducted prior to 2000 failed to find blood sulfide in micromolar concentrations while others showed that radiolabeled ³⁵S-sulfide is rapidly removed from blood and that mammals have a relatively high capacity to metabolize exogenously administered sulfide. Very recent studies using H₂S gas-sensing electrodes to directly measure sulfide in plasma or blood, or HPLC analysis of head-space gas, have also indicated that sulfide does not circulate at micromolar levels and is rapidly consumed by blood or tissues. Third, micromolar concentrations of sulfide in blood or exhaled air should be, but are not, malodorous. Fourth, estimates of dietary sulfur necessary to sustain micromolar levels of plasma sulfide greatly exceed the daily intake. Collectively, these studies imply that many of the biological effects of sulfide are only achieved at supra-physiological concentrations and they question whether circulating sulfide is a physiologically relevant signaling molecule. This review examines the blood/plasma sulfide measurements that have been reported over the past 30 years from the perspective of the analytical methods used and the potential sources of error.     

Prodrugs of sulfide and persulfide species: Implications in their different pharmacological activities

Reactive sulfur species (RSS), such as H₂S, hydrogen polysulfide (H₂S_n, n ≥ 2), and hydropersulfides (RSS_nH, n ≥ 1), are known to mediate diverse signaling pathways and possess a plethora of exciting therapeutic opportunities. Historically, due to the rapid inter-conversion among those species in vivo, the biological differences of distinct sulfur species were often overlooked. These species were considered to enrich the global sulfur pool in almost an equal fashion. However, advancement in this field has revealed that sulfur species at different oxidation states result in different pharmacological effects including scavenging reactive oxygen species (ROS), activating ion channels, and exhibiting analgesic effects. Here, we summarize recent advances in studying the biological and pharmacological differences of distinct sulfur species; discuss this phenomenon from the view of chemical properties and sulfur signaling pathways; and lay out a roadmap to transforming such new knowledge into general principles in developing sulfur-based therapeutics.     

Endogenous production of hydrogen sulfide in isolated bovine eye

Hydrogen sulfide (H₂S) is a novel gasotransmitter with physiological and pathological functions in vascular homeostasis, cardiovascular system and central nervous system. In the present study, we determined the endogenous levels of H₂S in various tissues of the bovine eye. We also examined the basal levels of H₂S in response to donors (sodium hydrosulfide, NaHS and sodium sulfide, Na₂S), substrate (l-cysteine), inhibitors (propargylglycine, PAG and aminooxyacetic acid, AOA) and activator (S-adenosyl-l-methionine, SAM) of this gas in the bovine retina. H₂S was measured using a well established spectrophotometric method. The highest concentration of endogenous H₂S was detected in cornea (19 ± 2.85 nmoles/mg protein, n = 6) and retina (17 ± 2.1 nmoles/mg protein, n = 6). Interestingly, H₂S was not present in vitreous humor. The inhibitors of CSE and CBS; PAG (1 mM) and AOA (1 mM), significantly attenuated the production of H₂S in the bovine retina by 56.8 and 42%, respectively. On the other hand the activator of CBS; SAM (100 μM), H₂S donors; NaHS (1 μM) and Na₂S (100 μM), significantly increased endogenous levels of H₂S in bovine retina. l-cysteine (10–300 μM) produced a significant (P < 0.05) concentration-dependent increase in H₂S levels reaching a maximal at 300 μM. We conclude that H₂S is endogenously produced in various tissues of the isolated bovine eye. Moreover, endogenous levels of H₂S are enhanced in the presence of substrate (l-cysteine), an activator of CBS (SAM) and H₂S donors but are blocked by inhibitors of enzymes that synthesize this gas in neural retina.     

Hydrogen sulfide and nitric oxide metabolites in the blood of free-ranging brown bears and their potential roles in hibernation

During winter hibernation, brown bears (Ursus arctos) lie in dens for half a year without eating while their basal metabolism is largely suppressed. To understand the underlying mechanisms of metabolic depression in hibernation, we measured type and content of blood metabolites of two ubiquitous inhibitors of mitochondrial respiration, hydrogen sulfide (H₂S) and nitric oxide (NO), in winter-hibernating and summer-active free-ranging Scandinavian brown bears. We found that levels of sulfide metabolites were overall similar in summer-active and hibernating bears but their composition in the plasma differed significantly, with a decrease in bound sulfane sulfur in hibernation. High levels of unbound free sulfide correlated with high levels of cysteine (Cys) and with low levels of bound sulfane sulfur, indicating that during hibernation H₂S, in addition to being formed enzymatically from the substrate Cys, may also be regenerated from its oxidation products, including thiosulfate and polysulfides. In the absence of any dietary intake, this shift in the mode of H₂S synthesis would help preserve free Cys for synthesis of glutathione (GSH), a major antioxidant found at high levels in the red blood cells of hibernating bears. In contrast, circulating nitrite and erythrocytic S-nitrosation of glyceraldehyde-3-phosphate dehydrogenase, taken as markers of NO metabolism, did not change appreciably. Our findings reveal that remodeling of H₂S metabolism and enhanced intracellular GSH levels are hallmarks of the aerobic metabolic suppression of hibernating bears.     

The reaction of H(2)S with oxidized thiols: generation of persulfides and implications to H(2)S biology

Hydrogen sulfide is an endogenously generated molecule with many reported physiological functions. Although several biological targets have been proposed, the biochemical mechanisms by which it elicits activity are not established. Thus, in an effort to begin to delineate the fundamental biological chemistry of H₂S, we have examined the reaction of H₂S with oxidized thiols and thiol proteins in order to determine whether persulfide formation occurs, is stable and how this may affect protein function. We have found that persulfides are easily generated, relatively stable and can alter enzyme activity. Moreover, we have begun to develop methodology for in situ generation of persulfides to facilitate further study of this potentially important species.     

H2S and its role in redox signaling

Hydrogen sulfide (H₂S) has emerged as an important gaseous signaling molecule that is produced endogenously by enzymes in the sulfur metabolic network. H₂S exerts its effects on multiple physiological processes important under both normal and pathological conditions. These functions include neuromodulation, regulation of blood pressure and cardiac function, inflammation, cellular energetics and apoptosis. Despite the recognition of its biological importance and its beneficial effects, the mechanism of H₂S action and the regulation of its tissue levels remain unclear in part owing to its chemical and physical properties that render handling and analysis challenging. Furthermore, the multitude of potential H₂S effects has made it difficult to dissect its signaling mechanism and to identify specific targets. In this review, we focus on H₂S metabolism and provide an overview of the recent literature that sheds some light on its mechanism of action in cellular redox signaling in health and disease. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.     

Structural Transformation and Physical Properties of a Hydrogel-Forming Peptide Studied by NMR,Transmission Electron Microscopy,and Dynamic Rheometer

Peptide-based hydrogels are attractive biological materials. Study of their self-assembly pathways from their monomer structures is important not only for undertaking the rational design of peptide-based materials, but also for understanding their biological functions and the mechanism of many human diseases relative to protein aggregation. In this work, we have monitored the conformation, morphological, and mechanical properties of a hydrogel-forming peptide during hydrogelation in different dimethylsulfoxide (DMSO)/H₂O solutions. The peptide shows nanofiber morphologies in DMSO/H₂O solution with a ratio lower than 4:1. Increased water percentage in the solution enhanced the hydrogelation rate and gel strength. One-dimensional and two-dimensional proton NMR and electron microscopy studies performed on the peptide in DMSO/H₂O solution with different ratios indicate that the peptide monomer tends to adopt a more helical structure during the hydrogelation as the DMSO/H₂O ratio is reduced. Interestingly, at the same DMSO/H₂O ratio, adding Ca²⁺ not only promotes peptide hydrogelation and gel strength, but also leads to special shear-thinning and recovery properties of the hydrogel. Without changing the peptide conformation, Ca²⁺ binds to the charged Asp residues and induces the change of interfiber interactions that play an important role in hydrogel properties.     

The role of mitochondrial reactive oxygen species,NO and H2S in ischaemia/reperfusion injury and cardioprotection

Redox signalling in mitochondria plays an important role in myocardial ischaemia/reperfusion (I/R) injury and in cardioprotection. Reactive oxygen and nitrogen species (ROS/RNS) modify cellular structures and functions by means of covalent changes in proteins including among others S‐nitros(yl)ation by nitric oxide (NO) and its derivatives, and S‐sulphydration by hydrogen sulphide (H₂S). Many enzymes are involved in the mitochondrial formation and handling of ROS, NO and H₂S under physiological and pathological conditions. In particular, the balance between formation and removal of reactive species is impaired during I/R favouring their accumulation. Therefore, various interventions aimed at decreasing mitochondrial ROS accumulation have been developed and have shown cardioprotective effects in experimental settings. However, ROS, NO and H₂S play also a role in endogenous cardioprotection, as in the case of ischaemic pre‐conditioning, so that preventing their increase might hamper self‐defence mechanisms. The aim of the present review was to provide a critical analysis of formation and role of reactive species, NO and H₂S in mitochondria, with a special emphasis on mechanisms of injury and protection that determine the fate of hearts subjected to I/R. The elucidation of the signalling pathways of ROS, NO and H₂S is likely to reveal novel molecular targets for cardioprotection that could be modulated by pharmacological agents to prevent I/R injury.     

Revisiting the Relation Between Species Diversity and Information Theory

The Shannon information function (H) has been extensively used in ecology as a statistic of species diversity. Yet, the use of Shannon diversity index has also been criticized, mainly because of its ambiguous ecological interpretation and because of its relatively great sensitivity to the relative abundances of species in the community. In my opinion, the major shortcoming of the traditional perspective (on the possible relation of species diversity with information theory) is that species need for an external receiver (the scientist or ecologist) to exist and transmit information. Because organisms are self-catalized replicating structures that can transmit genotypic information to offspring, it should be evident that any single species has two possible states or alternatives: to be or not to be. In other words, species have no need for an external receiver since they are their own receivers. Therefore, the amount of biological information (at the species scale) in a community with one only species would be log₂ 2¹ = 1 { \log }_{2} 2^{1} = 1 species, and not log₂ 1 = 0 { \log }_{2} 1 = 0 bits as in the traditional perspective. Moreover, species diversity appears to be a monotonic increasing function of log₂ 2^\textS { \log }_{2} 2^{{\text{S}}} (or S) when all species are equally probable (S being species richness), and not a function of log₂ \text S { \log }_{2} {\text{ S}} as in the traditional perspective. To avoid the noted shortcoming, we could use 2^H (instead of H) for calculating species diversity and species evenness (= 2^H/S). However, owing to the relatively great sensitivity of H to the relative abundances of species in the community, the value of species dominance (= 1 − 2^H/S) is unreasonably high when differences between dominant and subordinate species are considerable, thereby lowering the value of species evenness and diversity. This unsatisfactory behaviour is even more evident for Simpson index and related algorithms. I propose the use of other statistics for a better analysis of community structure, their relationship being: species evenness + species dominance = 1; species diversity × species uniformity = 1; and species diversity = species richness × species evenness.     

Hydrogen sulphide reduced the accumulation of lipid droplets in cardiac tissues of db/db mice via Hrd1 S-sulfhydration

Accumulation of lipid droplets (LDs) induces cardiac dysfunctions in type 2 diabetes patients. Recent studies have shown that hydrogen sulphide (H₂S) ameliorates cardiac functions in db/db mice, but its regulation on the formation of LDs in cardiac tissues is unclear. Db/db mice were injected with NaHS (40 μmol·kg^-1) for twelve weeks. H9c2 cells were treated with high glucose (40 mmol/L), oleate (200 µmol/L), palmitate (200 µmol/L) and NaHS (100 µmol/L) for 48 hours. Plasmids for the overexpression of wild-type Hrd1 and Hrd1 mutated at Cys115 were constructed. The interaction between Hrd1 and DGAT1 and DGAT2, the ubiquitylation level of DGAT1 and 2, the S-sulfhydration of Hrd1 were measured. Exogenous H₂S ameliorated the cardiac functions, decreased ER stress and reduced the number of LDs in db/db mice. Exogenous H₂S could elevate the ubiquitination level of DGAT 1 and 2 and increased the expression of Hrd1 in cardiac tissues of db/db mice. The S-sulfhydration of Hrd1 by NaHS enhanced the interaction between Hrd1 and DGAT1 and 2 to inhibit the formation of LD. Our findings suggested that H₂S modified Hrd1 S-sulfhydration at Cys115 to reduce the accumulation of LDs in cardiac tissues of db/db mice.     

Gase als zelluläre Signalstoffe. Gasotransmitter

Gasotransmitters – gases as cellular signalling molecules The gases nitric oxide (NO), carbon monoxide (CO) and hydrogen sulphide (H₂S) because of their capacity as signalling molecules have been now collectively termed “gasotransmitters”. These gases play an important role in inter‐ and intracellular signalling, as in the digestive, respiratory or urogenital tract, in controlling heart activity or in nerve function. Research now tries to work out functions and further details about the mechanism of action of these gases and their implications for physiology, pathophysiology and pharmacology. The most recent candidate, H2S, is found in high concentrations in the brain and in the testis and hence is involved in learning/memory and in reproductive functions. The development of new substances interfering with the production of H₂S or its targets may constitute an interesting pharmacological potential.     

Role of hydrogen sulfide donors in cancer development and progression

In recent years, a vast number of potential cancer therapeutic targets have emerged. However, developing efficient and effective drugs for the targets is of major concern. Hydrogen sulfide (H₂S), one of the three known gasotransmitters, is involved in the regulation of various cellular activities such as autophagy, apoptosis, migration, and proliferation. Low production of H₂S has been identified in numerous cancer types. Treating cancer cells with H₂S donors is the common experimental technique used to improve H₂S levels; however, the outcome depends on the concentration/dose, time, cell type, and sometimes the drug used. Both natural and synthesized donors are available for this purpose, although their effects vary independently ranging from strong cancer suppressors to promoters. Nonetheless, numerous signaling pathways have been reported to be altered following the treatments with H₂S donors which suggest their potential in cancer treatment. This review will analyze the potential of H₂S donors in cancer therapy by summarizing key cellular processes and mechanisms involved.