Hydrogen Sulfide and Persulfides Oxidation by Biologically Relevant Oxidizing Species

Synthesis, Characterization, and Reactivity of a Synthetic End-On Cobalt(II) Alkyl Persulfide Complex as a Model Platform for Thiolate Persulfidation

Persulfides (RSS-) are ubiquitous source of sulfides (S2-) in biology, and interactions between RSS- and bioinorganic metal centers play critical roles in biological hydrogen sulfide (H2S) biogenesis, signaling, and catabolism. Here, we report the use of contact-ion stabilized [Na(15-crown-5)][tBuSS] (1) as a simple synthon to access rare metal alkyl persulfide complexes and to investigate the reactivity of RSS- with transition metal centers to provide insights into metal thiolate persulfidation, including the fundamental difference between alkyl persulfides and alkyl thiolates. Reaction of 1 with [CoII(TPA)(OTf)]+ afforded the η1-alkyl persulfide complex [CoII(TPA)(SStBu)]+ (2), which was characterized by X-ray crystallography, UV-vis spectroscopy, and Raman spectroscopy. RSS- coordination to the Lewis acidic Co2+ center provided additional stability to the S-S bond, as evidenced by a significant increase in the Raman stretching frequency for 2 (vS-S = 522 cm-1, ΔvS-S = 66 cm-1). The effect of persulfidation on metal center redox potentials was further elucidated using cyclic voltammetry, in which the Co2+ → Co3+ oxidation potential of 2 (Ep,a = +89 mV vs SCE) is lowered by nearly 700 mV when compared to the corresponding thiolate complex [CoII(TPA)(StBu)]+ (3) (Ep,a = +818 mV vs SCE), despite persulfidation being generally seen as an oxidative post-translational modification. The reactivity of 2 toward reducing agents including PPh3, BH4-, and biologically relevant thiol reductant DTT led to different S2- output pathways, including formation of a dinuclear 2Co-2SH complex [CoII2(TPA)2(μ2-SH)2]2+(4).

Exploring the Mechanism of H2S Synthesis in Male Bactrian Camel Poll Glands Based on Data Independent Acquisition Proteomics and Non-Targeted Metabolomics

During estrus, the poll glands of male Bactrian Camels (Camelus Bactrianus) become slightly raised, exuding a large amount of pale yellow watery secretion with a characteristic odor that may contain hydrogen sulfide (H2S). However, whether H2S can be synthesized in the poll glands of male Bactrian Camels and its role in inducing camel estrus remains unclear. This study aimed to identify differentially expressed proteins (DEPs) and signaling pathways in the poll gland tissues of male Bactrian Camels using data independent acquisition (DIA) proteomics. Additionally, gas chromatography-mass spectrometry (GC-MS) was performed to identify differentially expressed metabolites (DEMs) in the neck hair containing secretions during estrus in male Bactrian Camels, to explore the specific expression patterns and mechanisms in the poll glands of camels during estrus. The results showed that cystathionine-γ-lyase (CTH) and cystathionine-β-synthase (CBS), which are closely related to H2S synthesis in camel poll glands during estrus, were mainly enriched in glycine, serine, and threonine metabolism, amino acid biosynthesis, and metabolic pathways. In addition, both enzymes were widely distributed and highly expressed in the acinar cells of poll gland tissues in camels during estrus. Meanwhile, the neck hair secretion contains high levels of amino acids, especially glycine, serine, threonine, and cystathionine, which are precursors for H2S biosynthesis. These results demonstrate that the poll glands of male Bactrian Camels can synthesize and secrete H2S during estrus. This study provides a basis for exploring the function and mechanism of H2S in the estrus of Bactrian Camels.

Widespread S-persulfidation in activated macrophages as a protective mechanism against oxidative-inflammatory stress

Acute inflammatory responses often involve the production of reactive oxygen and nitrogen species by innate immune cells, particularly macrophages. How activated macrophages protect themselves in the face of oxidative-inflammatory stress remains a long-standing question. Recent evidence implicates reactive sulfur species (RSS) in inflammatory responses; however, how endogenous RSS affect macrophage function and response to oxidative and inflammatory insults remains poorly understood. In this study, we investigated the endogenous pathways of RSS biogenesis and clearance in macrophages, with a particular focus on exploring how hydrogen sulfide (H2S)-mediated S-persulfidation influences macrophage responses to oxidative-inflammatory stress. We show that classical activation of mouse or human macrophages using lipopolysaccharide and interferon-γ (LPS/IFN-γ) triggers substantial production of H2S/RSS, leading to widespread protein persulfidation. Biochemical and proteomic analyses revealed that this surge in cellular S-persulfidation engaged ∼2% of total thiols and modified over 800 functionally diverse proteins. S-persulfidation was found to be largely dependent on the cystine importer xCT and the H2S-generating enzyme cystathionine γ-lyase and was independent of changes in the global proteome. We further investigated the role of the sulfide-oxidizing enzyme sulfide quinone oxidoreductase (SQOR), and found that it acts as a negative regulator of S-persulfidation. Elevated S-persulfidation following LPS/IFN-γ stimulation or SQOR inhibition was associated with increased resistance to oxidative stress. Upregulation of persulfides also inhibited the activation of the macrophage NLRP3 inflammasome and provided protection against inflammatory cell death. Collectively, our findings shed light on the metabolism and effects of RSS in macrophages and highlight the crucial role of persulfides in enabling macrophages to withstand and alleviate oxidative-inflammatory stress.

Sodium nitroprusside modulates oxidative and nitrosative processes in Lycopersicum esculentum L. under drought stress

KEY MESSAGE

Sodium nitroprusside mediates drought stress responses in tomatoes by modulating nitrosative and oxidative pathways, highlighting the interplay between nitric oxide, hydrogen sulfide, and antioxidant systems for enhanced drought tolerance. While nitric oxide (NO), a signalling molecule, enhances plant tolerance to abiotic stresses, its precise contribution to improving tomato tolerance to drought stress (DS) through modulating oxide-nitrosative processes is not yet fully understood. We aimed to examine the interaction of NO and nitrosative signaling, revealing how sodium nitroprusside (SNP) could mitigate the effects of DS on tomatoes. DS-seedlings endured 12% polyethylene glycol (PEG) in a 10% nutrient solution (NS) for 2 days, then transitioned to half-strength NS for 10 days alongside control plants. DS reduced total plant dry weight, chlorophyll a and b, Fv/Fm, leaf water potential (ΨI), and relative water content, but improved hydrogen peroxide (H2O2), proline, and NO content. The SNP reduced the DS-induced H2O2 generation by reducing thiol (-SH) and the carbonyl (-CO) groups. SNP increased not only NO but also the activity of L-cysteine desulfhydrase (L-DES), leading to the generation of H2S. Decreases in S-nitrosoglutathione reductase (GSNOR) and NADPH oxidase (NOX) suggest a potential regulatory mechanism in which S-nitrosylation [formation of S-nitrosothiol (SNO)] may influence protein function and signaling pathways during DS. Moreover, SNP improved ascorbate (AsA) and glutathione (GSH) and reduced oxidized glutathione (GSSG) levels in tomato plants under drought. Furthermore, the interaction of NO and H2S, mediated by L-DES activity, may serve as a vital cross-talk mechanism impacting plant responses to DS. Understanding these signaling interactions is crucial for developing innovative drought-tolerance strategies in crops.

Fluctuating redox conditions accelerate the electron storage and transfer in magnetite and production of dark hydroxyl radicals

Magnetite (Fe3O4), known as a geo-battery that can store and transfer electrons, often co-occurs with sulfide in subsurface environments with fluctuating redox conditions. However, little is known about how fluctuating redox conditions (e.g., sulfidation-oxidation) affect the electron storage and transfer in Fe3O4 that was associated with the production of dark hydroxyl radicals (⋅OH) and the oxidation of dissolved organic matter (DOM). This study revealed that Fe3O4 sulfidated by sulfide (S-Fe3O4) at neutral pH exhibited higher ⋅OH production upon oxygenation than Fe3O4, in which the cumulative ⋅OH concentration increased with increasing initial S/Fe ratio (≤ 0.50), sulfidation duration and number of sulfidation-oxidation cycle. X-ray photoelectron spectroscopy and wet-chemical analyses of Fe and S species of S-Fe3O4 showed that sulfidation enables electron storage in Fe3O4 by increasing both structural and surface Fe(II). Sulfide was converted into S0, acid volatile sulfur (AVS), and chromium-reducible sulfur (CRS) during Fe3O4 sulfidation. S-Fe3O4 with lower AVS/CRS ratio exhibited higher reactivity to produce ⋅OH, indicating the important role of CRS in transferring electrons from Fe(II) to O2. Based on quenching experiments and electron paramagnetic resonance analysis, a one-step two-electron transfer mechanism was proposed for O2 reduction during S-Fe3O4 oxygenation, and surface-bound rather than free ⋅OH were identified as the primary reactive oxygen species. The ⋅OH from S-Fe3O4 oxygenation was shown to be efficient in degradation of DOM. Overall, these results suggested that sulfidation-oxidation can accelerate the electron storage and transfer in Fe3O4 for dark ⋅OH production, having an important impact on the carbon cycling in subsurface environments.

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.

Regulation of Mitochondrial Respiration by Hydrogen Sulfide

Hydrogen sulfide (H2S), the third gasotransmitter, has positive roles in animals and plants. Mitochondria are the source and the target of H2S and the regulatory hub in metabolism, stress, and disease. Mitochondrial bioenergetics is a vital process that produces ATP and provides energy to support the physiological and biochemical processes. H2S regulates mitochondrial bioenergetic functions and mitochondrial oxidative phosphorylation. The article summarizes the recent knowledge of the chemical and biological characteristics, the mitochondrial biosynthesis of H2S, and the regulatory effects of H2S on the tricarboxylic acid cycle and the mitochondrial respiratory chain complexes. The roles of H2S on the tricarboxylic acid cycle and mitochondrial respiratory complexes in mammals have been widely studied. The biological function of H2S is now a hot topic in plants. Mitochondria are also vital organelles regulating plant processes. The regulation of H2S in plant mitochondrial functions is gaining more and more attention. This paper mainly summarizes the current knowledge on the regulatory effects of H2S on the tricarboxylic acid cycle (TCA) and the mitochondrial respiratory chain. A study of the roles of H2S in mitochondrial respiration in plants to elucidate the botanical function of H2S in plants would be highly desirable.

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.

Characterization of Glutathione Dithiophosphates as Long-Acting H2S Donors

Considering the important cytoprotective and signaling roles but relatively narrow therapeutic index of hydrogen sulfide (H₂S), advanced H₂S donors are required to achieve a therapeutic effect. In this study, we proposed glutathione dithiophosphates as new combination donors of H₂S and glutathione. The kinetics of H₂S formation in dithiophosphate solutions suggested a continuous H₂S release by the donors, which was higher for the dithiophosphate of reduced glutathione than oxidized glutathione. The compounds, unlike NaHS, inhibited the proliferation of C2C12 myoblasts at submillimolar concentrations due to an efficient increase in intracellular H₂S. The H₂S donors more profoundly affected reactive oxygen species and reduced glutathione levels in C2C12 myocytes, in which these parameters were elevated compared to myoblasts. Oxidized glutathione dithiophosphate as well as control donors exerted antioxidant action toward myocytes, whereas the effect of reduced glutathione dithiophosphate at (sub-)micromolar concentrations was rather modulating. This dithiophosphate showed an enhanced negative inotropic effect mediated by H₂S upon contraction of the atrial myocardium, furthermore, its activity was prolonged and reluctant for washing. These findings identify glutathione dithiophosphates as redox-modulating H₂S donors with long-acting profile, which are of interest for further pharmacological investigation.

Emerging Chemical Biology of Protein Persulfidation

Significance: Protein persulfidation (the formation of RSSH), an evolutionarily conserved oxidative posttranslational modification in which thiol groups in cysteine residues are converted into persulfides, has emerged as one of the main mechanisms through which hydrogen sulfide (H2S) conveys its signaling. Recent Advances: New methodological advances in persulfide labeling started unraveling the chemical biology of this modification and its role in (patho)physiology. Some of the key metabolic enzymes are regulated by persulfidation. RSSH levels are important for the cellular defense against oxidative injury, and they decrease with aging, leaving proteins vulnerable to oxidative damage. Persulfidation is dysregulated in many diseases. Critical Issues: A relatively new field of signaling by protein persulfidation still has many unanswered questions: the mechanism(s) of persulfide formation and transpersulfidation and the identification of "protein persulfidases," the improvement of methods to monitor RSSH changes and identify protein targets, and understanding the mechanisms through which this modification controls important (patho)physiological functions. Future Directions: Deep mechanistic studies using more selective and sensitive RSSH labeling techniques will provide high-resolution structural, functional, quantitative, and spatiotemporal information on RSSH dynamics and help with better understanding how H2S-derived protein persulfidation affects protein structure and function in health and disease. This knowledge could pave the way for targeted drug design for a wide variety of pathologies. Antioxid. Redox Signal. 39, 19-39.

Insights into self-degradation of cysteine esters and amides under physiological conditions yield new cleavable chemistry

An unprecedented H₂S release from cysteine esters and amides (CysO/NHR) under physiological conditions was discovered and the plausible mechanism was proposed. Alkylation of the amino moiety of cysteine esters enables the H₂S release to be tuned and further provides support to the mechanistic insights. This discovery not only provides new insights into several fundamental science issues including non-enzymatic H₂S-produced pathways, but also inspires new tunable cleavable motifs for sustained release of arylthiols and even for prodrug design.

Mitochondrial H2S donor AP39 induces stomatal closure by modulating guard cell mitochondrial activity

Hydrogen sulfide (H2S) is a gaseous signaling molecule involved in numerous physiological processes in plants, including gas exchange with the environment through the regulation of stomatal pore width. Guard cells (GCs) are pairs of specialized epidermal cells that delimit stomatal pores and have a higher mitochondrial density and metabolic activity than their neighboring cells. However, there is no clear evidence on the role of mitochondrial activity in stomatal closure induction. In this work, we showed that the mitochondrial-targeted H2S donor AP39 induces stomatal closure in a dose-dependent manner. Experiments using inhibitors of the mitochondrial electron transport chain (mETC) or insertional mutants in cytochrome c (CYTc) indicated that the activity of mitochondrial CYTc and/or complex IV are required for AP39-dependent stomatal closure. By using fluorescent probes and genetically encoded biosensors we reported that AP39 hyperpolarized the mitochondrial inner potential (Δψm) and increased cytosolic ATP, cytosolic hydrogen peroxide levels, and oxidation of the glutathione pool in GCs. These findings showed that mitochondrial-targeted H2S donors induce stomatal closure, modulate guard cell mETC activity, the cytosolic energetic and oxidative status, pointing to an interplay between mitochondrial H2S, mitochondrial activity, and stomatal closure.

Oxidants and Antioxidants in the Redox Biochemistry of Human Red Blood Cells

Red blood cells (RBCs) are exposed to both external and internal sources of oxidants that challenge their integrity and compromise their physiological function and supply of oxygen to tissues. Autoxidation of oxyhemoglobin is the main source of endogenous RBC oxidant production, yielding superoxide radical and then hydrogen peroxide. In addition, potent oxidants from other blood cells and the surrounding endothelium can reach the RBCs. Abundant and efficient enzymatic systems and low molecular weight antioxidants prevent most of the damage to the RBCs and also position the RBCs as a sink of vascular oxidants that allow the body to maintain a healthy circulatory system. Among the antioxidant enzymes, the thiol-dependent peroxidase peroxiredoxin 2, highly abundant in RBCs, is essential to keep the redox balance. A great part of the RBC antioxidant activity is supported by an active glucose metabolism that provides reducing power in the form of NADPH via the pentose phosphate pathway. There are several RBC defects and situations that generate oxidative stress conditions where the defense mechanisms are overwhelmed, and these include glucose-6-phosphate dehydrogenase deficiencies (favism), hemoglobinopathies like sickle cell disease and thalassemia, as well as packed RBCs for transfusion that suffer from storage lesions. These oxidative stress-associated pathologies of the RBCs underline the relevance of redox balance in these anucleated cells that lack a mechanism of DNA-inducible antioxidant response and rely on a complex and robust network of antioxidant systems.

The Human Mercaptopyruvate Sulfurtransferase TUM1 Is Involved in Moco Biosynthesis, Cytosolic tRNA Thiolation and Cellular Bioenergetics in Human Embryonic Kidney Cells

Sulfur is an important element that is incorporated into many biomolecules in humans. The incorporation and transfer of sulfur into biomolecules is, however, facilitated by a series of different sulfurtransferases. Among these sulfurtransferases is the human mercaptopyruvate sulfurtransferase (MPST) also designated as tRNA thiouridine modification protein (TUM1). The role of the human TUM1 protein has been suggested in a wide range of physiological processes in the cell among which are but not limited to involvement in Molybdenum cofactor (Moco) biosynthesis, cytosolic tRNA thiolation and generation of H₂S as signaling molecule both in mitochondria and the cytosol. Previous interaction studies showed that TUM1 interacts with the L-cysteine desulfurase NFS1 and the Molybdenum cofactor biosynthesis protein 3 (MOCS3). Here, we show the roles of TUM1 in human cells using CRISPR/Cas9 genetically modified Human Embryonic Kidney cells. Here, we show that TUM1 is involved in the sulfur transfer for Molybdenum cofactor synthesis and tRNA thiomodification by spectrophotometric measurement of the activity of sulfite oxidase and liquid chromatography quantification of the level of sulfur-modified tRNA. Further, we show that TUM1 has a role in hydrogen sulfide production and cellular bioenergetics.

Synthesis and neuroprotective effects of H2S-donor-peptide hybrids on hippocampal neuronal cells

Hydrogen sulfide (H₂S) has emerged as an endogenous signaling molecule that functions in many physiological and pathological processes of human cells in health and disease, including neuromodulation and neuroprotection, inflammation, angiogenesis, and vasorelaxation. The limited clinical applications of current H₂S donors have led to the development of H₂S donor hybrid compounds that combine current H₂S donors with bioactive molecules. Finely tuned multi-targeting hybrid molecules have been shown to have complementary neuroprotective effects against reactive oxygen species (ROS)-induced oxidative stress. In this study, we developed hybrid molecules combining a dithiolethione-based slow-releasing H₂S donor that exerts neuroprotective effects, with the tripeptides glycyl-L-histidyl-l-lysine (GHK) and L-alanyl-L-cystinyl-l-glutamine (ACQ), two natural products that exhibit powerful antioxidant effects. In particular, a hybrid combination of a dithiolethione-based slow-releasing H₂S donor and ACQ exhibited significant neuroprotective effects against glutamate-induced oxidative damage in HT22 hippocampal neuronal cells. This hybrid remarkably suppressed Ca²⁺ accumulation and ROS production. Furthermore, it efficiently inhibited apoptotic neuronal cell death by blocking apoptosis-inducing factor release and its translocation to the nucleus. These results indicate that the hybrid efficiently inhibited apoptotic neuronal cell damage by complementary neuroprotective actions.

Generation and Physiology of Hydrogen Sulfide and Reactive Sulfur Species in Bacteria

Sulfur is not only one of the most abundant elements on the Earth, but it is also essential to all living organisms. As life likely began and evolved in a hydrogen sulfide (H₂S)-rich environment, sulfur metabolism represents an early form of energy generation via various reactions in prokaryotes and has driven the sulfur biogeochemical cycle since. It has long been known that H₂S is toxic to cells at high concentrations, but now this gaseous molecule, at the physiological level, is recognized as a signaling molecule and a regulator of critical biological processes. Recently, many metabolites of H₂S, collectively called reactive sulfur species (RSS), have been gradually appreciated as having similar or divergent regulatory roles compared with H₂S in living organisms, especially mammals. In prokaryotes, even in bacteria, investigations into generation and physiology of RSS remain preliminary and an understanding of the relevant biological processes is still in its infancy. Despite this, recent and exciting advances in the fields are many. Here, we discuss abiotic and biotic generation of H₂S/RSS, sulfur-transforming enzymes and their functioning mechanisms, and their physiological roles as well as the sensing and regulation of H₂S/RSS.

Reactive sulfur species and their significance in health and disease

Reactive sulfur species (RSS) have been recognized in the last two decades as very important molecules in redox regulation. They are involved in metabolic processes and, in this way, they are responsible for maintenance of health. This review summarizes current information about the essential biological RSS, including H₂S, low molecular weight persulfides, protein persulfides as well as organic and inorganic polysulfides, their synthesis, catabolism and chemical reactivity. Moreover, the role of RSS disturbances in various pathologies including vascular diseases, chronic kidney diseases, diabetes mellitus Type 2, neurological diseases, obesity, chronic obstructive pulmonary disease and in the most current problem of COVID-19 is presented. The significance of RSS in aging is also mentioned. Finally, the possibilities of using the precursors of various forms of RSS for therapeutic purposes are discussed.

Hydropersulfides Inhibit Lipid Peroxidation and Protect Cells from Ferroptosis

Hydropersulfides (RSSH) are believed to serve important roles in vivo, including as scavengers of damaging oxidants and electrophiles. The α-effect makes RSSH not only much better nucleophiles than thiols (RSH), but also much more potent H-atom transfer agents. Since HAT is the mechanism of action of the most potent small-molecule inhibitors of phospholipid peroxidation and associated ferroptotic cell death, we have investigated their reactivity in this context. Using the fluorescence-enabled inhibited autoxidation (FENIX) approach, we have found RSSH to be highly reactive toward phospholipid-derived peroxyl radicals (k_inh = 2 × 10⁵ M^-1 s^-1), equaling the most potent ferroptosis inhibitors identified to date. Related (poly)sulfide products resulting from the rapid self-reaction of RSSH under physiological conditions (e.g., disulfide, trisulfide, H₂S) are essentially unreactive, but combinations from which RSSH can be produced in situ (i.e., polysulfides with H₂S or thiols with H₂S₂) are effective. In situ generation of RSSH from designed precursors which release RSSH via intramolecular substitution or hydrolysis improve the radical-trapping efficiency of RSSH by minimizing deleterious self-reactions. A brief survey of structure-reactivity relationships enabled the design of new precursors that are more efficient. The reactivity of RSSH and their precursors translates from (phospho)lipid bilayers to cell culture (mouse embryonic fibroblasts), where they were found to inhibit ferroptosis induced by inactivation of glutathione peroxidase-4 (GPX4) or deletion of the gene encoding it. These results suggest that RSSH and the pathways responsible for their biosynthesis may act as a ferroptosis suppression system alongside the recently discovered FSP1/ubiquinone and GCH1/BH₄/DHFR systems.

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

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

Esterase-Activated Perthiocarbonate Persulfide Donors Provide Insights into Persulfide Persistence and Stability

Persulfides (RSSH) are important reactive sulfur species (RSS) that are intertwined with the biological functions of hydrogen sulfide (H₂S). The direct study of persulfides is difficult, however, due to their both nucleophilic and electrophilic character, which leads to the generation of an equilibrium of different RSS. To investigate the effects of persulfides directly, especially in biological systems, persulfide donors are needed to generate persulfides in situ. Here, we report the synthesis of esterase-activated perthiocarbonate persulfide donors and investigate the effects of structural modifications on persulfide release. Although steric bulk of the ester did not significantly alter persulfide release kinetics, increased steric bulk of the thiol increased the persulfide release rate. In addition, we found that the steric bulk and identity of the thiol significantly impact persulfide persistence. Further mechanistic investigations into different competing reaction pathways from perthiocarbonates revealed that multiple RSS can be delivered (i.e., H₂S, COS, or RSSH) depending on the persulfide donor structure and activator identity.

Radiolysis Studies of Oxidation and Nitration of Tyrosine and Some Other Biological Targets by Peroxynitrite-Derived Radicals

The widespread interest in free radicals in biology extends far beyond the effects of ionizing radiation, with recent attention largely focusing on reactions of free radicals derived from peroxynitrite (i.e., hydroxyl, nitrogen dioxide, and carbonate radicals). These radicals can easily be generated individually by reactions of radiolytically-produced radicals in aqueous solutions and their reactions can be monitored either in real time or by analysis of products. This review first describes the general principles of selective radical generation by radiolysis, the yields of individual species, the advantages and limitations of either pulsed or continuous radiolysis, and the quantitation of oxidizing power of radicals by electrode potentials. Some key reactions of peroxynitrite-derived radicals with potential biological targets are then discussed, including the characterization of reactions of tyrosine with a model alkoxyl radical, reactions of tyrosyl radicals with nitric oxide, and routes to nitrotyrosine formation. This is followed by a brief outline of studies involving the reactions of peroxynitrite-derived radicals with lipoic acid/dihydrolipoic acid, hydrogen sulphide, and the metal chelator desferrioxamine. For biological diagnostic probes such as ‘spin traps’ to be used with confidence, their reactivities with radical species have to be characterized, and the application of radiolysis methods in this context is also illustrated.

Methods for Suppressing Hydrogen Sulfide in Biological Systems

Significance: Hydrogen sulfide (H₂S) plays critical roles in redox biology, and its regulatory effects are tightly controlled by its cellular location and concentration. The imbalance of H₂S is believed to contribute to some pathological processes. Recent Advances: Downregulation of H₂S requires chemical tools such as inhibitors of H₂S-producing enzymes and H₂S scavengers. Recent efforts have discovered some promising inhibitors and scavengers. These advances pave the road toward better understanding of the functions of H₂S. Critical Issues: Precise H₂S downregulation is challenging. The potency and specificity of current inhibitors are still far from ideal. H₂S-producing enzymes are involved in complex sulfur metabolic pathways and ubiquitously present in biological matrices. The inhibition of these enzymes can cause unwanted side effects. H₂S scavengers allow targeted H₂S clearance, but their options are still limited. In addition, the scavenging process often results in biologically active by-products. Future Directions: Further development of potent and specific inhibitors for H₂S-producing enzymes is needed. Scavengers that can rapidly and selectively remove H₂S while generating biocompatible by-products are needed. Potential therapeutic applications of scavengers and inhibitors are worth exploring. Antioxid. Redox Signal. 36, 294-308.

The Biological/Physiological Utility of Hydropersulfides (RSSH) and Related Species: What Is Old Is New Again

Significance: Hydrogen sulfide (H₂S) is reported to be an important mediator involved in numerous physiological processes. H₂S and hydropersulfides (RSSH) species are intimately linked biochemically. Therefore, interest in the mechanisms of the biological activity of H₂S has led to investigations of the chemical biology of RSSH since they are likely to coexist in a biological system. Currently it is hypothesized that RSSH may be responsible for a least part of the observed H₂S-mediated biology/physiology. Recent Advances: It has been recently touted that thiols (RSH) and RSSH have some important differences in terms of their chemical biology and that the generation of RSSH from RSH is purposeful to exploit these chemical differences as a response to a physiological or biological stress. This transformation may represent an unappreciated/unrecognized biological mechanism for dealing with cellular stresses. Critical Issues: Although recent studies indicate a diverse and potentially important chemical biology associated with RSSH species, these ideas have their foundations in early studies (some over 60 years old). It is vital to recognize the nature of this early work to fully appreciate the current ideas regarding RSSH biology. Importantly, these early studies were performed before the realization of purposeful H₂S biosynthesis (before 1996). Future Directions: Taking clues from the past studies of RSSH chemistry and biology, progress in delineating the chemical biology of RSSH will continue. Determination of the possible relevance of RSSH chemical biology to signaling and cellular physiology will be a primary focus of many future studies. Antioxid. Redox Signal. 36, 244-255.

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.

Sulfane Sulfur Compounds as Source of Reappearance of Reductive Off-Odors in Wine

Reactive compounds with one or more sulfane sulfur atoms can be an important source of reductive off-odors in wine. These substances contain labile sulfur, which can participate in microbiological (enzymatic) and chemical transformations (including in the post-bottling period), releasing malodorous hydrogen sulfide (H2S) and its derivatives (MeSH, EtSH, etc.). The following sulfane sulfur compounds were considered in this review as important precursors in the wine chemistry of reductive aromas: elemental sulfur (S8), persulfides (R-S-S-H), polysulfanes (R-Sn-R(′)), polythionates (−O3S-Sn-SO3−), thiosulfate (S2O32−) and derivatives of (poly)sulfane monosulfonic acids (R-Sn-SO3H). This review discusses the formation of these compounds, their reactivity and chemical transformations in wine, including reactions of nucleophilic substitution. In particular, the reactions of thiolysis, thiosulfatolysis and sulfitolysis of sulfane sulfur compounds are described, which lead in the end to reductive aroma compounds. In this way, the review attempts to shed light on some of the mysteries in the field of sulfur chemistry in wine and the reappearance of reductive off-odors after bottling.

Extracellular cysteines C226 and C232 mediate hydrogen sulfide-dependent inhibition of Orai3-mediated store-operated calcium entry

The gaseous signaling molecule hydrogen sulfide (H₂S) physiologically regulates store-operated Ca²⁺ entry (SOCE). The SOCE machinery consists of the plasma membrane-localized Orai channels (Orai1-3) and endoplasmic reticulum-localized stromal interaction molecule (STIM)1 and STIM2 proteins. H₂S inhibits Orai3- but not Orai1- or Orai2-mediated SOCE. The current objective was to define the mechanism by which H₂S selectively modifies Orai3. We measured SOCE and STIM1/Orai3 dynamics and interactions in HEK293 cells exogenously expressing fluorescently tagged human STIM1 and Orai3 in the presence and absence of the H₂S donor GYY4137. Two cysteines (C226 and C232) are present in Orai3 that are absent in the Orai1 and Orai2. When we mutated either of these cysteines to serine, alone or in combination, SOCE inhibition by H₂S was abolished. We also established that inhibition was dependent on an interaction with STIM1. To further define the effects of H₂S on STIM1/Orai3 interaction, we performed a series of fluorescence recovery after photobleaching (FRAP), colocalization, and fluorescence resonance energy transfer (FRET) experiments. Treatment with H₂S did not affect the mobility of Orai3 in the membrane, nor did it influence STIM1/Orai3 puncta formation or STIM1-Orai3 protein-protein interactions. These data support a model in which H₂S modification of Orai3 at cysteines 226 and 232 limits SOCE evoked upon store depletion and STIM1 engagement, by a mechanism independent of the interaction between Orai3 and STIM1.

COVID-19: A Redox Disease-What a Stress Pandemic Can Teach Us About Resilience and What We May Learn from the Reactive Species Interactome About Its Treatment

Significance: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus causing coronavirus disease 2019 (COVID-19), affects every aspect of human life by challenging bodily, socioeconomic, and political systems at unprecedented levels. As vaccines become available, their distribution, safety, and efficacy against emerging variants remain uncertain, and specific treatments are lacking. Recent Advances: Initially affecting the lungs, COVID-19 is a complex multisystems disease that disturbs the whole-body redox balance and can be long-lasting (Long-COVID). Numerous risk factors have been identified, but the reasons for variations in susceptibility to infection, disease severity, and outcome are poorly understood. The reactive species interactome (RSI) was recently introduced as a framework to conceptualize how cells and whole organisms sense, integrate, and accommodate stress. Critical Issues: We here consider COVID-19 as a redox disease, offering a holistic perspective of its effects on the human body, considering the vulnerability of complex interconnected systems with multiorgan/multilevel interdependencies. Host/viral glycan interactions underpin SARS-CoV-2's extraordinary efficiency in gaining cellular access, crossing the epithelial/endothelial barrier to spread along the vascular/lymphatic endothelium, and evading antiviral/antioxidant defences. An inflammation-driven "oxidative storm" alters the redox landscape, eliciting epithelial, endothelial, mitochondrial, metabolic, and immune dysfunction, and coagulopathy. Concomitantly reduced nitric oxide availability renders the sulfur-based redox circuitry vulnerable to oxidation, with eventual catastrophic failure in redox communication/regulation. Host nutrient limitations are crucial determinants of resilience at the individual and population level. Future Directions: While inflicting considerable damage to health and well-being, COVID-19 may provide the ultimate testing ground to improve the diagnosis and treatment of redox-related stress diseases. "Redox phenotyping" of patients to characterize whole-body RSI status as the disease progresses may inform new therapeutic approaches to regain redox balance, reduce mortality in COVID-19 and other redox diseases, and provide opportunities to tackle Long-COVID. Antioxid. Redox Signal. 35, 1226-1268.

H2S signaling and extracellular matrix remodeling in cardiovascular diseases: A tale of tense relationship

Extracellular matrix (ECM) is a non-cellular three-dimensional macromolecular network that not only provides mechanical support but also transduces essential molecular signals in organ functions. ECM is constantly remodeled to control tissue homeostasis, responsible for cell adhesion, cell migration, cell-to-cell communication, and cell differentiation, etc. The dysregulation of ECM components contributes to various diseases, including cardiovascular diseases, fibrosis, cancer, and neurodegenerative diseases, etc. Aberrant ECM remodeling is initiated by various stress, such as oxidative stress, inflammation, ischemia, and mechanical stress, etc. Hydrogen sulfide (H₂S) is a gasotransmitter that exhibits a wide variety of cytoprotective and physiological functions through its anti-oxidative and anti-inflammatory actions. Amounting research shows that H₂S can attenuate aberrant ECM remodeling. In this review, we discussed the implications and mechanisms of H₂S in the regulation of ECM remodeling in cardiovascular diseases, and highlighted the potential of H₂S in the prevention and treatment of cardiovascular diseases through attenuating adverse ECM remodeling.

A Case for Hydrogen Sulfide Metabolism as an Oxygen Sensing Mechanism

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

An evolutionary perspective on the interplays between hydrogen sulfide and oxygen in cellular functions

The physiological effects of the endogenously generated hydrogen sulfide (H2S) have been extensively studied in recent years. This review summarized the role of H2S in the origin of life and H2S metabolism in organisms from bacteria to vertebrates, examined the relationship between H2S and oxygen from an evolutionary perspective and emphasized the oxygen-dependent manner of H2S signaling in various physiological and pathological processes. H2S and oxygen are inextricably linked in various cellular functions. H2S is involved in aerobic respiration and stimulates oxidative phosphorylation and ATP production within the cell. Besides, H2S has protective effects on ischemia and reperfusion injury in several organs by acting as an oxygen sensor. Also, emerging evidence suggests the role of H2S is in an oxygen-dependent manner. All these findings indicate the subtle relationship between H2S and oxygen and further explain why H2S, a toxic molecule thriving in an anoxia environment several billion years ago, still affects homeostasis today despite the very low content in the body.

A Review of Chemical Tools for Studying Small Molecule Persulfides: Detection and Delivery

Hydrogen sulfide (H₂S) has gained significant attention as a potent bioregulator in the redox metabolome, but it is just one of many reactive sulfur species (RSS). Recently, small molecule persulfides (structure RSSH) have emerged as RSS of particular interest due to their enhanced antioxidant abilities compared to H₂S and their ability to directly convert protein thiols into protein persulfides, suggesting that persulfides may have distinct physiological functions from H₂S. However, persulfides exhibit instability and cross-reactivity that hampers the elucidation of their precise biological roles. As such, chemists have designed chemical tools and techniques to facilitate the study of persulfides under various conditions. These molecules and methods include persulfide trapping reagents and sensors, as well as compounds that degrade in response to various triggers to release persulfides, termed persulfide donors. There now exist a variety of persulfide donor classes, some of which possess tissue-targeting capabilities designed to mimic localized endogenous production of RSS. This Review briefly covers the physicochemical properties of persulfides, the endogenous production of small molecule persulfides, and their reactions with protein thiols and other reactive species. These introductory sections are followed by a discussion of chemical tools used in persulfide chemical biology, with critical analysis of recent advancements in the field and commentary on potential directions for future research.

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.

Biosynthesis, Quantification and Genetic Diseases of the Smallest Signaling Thiol Metabolite: Hydrogen Sulfide

Hydrogen sulfide (H2S) is a gasotransmitter and the smallest signaling thiol metabolite with important roles in human health. The turnover of H2S in humans is mainly governed by enzymes of sulfur amino acid metabolism and also by the microbiome. As is the case with other small signaling molecules, disease-promoting effects of H2S largely depend on its concentration and compartmentalization. Genetic defects that impair the biogenesis and catabolism of H2S have been described; however, a gap in knowledge remains concerning physiological steady-state concentrations of H2S and their direct clinical implications. The small size and considerable reactivity of H2S renders its quantification in biological samples an experimental challenge. A compilation of methods currently employed to quantify H2S in biological specimens is provided in this review. Substantial discrepancy exists in the concentrations of H2S determined by different techniques. Available methodologies permit end-point measurement of H2S concentration, yet no definitive protocol exists for the continuous, real-time measurement of H2S produced by its enzymatic sources. We present a summary of available animal models, monogenic diseases that impair H2S metabolism in humans including structure-function relationships of pathogenic mutations, and discuss possible approaches to overcome current limitations of study.

Inhibitory effect of polysulfide, an endogenous sulfur compound, on oxidative stress-induced TRPA1 activation

Polysulfide (PS), an endogenous sulfur compound, generated by oxidation of hydrogen sulfide, has a stimulatory action on the nociceptive TRPA1 channel. TRPA1 is also activated by reactive oxygen species such as hydrogen peroxide (H₂O₂) produced during inflammation. Here, we examined the effect of PS on H₂O₂-induced responses in native and heterologously expressed TRPA1 using a cell-based calcium assay. We also carried out behavioral experiments in vivo. In mouse sensory neurons, H₂O₂ elicited early TRPA1-dependent and late TRPA1-independent increases of [Ca²⁺]_i. The former was suppressed by the pretreatment with PS. In cells heterologously expressed TRPA1, PS suppressed [Ca²⁺]_i responses to H₂O₂. Simultaneous measurement of [Ca²⁺]_i and the intracellular PS level revealed that scavenging effect of PS was not related to the inhibitory effect. Removal of extracellular Ca²⁺, a calmodulin inhibitor and dithiothreitol attenuated the inhibitory effect of PS. Pretreatment with PS diminished nociceptive behaviors induced by H₂O₂. The present data suggest that PS suppresses oxidative stress-induced TRPA1 activation due to cysteine modification and Ca²⁺/calmodulin signaling. Thus, endogenous sulfurs may have regulatory roles in nociception via functional changes in TRPA1 under inflammatory conditions.

Dithiophosphate-Induced Redox Conversions of Reduced and Oxidized Glutathione

Phosphorus species are potent modulators of physicochemical and bioactive properties of peptide compounds. O,O-diorganyl dithiophoshoric acids (DTP) form bioactive salts with nitrogen-containing biomolecules; however, their potential as a peptide modifier is poorly known. We synthesized amphiphilic ammonium salts of O,O-dimenthyl DTP with glutathione, a vital tripeptide with antioxidant, protective and regulatory functions. DTP moiety imparted radical scavenging activity to oxidized glutathione (GSSG), modulated the activity of reduced glutathione (GSH) and profoundly improved adsorption and electrooxidation of both glutathione salts on graphene oxide modified electrode. According to NMR spectroscopy and GC–MS, the dithiophosphates persisted against immediate dissociation in an aqueous solution accompanied by hydrolysis of DTP moiety into phosphoric acid, menthol and hydrogen sulfide as well as in situ thiol-disulfide conversions in peptide moieties due to the oxidation of GSH and reduction of GSSG. The thiol content available in dissolved GSH dithiophosphate was more stable during air oxidation compared with free GSH. GSH and the dithiophosphates, unlike DTP, caused a thiol-dependent reduction of MTS tetrazolium salt. The results for the first time suggest O,O-dimenthyl DTP as a redox modifier for glutathione, which releases hydrogen sulfide and induces biorelevant redox conversions of thiol/disulfide groups.

Hydrogen sulphide as a guard cell network regulator

Hydrogen sulphide (H₂ S) is an endogenously produced gasotransmitter that has rapidly emerged as an active signalling component of several plant processes, stomatal movement regulation among them. The guard cells (GCs), pairs of cells that neighbour the stomatal pores, transduce endogenous and environmental signals, through signalling network, to control stomatal pore size. In this complex network, which has become a model system for plant signalling, few highly connected components form a core that links most of the pathways. The evidence summarized in this insight, on the interplay between H₂ S and different key components of the GC networks, points towards H₂ S as a regulator of the GC core signalling pathway.

Label-Free Quantitative Proteomic Analysis of Nitrogen Starvation in Arabidopsis Root Reveals New Aspects of H2S Signaling by Protein Persulfidation

Hydrogen sulfide (H2S)-mediated signaling pathways regulate many physiological and pathophysiological processes in mammalian and plant systems. The molecular mechanism by which hydrogen sulfide exerts its action involves the posttranslational modification of cysteine residues to form a persulfidated thiol motif. We developed a comparative and label-free quantitative proteomic analysis approach for the detection of endogenous persulfidated proteins in N-starved Arabidopsis thaliana roots by using the tag-switch method. In this work, we identified 5214 unique proteins from root tissue that were persulfidated, 1674 of which were quantitatively analyzed and found to show altered persulfidation levels in vivo under N deprivation. These proteins represented almost 13% of the entire annotated proteome in Arabidopsis. Bioinformatic analysis revealed that persulfidated proteins were involved in a wide range of biological functions, regulating important processes such as primary metabolism, plant responses to stresses, growth and development, RNA translation and protein degradation. Quantitative mass spectrometry analysis allowed us to obtain a comprehensive view of hydrogen sulfide signaling via changes in the persulfidation levels of key protein targets involved in ubiquitin-dependent protein degradation and autophagy, among others.

Harnessing the Benefits of Endogenous Hydrogen Sulfide to Reduce Cardiovascular Disease

Cardiovascular disease is the leading cause of death in the U.S. While various studies have shown the beneficial impact of exogenous hydrogen sulfide (H₂S)-releasing drugs, few have demonstrated the influence of endogenous H₂S production. Modulating the predominant enzymatic sources of H₂S-cystathionine-β-synthase, cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase-is an emerging and promising research area. This review frames the discussion of harnessing endogenous H₂S within the context of a non-ischemic form of cardiomyopathy, termed diabetic cardiomyopathy, and heart failure. Also, we examine the current literature around therapeutic interventions, such as intermittent fasting and exercise, that stimulate H₂S production.

Regulation of redox signaling by reactive sulfur species

Reactive sulfur species, such as cysteine persulfide, are produced endogenously at significant levels in cells and have rapidly emerged as common biomolecules. By virtue of improved analytical methods for detecting reactive persulfides, it has been demonstrated that these reactive molecules exhibit unique chemical properties and are present in various forms in vivo. Accumulating evidence has suggested that persulfides may be involved in a variety of biological processes, such as anti­oxidant and anti-inflammatory responses, biosynthesis of sulfur-containing molecules, mitochondrial energy metabolism via sulfur respiration, and cytoprotection via regulation of redox signal transduction induced by endogenous and exogenous electrophiles. Elucidation of the persulfide-dependent metabolism of redox signals is expected to facilitate our understanding of the importance of persulfides in regulating redox signals.

Trisulfide linked cholesteryl PEG conjugate attenuates intracellular ROS and collagen-1 production in a breast cancer co-culture model

The progression of cancer has been closely-linked with augmentation of cellular reactive oxygen species (ROS) levels and ROS-associated changes in the tumour microenvironment (TME), including alterations to the extracellular matrix and associated low drug uptake. Herein we report the application of a co-culture model to simulate the ROS based cell-cell interactions in the TME using fibroblasts and breast cancer cells, and describe how novel reactive polymers can be used to modulate those interactions. Under the co-culture conditions, both cell types exhibited modifications in behaviour, including significant overproduction of ROS in the cancer cells, and elevation of the collagen-1 secretion and stained actin filament intensity in the fibroblasts. To examine the potential of using reactive antioxidant polymers to intercept ROS communication and thereby manipulate the TME, we employed H2S-releasing macromolecular conjugates which have been previously demonstrated to mitigate ROS production in HEK cells. The specific conjugate used, mPEG-SSS-cholesteryl (T), significantly reduced ROS levels in co-cultured cancer cells by approximately 50%. This reduction was significantly greater than that observed with the other positive antioxidant controls. Exposure to T was also found to downregulate levels of collagen-1 in the co-cultured fibroblasts, while exhibiting less impact on cells in mono-culture. This would suggest a possible downstream effect of ROS-mitigation by T on stromal-tumour cell signalling. Since fibroblast-derived collagens modulate crucial steps in tumorigenesis, this ROS-associated effect could potentially be harnessed to slow cancer progression. The model may also be beneficial for interrogating the impact of antioxidants on naturally enhanced ROS levels, rather than relying on the application of exogenous oxidants to simulate elevated ROS levels.

Pharmacological induction of mesenchymal-epithelial transition via inhibition of H2S biosynthesis and consequent suppression of ACLY activity in colon cancer cells

Hydrogen sulfide (H₂S) is an important endogenous gaseous transmitter mediator, which regulates a variety of cellular functions in autocrine and paracrine manner. The enzymes responsible for the biological generation of H₂S include cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST). Increased expression of these enzymes and overproduction of H₂S has been implicated in essential processes of various cancer cells, including the stimulation of metabolism, maintenance of cell proliferation and cytoprotection. Cancer cell identity is characterized by so-called "transition states". The progression from normal (epithelial) to transformed (mesenchymal) state is termed epithelial-to-mesenchymal transition (EMT) whereby epithelial cells lose their cell-to-cell adhesion capacity and gain mesenchymal characteristics. The transition process can also proceed in the opposite direction, and this process is termed mesenchymal-to-epithelial transition (MET). The current project was designed to determine whether inhibition of endogenous H₂S production in colon cancer cells affects the EMT/MET balance in vitro. Inhibition of H₂S biosynthesis in HCT116 human colon cancer cells was achieved either with aminooxyacetic acid (AOAA) or 2-[(4-hydroxy-6-methylpyrimidin-2-yl)sulfanyl]-1-(naphthalen-1-yl)ethan-1-one (HMPSNE). These inhibitors induced an upregulation of E-cadherin and Zonula occludens-1 (ZO-1) expression and downregulation of fibronectin expression, demonstrating that H₂S biosynthesis inhibitors can produce a pharmacological induction of MET in colon cancer cells. These actions were functionally reflected in an inhibition of cell migration, as demonstrated in an in vitro "scratch wound" assay. The mechanisms involved in the action of endogenously produced H₂S in cancer cells in promoting (or maintaining) EMT (or tonically inhibiting MET) relate, at least in part, in the induction of ATP citrate lyase (ACLY) protein expression, which occurs via upregulation of ACLY mRNA (via activation of the ACLY promoter). ACLY in turn, regulates the Wnt-β-catenin pathway, an essential regulator of the EMT/MET balance. Taken together, pharmacological inhibition of endogenous H₂S biosynthesis in cancer cells induces MET. We hypothesize that this may contribute to anti-cancer / anti-metastatic effects of H₂S biosynthesis inhibitors.

Recent progress of redox-responsive polymeric nanomaterials for controlled release

This perspective focuses on the development of redox-responsive polymeric nanomaterials for controlled payload release within the last four years.

Trisulfide-Bearing PEG Brush Polymers Donate Hydrogen Sulfide and Ameliorate Cellular Oxidative Stress

A potential approach to combat cellular dysfunction is to manipulate cell communication and signaling pathways to restore physiological functions while protecting unaffected cells. For instance, delivering the signaling molecule H₂S to certain cells has been shown to restore cell viability and re-normalize cell behavior. We have previously demonstrated the ability to incorporate a trisulfide-based H₂S-donating moiety into linear polymers with good in vitro releasing profiles and demonstrated their potential for ameliorating oxidative stress. Herein, we report two novel series of brush polymers decorated with higher numbers of H₂S-releasing segments. These materials contain two trisulfide-based monomers co-polymerized with oligo(ethylene glycol methyl ether methacrylate) via reversible addition-fragmentation chain-transfer polymerization. The macromolecules were characterized to have a range of trisulfide densities with similar, well-defined molecular weight distribution, good H₂S-releasing profiles, and high cellular tolerance. Using an amperometric technique, the H₂S liberated and total sulfide release were found to depend on concentrations and chemical nature of triggering molecules (glutathione and cysteine) and, importantly, the position of reactive groups within the brush structure. Notably, when introduced to cells at well-tolerated doses, two macromolecular donors which have the same proportion as of the H₂S-donating monomer (30%) but differ in releasing moiety location show similar cellular H₂S-releasing kinetics. These donors can restore reactive oxygen species levels to baseline values, when polymer pretreated cells are exposed to exogenous oxidants (H₂O₂). Our work opens up a new aspect in preparing H₂S macromolecule donors and their application to arresting cellular oxidative cascades.

Sulfur Metabolism Under Stress

Significance: In humans, imbalances in the reduction-oxidation (redox) status of cells are associated with many pathological states. In addition, many therapeutics and prophylactics used as interventions for diverse pathologies either directly modulate oxidant levels or otherwise influence endogenous cellular redox systems. Recent Advances: The cellular machineries that maintain redox homeostasis or that function within antioxidant defense systems rely heavily on the regulated reactivities of sulfur atoms either within or derived from the amino acids cysteine and methionine. Recent advances have substantially advanced our understanding of the complex and essential chemistry of biological sulfur-containing molecules. Critical Issues: The redox machineries that maintain cellular homeostasis under diverse stresses can consume large amounts of energy to generate reducing power and/or large amounts of sulfur-containing nutrients to replenish or sustain intracellular stores. By understanding the metabolic pathways underlying these responses, one can better predict how to protect cells from specific stresses. Future Directions: Here, we summarize the current state of knowledge about the impacts of different stresses on cellular metabolism of sulfur-containing molecules. This analysis suggests that there remains more to be learned about how cells use sulfur chemistry to respond to stresses, which could in turn lead to advances in therapeutic interventions for some exposures or conditions.

Insights on Potential Formation Damage Mechanisms Associated with the Use of Gel Breakers in Hydraulic Fracturing

Hydraulic fracturing using water-soluble polymers has been extensively used to enhance the productivity of oil and gas wells. However, the production enhancement can be significantly impaired due to polymer residue generated within the proppant pack in the created fractures. This work describes an approach to establish a suitable fracturing fluid cleanup process by characterizing broken polymer residues generated from the use of different gel breaker types. Commonly used gel breakers such as inorganic oxidizers (bromate and persulfate salts), specific enzymes, and acids were evaluated in this work. The influence of each gel breaker was examined using High-Pressure/High-Temperature (HP/HT) rheometer, aging cells, zeta potential, Gel Permeation Chromatography (GPC), and Environmental Scanning Electron Microscope/Energy Dispersive X-ray Spectroscopy (ESEM/EDS). Experiments were performed on a carboxymethylhydroxypropyl guar (CMHPG) fracturing fluid at temperatures up to 300 °F. The developed GPC methodology showed that the size of the broken polymer chains was mainly dependent on the type of gel breakers used. Moreover, laboratory tests have revealed that some gel breakers may negatively influence the performance of polymeric clay stabilizers. Additionally, this work showed damaging precipitations that can be generated due to the interactions of gel breakers with H₂S.

SG1002 and Catenated Divalent Organic Sulfur Compounds as Promising Hydrogen Sulfide Prodrugs

Significance: Sulfur has a critical role in protein structure/function and redox status/signaling in all living organisms. Although hydrogen sulfide (H2S) and sulfane sulfur (SS) are now recognized as central players in physiology and pathophysiology, the full scope and depth of sulfur metabolome's impact on human health and healthy longevity has been vastly underestimated and is only starting to be grasped. Since many pathological conditions have been related to abnormally low levels of H2S/SS in blood and/or tissues, and are amenable to treatment by H2S supplementation, development of safe and efficacious H2S donors deserves to be undertaken with a sense of urgency; these prodrugs also hold the promise of becoming widely used for disease prevention and as antiaging agents. Recent Advances: Supramolecular tuning of the properties of well-known molecules comprising chains of sulfur atoms (diallyl trisulfide [DATS], S8) was shown to lead to improved donors such as DATS-loaded polymeric nanoparticles and SG1002. Encouraging results in animal models have been obtained with SG1002 in heart failure, atherosclerosis, ischemic damage, and Duchenne muscular dystrophy; with TC-2153 in Alzheimer's disease, schizophrenia, age-related memory decline, fragile X syndrome, and cocaine addiction; and with DATS in brain, colon, gastric, and breast cancer. Critical Issues: Mode-of-action studies on allyl polysulfides, benzyl polysulfides, ajoene, and 12 ring-substituted organic disulfides and thiosulfonates led several groups of researchers to conclude that the anticancer effect of these compounds is not mediated by H2S and is only modulated by reactive oxygen species, and that their central model of action is selective protein S-thiolation. Future Directions: SG1002 is likely to emerge as the H2S donor of choice for acquiring knowledge on this gasotransmitter's effects in animal models, on account of its unique ability to efficiently generate H2S without byproducts and in a slow and sustained mode that is dose independent and enzyme independent. Efficient tuning of H2S donation characteristics of DATS, dibenzyl trisulfide, and other hydrophobic H2S prodrugs for both oral and parenteral administration will be achieved not only by conventional structural modification of a lead molecule but also through the new "supramolecular tuning" paradigm.

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.

Monitoring the Redox Status in Multiple Sclerosis

Worldwide, over 2.2 million people suffer from multiple sclerosis (MS), a multifactorial demyelinating disease of the central nervous system. MS is characterized by a wide range of motor, autonomic, and psychobehavioral symptoms, including depression, anxiety, and dementia. The blood, cerebrospinal fluid, and postmortem brain samples of MS patients provide evidence on the disturbance of reduction-oxidation (redox) homeostasis, such as the alterations of oxidative and antioxidative enzyme activities and the presence of degradation products. This review article discusses the components of redox homeostasis, including reactive chemical species, oxidative enzymes, antioxidative enzymes, and degradation products. The reactive chemical species cover frequently discussed reactive oxygen/nitrogen species, infrequently featured reactive chemicals such as sulfur, carbonyl, halogen, selenium, and nucleophilic species that potentially act as reductive, as well as pro-oxidative stressors. The antioxidative enzyme systems cover the nuclear factor erythroid-2-related factor 2 (NRF2)-Kelch-like ECH-associated protein 1 (KEAP1) signaling pathway. The NRF2 and other transcriptional factors potentially become a biomarker sensitive to the initial phase of oxidative stress. Altered components of the redox homeostasis in MS were discussed in search of a diagnostic, prognostic, predictive, and/or therapeutic biomarker. Finally, monitoring the battery of reactive chemical species, oxidative enzymes, antioxidative enzymes, and degradation products helps to evaluate the redox status of MS patients to expedite the building of personalized treatment plans for the sake of a better quality of life.