Analysis and characterization of sulfane sulfur

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

Versatile roles of cysteine persulfides in tumor biology

Rewiring the transsulfuration pathway is recognized as a rapid adaptive metabolic response to environmental conditions in cancer cells to support their increased cysteine demand and to produce Reactive Sulfur Species (RSS) including hydrogen sulfide (H2S) and cysteine persulfide. This can directly (via RSS) or indirectly (by supplying Cys) trigger chemical or enzyme catalyzed persulfidation on critical protein cysteine residues to protect them from oxidative damage and to orchestrate protein functions, and thereby contribute to cancer cell plasticity. In this review key aspects of persulfide-mediated biological processes are highlighted and critically discussed in relation to cancer cell survival, bioenergetics, proliferation as well as in tumor angiogenesis, adaptation to hypoxia and oxidative stress, and regulation of epithelial to mesenchymal transition.

Hypoxia increases persulfide and polysulfide formation by AMP kinase dependent cystathionine gamma lyase phosphorylation

Hydropersulfide and hydropolysulfide metabolites are increasingly important reactive sulfur species (RSS) regulating numerous cellular redox dependent functions. Intracellular production of these species is known to occur through RSS interactions or through translational mechanisms involving cysteinyl t-RNA synthetases. However, regulation of these species under cell stress conditions, such as hypoxia, that are known to modulate RSS remain poorly understood. Here we define an important mechanism of increased persulfide and polysulfide production involving cystathionine gamma lyase (CSE) phosphorylation at serine 346 and threonine 355 in a substrate specific manner, under acute hypoxic conditions. Hypoxic phosphorylation of CSE occurs in an AMP kinase dependent manner increasing enzyme activity involving unique inter- and intramolecular interactions within the tetramer. Importantly, both cellular hypoxia and tissue ischemia result in AMP Kinase dependent CSE phosphorylation that regulates blood flow in ischemic tissues. Our findings reveal hypoxia molecular signaling pathways regulating CSE dependent persulfide and polysulfide production impacting tissue and cellular response to stress.

How super is supersulfide?: Reconsidering persulfide reactivity in cellular biology

In an attempt to understand the cellular mechanisms of H2S signalling, recent research has focused on supersulfide (i.e., alkyl and inorganic hydropersulfide) formation and subsequent reactivity. While our understanding of supersulfides in biology has rapidly advanced, there are some chemical features of this unique functional group that require re-evaluation. Persulfides, such as glutathione hydropersulfide, have been called "supersulfide" as it is assumed that the alkyl hydropersulfide (RSSH) functional group is a superior nucleophile compared to the corresponding thiol (RSH) due to the alpha effect. However, recent quantum mechanical calculations and experimental data show that persulfides are not "super" nucleophiles, but rather potent electrophiles in cellular biology. It is proposed here that persulfides, via their electrophilic signalling effects, induces a cellular hormesis effect, which may explain the observed effects of altered RSSH production. Therefore, the electrophilic and thiol oxidant properties of persulfides should considered in cellular biology.

Sensing and regulation of reactive sulfur species (RSS) in bacteria

The infected host deploys generalized oxidative stress caused by small inorganic reactive molecules as antibacterial weapons. An emerging consensus is that hydrogen sulfide (H2S) and forms of sulfur with sulfur-sulfur bonds termed reactive sulfur species (RSS) provide protection against oxidative stressors and antibiotics, as antioxidants. Here, we review our current understanding of RSS chemistry and its impact on bacterial physiology. We start by describing the basic chemistry of these reactive species and the experimental approaches developed to detect them in cells. We highlight the role of thiol persulfides in H2S-signaling and discuss three structural classes of ubiquitous RSS sensors that tightly regulate cellular H2S/RSS levels in bacteria, with a specific focus on the chemical specificity of these sensors.

Disulfides form persulfides at alkaline pH leading to potential overestimations in the cold cyanolysis method

It is well established that proteins and peptides can release sulfur under alkaline treatment, mainly through the β-elimination of disulfides with the concomitant formation of persulfides and dehydroalanine derivatives. In this study, we evaluated the formation of glutathione persulfide (GSSH/GSS-) by exposure of glutathione disulfide (GSSG) to alkaline conditions. The kinetics of the reaction between GSSG and HO- was investigated by UV-Vis absorbance, reaction with 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB), and cold cyanolysis, obtaining an apparent second-order rate constant of ∼10-3 M-1 s-1 at 25 °C. The formation of GSSH and the dehydroalanine derivative was confirmed by HPLC and/or mass spectrometry. However, the mixtures did not equilibrate in a timescale of hours, and additional species, including thiol and diverse sulfane sulfur compounds were also formed, probably through further reactions of the persulfide. Cold cyanolysis is frequently used to quantify persulfides, since it measures sulfane sulfur. This method involves a step in which the sample to be analyzed is incubated with cyanide at alkaline pH. When cold cyanolysis was applied to samples containing GSSG, sulfane sulfur products that were not present in the original sample were measured. Thus, our results reveal the risk of overestimating the amount of sulfane sulfur compounds in samples that contain disulfides due to their decay to persulfides and other sulfane sulfur compounds at alkaline pH. Overall, our study highlights that the β-elimination of disulfides is a potential source of persulfides, although we do not recommend the preparation of GSSH from incubation of GSSG in alkali. Our study also highlights the importance of being cautious when doing and interpreting cold cyanolysis experiments.

Sulfur metabolic response in macrophage limits excessive inflammatory response by creating a negative feedback loop

The excessive inflammatory response of macrophages plays a vital role in the pathogenesis of various diseases. The dynamic metabolic alterations in macrophages, including amino acid metabolism, are known to orchestrate their inflammatory phenotype. To explore a new metabolic pathway that regulates the inflammatory response, we examined metabolome changes in mouse peritoneal macrophages (PMs) in response to lipopolysaccharide (LPS) and found a coordinated increase of cysteine and its related metabolites, suggesting an enhanced demand for cysteine during the inflammatory response. Because Slc7a11, which encodes a cystine transporter xCT, was remarkably upregulated upon the pro-inflammatory challenge and found to serve as a major channel of cysteine supply, we examined the inflammatory behavior of Slc7a11 knockout PMs (xCT-KO PMs) to clarify an impact of the increased cysteine demand on inflammation. The xCT-KO PMs exhibited a prolonged upregulation of pro-inflammatory genes, which was recapitulated by cystine depletion in the culture media of wild-type PMs, suggesting that cysteine facilitates the resolution of inflammation. Detailed analysis of the sulfur metabolome revealed that supersulfides, such as cysteine persulfide, were increased in PMs in response to LPS, which was abolished in xCT-KO PMs. Supplementation of N-acetylcysteine tetrasulfide (NAC-S2), a supersulfide donor, attenuated the pro-inflammatory gene expression in xCT-KO PMs. Thus, activated macrophages increase cystine uptake via xCT and produce supersulfides, creating a negative feedback loop to limit excessive inflammation. Our study highlights the finely tuned regulation of macrophage inflammatory response by sulfur metabolism.

Contribution of NRF2 to sulfur metabolism and mitochondrial activity

NF-E2-related factor 2 (NRF2) plays a crucial role in the maintenance of cellular homeostasis by regulating various enzymes and proteins that are involved in the redox reactions utilizing sulfur. While substantial impacts of NRF2 on mitochondrial activity have been described, the precise mechanism by which NRF2 regulates mitochondrial function is still not fully understood. Here, we demonstrated that NRF2 increased intracellular persulfides by upregulating the cystine transporter xCT encoded by Slc7a11, a well-known NRF2 target gene. Persulfides have been shown to play an important role in mitochondrial function. Supplementation with glutathione trisulfide (GSSSG), which is a form of persulfide, elevated the mitochondrial membrane potential (MMP), increased the oxygen consumption rate (OCR) and promoted ATP production. Persulfide-mediated mitochondrial activation was shown to require the mitochondrial sulfur oxidation pathway, especially sulfide quinone oxidoreductase (SQOR). Consistently, NRF2-mediated mitochondrial activation was also dependent on SQOR activity. This study clarified that the facilitation of persulfide production and sulfur metabolism in mitochondria by increasing cysteine availability is one of the mechanisms for NRF2-dependent mitochondrial activation.

Cystathionine β-synthase overexpression drives metastatic dissemination in pancreatic ductal adenocarcinoma via inducing epithelial-to-mesenchymal transformation of cancer cells

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest of all cancer types with a constant rise in global incidence. Therefore, better understanding of PDAC biology, in order to design more efficient diagnostic and treatment modalities, is a priority. Here we found that the expression levels of cystathionine β-synthase (CBS), a transsulfuration enzyme, is markedly elevated in metastatic PDAC cells compared to cell lines isolated from non-metastatic primary tumors. On human immunohistochemical samples from PDAC patients we also found higher CBS staining in cancerous ductal cells compared to in non-tumor tissue, which was further elevated in the lymph node metastasis of the same patients. In mice, orthotopically injected CBS-silenced T3M4 cells induced fewer liver metastases compared to control cells indicating important roles for CBS in PDAC cancer cell invasion and malignant transformation. Wound healing and colony formation assays in cell culture confirmed that CBS-deficient metastatic T3M4 and non-metastatic BxPC3 primary tumor cells migrate slower and have impaired anchorage-independent growth capacities compared to control T3M4 cells. CBS silencing in T3M4 cells lowered WNT5a and SNAI1 gene expression down to levels that were observed in BxPC3 cells as well as resulted in an increase in E-cadherin and a decrease in Vimentin signals in mouse tumors when injected orthotopically. These observations suggested a primary role for the epithelial to mesenchymal transformation of cancer cells in CBS-mediated tumor aggressiveness. Under normal conditions, STAT3, an upstream regulator of Wnt signaling pathways, was less phosphorylated and more oxidized in shCBS T3M4 and BxPC3 compared to control T3M4 cells, which is consistent with decreased transcriptional activity at lower CBS levels due to less protection against oxidation. Sulfur metabolome analyses suggested that this CBS-mediated protection against oxidative modifications is likely to be related to persulfide/sulfide producing activities of the enzyme rather than its canonical function to produce cystathionine for cysteine synthesis. Taken together, CBS overexpression through regulation of the EMT plays a significant role in PDAC cancer cell invasion and metastasis.

Cysteine trisulfide oxidizes protein thiols and induces electrophilic stress in human cells

The cellular effects of hydrogen sulfide (H₂S) signaling may be partially mediated by the formation of alkyl persulfides from thiols, such as glutathione and protein cysteine residues. Persulfides are potent nucleophiles and reductants and therefore potentially an important endogenous antioxidant or protein post-translational modification. To directly study the cellular effects of persulfides, cysteine trisulfide (Cys-S₃) has been proposed as an in situ persulfide donor, as it reacts with cellular thiols to generate cysteine persulfide (Cys-S-S^-). Numerous pathways sense and respond to electrophilic cellular stressors to inhibit cellular proliferation and induce apoptosis, however the effect of Cys-S₃ on the cellular stress response has not been addressed. Here we show that Cys-S₃ inhibited cellular metabolism and proliferation and rapidly induced cellular- and ER-stress mechanisms, which were coupled to widespread protein-thiol oxidation. Cys-S₃ reacted with Na₂S to generate cysteine persulfide, which protected human cell lines from ER-stress. However this method of producing cysteine persulfide contains excess sulfide, which interferes with the direct analysis of persulfide donation. We conclude that cysteine trisulfide is a thiol oxidant that induces cellular stress and decreased proliferation.

A Fast and Ratiometric Method for Quantification of Cysteine-Bound Persulfides Based on Alkylation and Gel-Shift Assays

Cysteine-bound persulfides (Cys-SSH) in proteins are sulfur carrier intermediates in the synthesis of essential cofactors such as iron-sulfur clusters, molybdenum cofactor, vitamin (thiamine), and thionucleosides (thiolated tRNA). Protein-bound persulfides are also used for signaling purposes as a carrier of the "H₂S" signal. Several methods have been developed to detect and quantify cysteine-bound persulfides in protein and monitor their exchange. The main challenge in developing these techniques is to discriminate persulfidated cysteine from cysteine and other cysteine modifications. It is also critical to develop ratiometric methods to quantify the level of persulfidation in the protein of interest. We describe here a ratiometric method to label and quantify protein-bound persulfides relying on alkylation and gel-shift assays. This method is based on the derivation of cysteine and persulfides with "heavy" alkylating agents, followed by specific cleavage of the sulfur-sulfur bond of the alkylated persulfide by a reducing agent and separation of the alkylated species by electrophoresis. A persulfide is thus revealed by the appearance of a species lacking one alkylation unit under reducing conditions. We call this alkylation-reduction band-shift (ARBS) assay. Moreover, the quantification of the bands corresponding to the persulfidated and non-persulfidated species in the same lane provides a ratiometric quantification allowing determination of the level of persulfidation of individual cysteine. Other cysteine modifications such as disulfides, sulfenic, sulfinic, sulfonic acids, nitrosothiols, and sulfenamides preclude alkylation. Thus, they may appear as false positives, but they are ruled out by the analysis under nonreducing conditions since these species do not behave as persulfides under these conditions.

Lipophilic compounds in garlic decrease the toxicity of methylmercury by forming sulfur adducts

Garlic (Allium sativum L.) contains numerous sulfur compounds. We have previously found that reactive sulfur species such as glutathione persulfide, glutathione polysulfide, protein-bound persulfides, and hydrogen sulfide can bind to methylmercury to give bismethylmercury sulfide, which is less toxic than methylmercury. It was not clear, however, whether such reactive sulfur species are present in garlic. The aim of the study presented here was to determine whether garlic contains reactive sulfur species that can bind to methylmercury. We extracted garlic with organic solvents and then performed silica gel column chromatography to separate constituents that could cause bismethylmercury sulfide to form. We found numerous garlic constituents could bind to methylmercury to form bismethylmercury sulfide. A hexane extract of garlic decreased methylmercury cytotoxicity in vitro and body weight loss in mice. The results suggest that ingesting garlic may decrease methylmercury toxicity by causing the formation of sulfur adducts that inhibit adverse reactions.

Possible differences in the mechanism of malignant transformation of HaCaT cells by arsenite and its dimethyl metabolites, particularly dimethylthioarsenics

BACKGROUND

As a confirmed human carcinogen, arsenic can cause skin cancer, lung cancer, etc. However, its carcinogenic mechanism is still unclear. In recent years, the oxidative stress hypothesis has become widely accepted. In mammals it has been found that arsenic can be converted to dimethylarsinous acid (DMAIII) and dimethylmonothioarsinic acid (DMMTAV) through a series of methylation and redox reactions. DMAIII and DMMTAV are highly toxic.

METHODS

Human keratinocytes (HaCaT) were exposed to different concentrations of NaAsO2 (IAsIII), DMMTAV and DMAIII for 24 h. Reactive oxygen species (hydrogen peroxide and superoxide), oxidative damage markers (8-hydroxydeoxyguanosine and malondialdehyde), and antioxidant markers (glutathione and superoxide dismutase) were measured. In addition, sulfane sulfurs were measured in HaCaT cells and a cell-free system.

RESULTS

In the DMMTAV and DMAIII treatment groups, the levels of hydrogen peroxide and superoxide in HaCaT cells were higher than in the IAsIII treatment groups at the same dose. Levels of 8-OHdG and MDA in the DMMTAV and DMAIII treatment groups were also higher than those in the IAsIII treatment groups at the same dose. However, in the DMMTAV and DMAIII treatment groups, the levels of GSH and SOD activity were lower than that in the IAsIII treatment groups. In DMMTAV-treated HaCaT cells, sulfane sulfurs were produced. Further, it was found that DMMTAV could react with DMDTAV to form persulfide in the cell-free system, which may explain the mechanism of the formation of sulfane sulfurs in DMMTAV-treated HaCaT cells.

CONCLUSIONS

DMMTAV and DMAIII more readily induce reactive oxygen species (ROS) and cause oxidative damage in HaCaT cells than inorganic arsenic. Further, the persulfide formed by the reaction of DMMTAV and DMDTAV produced from the metabolism of DMMTAV may induce a stronger reductive defense mechanism than GSH against the intracellular oxidative stress of DMMTAV. However, the cells exposed to arsenite are transformed by the continuous nuclear translocation of Nrf2 due to oxidative stress, and the persulfide from dimethylthioarsenics may promote Nrf2 by the combination with thiol groups, especially redox control key protein, Keap1, eventually cause nuclear translocation of sustained Nrf2.

Effects of Thiosulfate as a Sulfur Source on Plant Growth, Metabolites Accumulation and Gene Expression in Arabidopsis and Rice

Plants are considered to absorb sulfur from their roots in the form of sulfate. In bacteria like Escherichia coli, thiosulfate is a preferred sulfur source. It is converted into cysteine (Cys). This transformation consumes less NADPH and ATP than sulfate assimilation into Cys. In Saccharomyces cerevisiae, thiosulfate promoted growth more than sulfate. In the present study, the availability of thiosulfate, the metabolite transformations and gene expressions it induces were investigated in Arabidopsis and rice as model dicots and monocots, respectively. In Arabidopsis, the thiosulfate-amended plants had lower biomass than those receiving sulfate when sulfur concentrations in the hydroponic medium were above 300 μM. In contrast, rice biomass was similar for plants raised on thiosulfate and sulfate at 300 μM sulfur. Therefore, both plants can use thiosulfate but it is a better sulfur source for rice. In both plants, thiosulfate levels significantly increased in roots following thiosulfate application, indicating that the plants absorbed thiosulfate into their root cells. Thiosulfate is metabolized in plants by a different pathway from that used for sulfate metabolism. Thiosulfate increases plant sulfide and cysteine persulfide levels which means that plants are in a more reduced state with thiosulfate than with sulfate. The microarray analysis of Arabidopsis roots revealed that 13 genes encoding Cys-rich proteins were upregulated more with thiosulfate than with sulfate. These results together with those of the widely targeted metabolomics analysis were used to proposes a thiosulfate assimilation pathway in plants.

Measurement of Protein Persulfidation: Improved Tag-Switch Method

Hydrogen sulfide (H₂S) is an endogenously produced signaling gasotransmitter, generated by the enzymes cystathionine γ-lyase, cystathionine β-synthase, and 3-mercaptopyruvate sulfurtransferase. The involvement of H₂S in numerous physiological, as well as pathophysiological conditions, was established over the past decade. However, the exact mechanism(s) of regulation of the biological functions by H₂S are under active investigations. It is proposed that the oxidative posttranslational modification of protein cysteine residues, known as persulfidation, could be the main mechanism of action of H₂S. Protein persulfides show similar reactivity to thiols, which represents one of the main obstacles in the development of a reliable method for detection of this specific protein modification. Subsequently, having a selective method for persulfide detection is of utmost importance in order to fully understand the physiological and pathophysiological role of H₂S. Several methods have been proposed for the detection of protein persulfidation, all of which are highlighted in this chapter. Furthermore, we provide a detailed description and protocol for the first selective persulfide labeling method, a tag-switch method, developed in our group.

Cysteine desulfurase is regulated by phosphorylation of Nfs1 in yeast mitochondria

The cysteine desulfurase Nfs1/Isd11 uses the amino acid cysteine as the substrate and its activity is absolutely required for contributing persulfide sulfur to the essential process of iron-sulfur (Fe-S) cluster assembly in mitochondria. Here we describe a novel regulatory process involving phosphorylation of Nfs1 in mitochondria. Phosphorylation enhanced cysteine desulfurase activity, while dephosphorylation decreased its activity. Nfs1 phosphopeptides were identified, and the corresponding phosphosite mutants showed impaired persulfide formation. Nfs1 pull down from mitochondria recovered an associated kinase activity, and Yck2, a kinase present in the pull down, was able to phosphorylate Nfs1 in vitro and stimulate cysteine desulfurase activity. Yck2 exhibited an eclipsed distribution in the mitochondrial matrix, although other cellular localizations have been previously described. Mitochondria lacking the Yck2 protein kinase (∆yck2) showed less phosphorylating activity for Nfs1. Compared with wild-type mitochondria, ∆yck2 mitochondria revealed slower persulfide formation on Nfs1 consistent with a role of Yck2 in regulating mitochondrial cysteine desulfurase activity. We propose that Nfs1 phosphorylation may provide a means of rapid adaptation to increased metabolic demand for sulfur and Fe-S clusters within mitochondria.

H2S and polysulfide metabolism: Conventional and unconventional pathways

It is now well established that hydrogen sulfide (H₂S) is an effector of a wide variety of physiological processes. It is also clear that many of the effects of H₂S are mediated through reactions with cysteine sulfur on regulatory proteins and most of these are not mediated directly by H₂S but require prior oxidation of H₂S and the formation of per- and polysulfides (H₂S_n, n = 2-8). Attendant with understanding the regulatory functions of H₂S and H₂S_n is an appreciation of the mechanisms that control, i.e., both increase and decrease, their production and catabolism. Although a number of standard "conventional" pathways have been described and well characterized, novel "unconventional" pathways are continuously being identified. This review summarizes our current knowledge of both the conventional and unconventional.

Analysis of endogenous H2S and H2Sn in mouse brain by high-performance liquid chromatography with fluorescence and tandem mass spectrometric detection

Previous studies indicated that bound sulfur species (BSS), including hydrogen polysulfide (H₂S_n), have various physiological functions in mammalian cells. Although H₂S_n molecules have been considered as secondary metabolites derived from hydrogen sulfide (H₂S) based on in vitro studies or predetermined reaction formula, the physiological form of BSS and their endogenous concentration remain unclear. In the present study, we aimed to improve the usual method using monobromobimane (mBB) followed by high performance liquid chromatographic (HPLC) analysis for HS^－ for simultaneous determination of H₂S, H₂S₂, H₂S₃ and cysteine persulfide in biological samples. We demonstrated that mBB derivatization of H₂S and H₂S_n standards under alkaline conditions (pH 9.5) induced significant decreases in H₂S₂ and H₂S₃ levels and a significant increase in the H₂S level in an incubation time-dependent manner. Conversely, the derivatization of mBB adducts of H₂S₂ and H₂S₃ were stable under neutral conditions (pH 7.0), which is physiologically relevant. Therefore, we re-examined the method using mBB and applied an improved method for the evaluation of H₂S, H₂S₂, and H₂S₃ in mouse brain under physiological pH conditions. The concentrations of H₂S and H₂S₂ were 0.030 ± 0.004μmol/g protein and 0.026 ± 0.002μmol/g protein, respectively. Although the level of H₂S₃ was below the quantification limit of this method, H₂S₃ was detected in mouse brain. Using the method established here, we reveal for the first time the existence of endogenous H₂S₂ and H₂S₃ in mammalian brain tissues. H₂S₂ and H₂S₃ exert anti-oxidant activity and anti-carbonyl stress effects through the regulation of redox balance in neuronal cells. Thus, our observations provide novel insights into the physiological functions of BSS in the brain and into neuronal diseases involved in redox imbalance.

Cysteine persulfides and polysulfides produced by exchange reactions with H2S protect SH-SY5Y cells from methylglyoxal-induced toxicity through Nrf2 activation

Many physiological functions of hydrogen sulfide (H₂S) have been reported in mammalian cells over the last 20 years. These physiological effects have been ascertained through in vitro treatment of cells with Na₂S or NaHS, both of which are precursors of H₂S. Since H₂S exists as HS⁻ in a neutral solution, a disulfide compound such as cystine could react with HS⁻ in culture medium as well as in the cell.

This study demonstrated that after the addition of Na₂S solution into culture medium, HS⁻ was transiently generated and disappeared immediately through the reaction between HS⁻ and cystine to form cysteine persulfides and polysulfides in the culture medium (bound sulfur mixture: BS-Mix). Furthermore, we found that the addition of Na₂S solution resulted in an increase of intracellular cysteine persulfide levels in SH-SY5Y cells. This alteration in intracellular persulfide was also observed in cystine-free medium.

Considering this reaction of HS⁻ as a precursor of BS-Mix, we highlighted the cytoprotective effect of Na₂S on human neuroblastoma SH-SY5Y cells against methylglyoxal (MG)-induced toxicity. BS-Mix produced with Na₂S in cystine-containing medium provided SH-SY5Y cells significant protective effect against MG-induced toxicity. However, the protective effect was attenuated in cystine-free medium. Moreover, we observed that Na₂S or BS-Mix activated the Keap1/Nrf2 system and increased glutathione (GSH) levels in the cell. In addition, the activation of Nrf2 is significantly attenuated in cystine-free medium.

These results suggested that Na₂S protects SH-SY5Y cells from MG cytotoxicity through the activation of Nrf2, mediated by cysteine persulfides and polysulfides that were generated by Na₂S addition.

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Neuronal cells were protected from methylglyoxal-induced toxicity by cysteine persulfides.

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H₂S immediately reacts with cystine to form persulfides and polysulfides in culture medium.

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Cysteine persulfides protect neuronal cells from carbonyl stress through the activation of Nrf2.

Nitrosopersulfide (SSNO-) targets the Keap-1/Nrf2 redox system

Nitric oxide (NO), hydrogen sulfide and polysulfides have been proposed to contribute to redox signaling by activating the Keap-1/Nrf2 stress response system. Nitrosopersulfide (SSNO^-) recently emerged as a bioactive product of the chemical interaction of NO or nitrosothiols with sulfide; upon decomposition it generates polysulfides and free NO, triggering the activation of soluble guanylate cyclase, inducing blood vessel relaxation in vitro and lowering blood pressure in vivo. Whether SSNO^- itself interacts with the Keap-1/Nrf2 system is unknown. We therefore sought to investigate the ability of SSNO^- to activate Nrf2-dependent processes in human vascular endothelial cells, and to compare the pharmacological effects of SSNO^- with those of its precursors NO and sulfide at multiple levels of target engagement. We here demonstrate that SSNO^- strongly increases nuclear levels, binding activity and transactivation activity of Nrf2, thereby increasing mRNA expression of Hmox-1, the gene encoding for heme oxygenase 1, without adversely affecting cell viability. Under all conditions, SSNO^- appeared to be more potent than its parent compounds, NO and sulfide. SSNO^--induced Nrf2 transactivation activity was abrogated by either NO scavenging with cPTIO or inhibition of thiol sulfuration by high concentrations of cysteine, implying a role for both persulfides/polysulfides and NO in SSNO^- mediated Nrf2 activation. Taken together, our studies demonstrate that the Keap-1/Nrf2 redox system is a biological target of SSNO^-, enriching the portfolio of bioactivity of this vasoactive molecule to also engage in the regulation of redox signaling processes. The latter suggests a possible role as messenger and/or mediator in cellular sensing and adaptations processes.

Improving the soluble expression and purification of recombinant human stem cell factor (SCF) in endotoxin-free Escherichia coli by disulfide shuffling with persulfide

We here present a new method for the expression and purification of recombinant human stem cell factor (rhSCF(164)) in endotoxin-free ClearColi(®) BL21(DE3) cells harboring codon-optimized Profinity eXact™-tagged hSCF cDNA. Previously, we demonstrated that co-expression with thioredoxin increased the solubility of rhSCF in Escherichia coli BL21(DE3), and addition of l-arginine enhanced chromatography performance by removing the endotoxin-masked surface of rhSCF. Initially, we tried to express rhSCF in an endotoxin-free strain using a thioredoxin co-expression system, which resulted in significantly lower expression, possibly due to the stress imposed by overexpressed thioredoxin or antibiotics susceptibility. Therefore, we developed a new expression system without thioredoxin. External redox coupling was tested using persulfides such as glutathione persulfide or cysteine persulfide for the in vivo-folding of hSCF in the cytoplasm. Persulfides improved the protein solubility by accelerating disulfide-exchange reactions for incorrectdisulfides during folding in E. coli. Furthermore, the persulfides enhanced the expression level, likely due to upregulation of the enzymatic activity of T7 RNA polymerase. The recombinant protein was purified via affinity chromatography followed by cleavage with sodium fluoride, resulting in complete proteolytic removal of the N-terminal tag. The endotoxin-free fusion protein from ClearColi(®) BL21(DE3) could bind to the resin in the standard protocol using sodium phosphate (pH 7.2). Furthermore, purified rhSCF enhanced the proliferation and maturation of the human mast cell line LAD2. Thus, we conclude that use of the protein expression system employing E. coli by disulfide shuffling with persulfide addition could be a very useful method for efficient protein production.

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

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

Kinetic and thermodynamic studies on the disulfide-bond reducing potential of hydrogen sulfide

The significance of persulfide species in hydrogen sulfide biology is increasingly recognized. However, the molecular mechanisms of their formation remain largely elusive. The obvious pathway of the reduction of biologically abundant disulfide moieties by sulfide was challenged on both thermodynamic and kinetic grounds. Using DTNB (5,5'-dithiobis-(2-nitrobenzoic acid), also known as Ellman's reagent) as a model disulfide we conducted a comprehensive kinetic study for its reaction with sulfide. The bimolecular reaction is relatively fast with a second-order rate constant of 889 ± 12 M(-1)s(-1) at pH = 7.4. pH dependence of the rate law revealed that the reaction proceeds via the bisulfide anion species with an initial nucleophilic thiol-disulfide exchange reaction to give 5-thio-2-nitrobenzoic acid (TNB) and TNB-persulfide with a pH independent second-order rate constant of 1090 ± 12 M(-1)s(-1). However, kinetic studies and stoichiometric analyses in a wide range of reactant ratios together with kinetic simulations revealed that it is a multistep process that proceeds via kinetically driven, practically irreversible reactions along the disulfide → persulfide → inorganic polysulfides axis. The kinetic model postulated here, which is fully consistent with the experimental data, suggests that the TNB-persulfide is further reduced by sulfide with a second-order rate constant in the range of 5 × 10(3) - 5 × 10(4) M(-1)s(-1) at pH 7.4 and eventually yields inorganic polysulfides and TNB. The reactions of cystine and GSSG with sulfide were found to be significantly slower and to occur via more complicated reaction schemes. (1)H NMR studies suggest that these reactions also generate Cys-persulfide and inorganic polysulfide species, but in contrast with DTNB, in consecutive equilibrium processes that are sensitive to changes in the reactant and product ratios. Collectively, our results demonstrate that the reaction of disulfides with sulfide is a highly system specific process from both thermodynamic and kinetic aspects, which together with the considerable steady-state concentrations of the reactants in biological systems signifies physiological relevance.

Shared-intermediates in the biosynthesis of thio-cofactors: Mechanism and functions of cysteine desulfurases and sulfur acceptors

Cysteine desulfurases utilize a PLP-dependent mechanism to catalyze the first step of sulfur mobilization in the biosynthesis of sulfur-containing cofactors. Sulfur activation and integration into thiocofactors involve complex mechanisms and intricate biosynthetic schemes. Cysteine desulfurases catalyze sulfur-transfer reactions from l-cysteine to sulfur acceptor molecules participating in the biosynthesis of thio-cofactors, including Fe-S clusters, thionucleosides, thiamin, biotin, and molybdenum cofactor. The proposed mechanism of cysteine desulfurases involves the PLP-dependent cleavage of the C-S bond from l-cysteine via the formation of a persulfide enzyme intermediate, which is considered the hallmark step in sulfur mobilization. The subsequent sulfur transfer reaction varies with the class of cysteine desulfurase and sulfur acceptor. IscS serves as a mecca for sulfur incorporation into a network of intertwined pathways for the biosynthesis of thio-cofactors. The involvement of a single enzyme interacting with multiple acceptors, the recruitment of shared-intermediates partaking roles in multiple pathways, and the participation of Fe-S enzymes denote the interconnectivity of pathways involving sulfur trafficking. In Bacillus subtilis, the occurrence of multiple cysteine desulfurases partnering with dedicated sulfur acceptors partially deconvolutes the routes of sulfur trafficking and assigns specific roles for these enzymes. Understanding the roles of promiscuous vs. dedicated cysteine desulfurases and their partnership with shared-intermediates in the biosynthesis of thio-cofactors will help to map sulfur transfer events across interconnected pathways and to provide insight into the hierarchy of sulfur incorporation into biomolecules. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Characterization of the mechanism of the NADH-dependent polysulfide reductase (Npsr) from Shewanella loihica PV-4: formation of a productive NADH-enzyme complex and its role in the general mechanism of NADH and FAD-dependent enzymes

The NADH-dependent polysulfide reductase (Npsr) from Shewanella loihica PV-4 is a member of the single cysteine-containing subset of the family of disulfide reductases represented by glutathione reductase. We have determined the kinetics of the reductive half-reaction of the enzyme with NADH using stopped-flow spectroscopy and kinetic isotope effects, and these results indicate that the reductive and oxidative half-reactions are both partially rate-limiting for enzyme turnover. During reaction with NADH, the reduced nucleotide appears to bind rapidly in an unproductive conformation, followed by the formation of a productive E·NADH complex and subsequent electron transfer to FAD. F161 of Npsr fills the space in which the nicotinamide ring of NADH would be expected to bind. We have shown that while this residue is not absolutely required for catalysis, it does assist in the forward commitment to catalysis through its role in the reductive half reaction, where it appears to enhance hydride transfer in the productive E·NADH complex. While the fluorescence and absorbance spectra of the stable redox forms of the wild-type and F161A mutant enzymes are similar, intermediates formed during reduction and turnover have different characteristics and appear to indicate that the enzyme-NADH complex formed just prior to hydride transfer on the F161A enzyme has weaker FAD-NADH interactions than the wild-type enzyme, consistent with a "looser" enzyme-NADH complex. The 2.7Å crystal structure of the F161A mutant was determined, and shows that the nicotinamide ring of NADH would have the expected freedom of motion in the more open NADH binding cavity.

S-sulfhydration/desulfhydration and S-nitrosylation/denitrosylation: a common paradigm for gasotransmitter signaling by H2S and NO

Sulfhydryl groups on protein Cys residues undergo an array of oxidative reactions and modifications, giving rise to a virtual redox zip code with physiological and pathophysiological relevance for modulation of protein structure and functions. While over two decades of studies have established NO-dependent S-nitrosylation as ubiquitous and fundamental for the regulation of diverse protein activities, proteomic methods for studying H2S-dependent S-sulfhydration have only recently been described and now suggest that this is also an abundant modification with potential for global physiological importance. Notably, protein S-sulfhydration and S-nitrosylation bear striking similarities in terms of their chemical and biological determinants, as well as reversal of these modifications via group-transfer to glutathione, followed by the removal from glutathione by enzymes that have apparently evolved to selectively catalyze denitrosylation and desulfhydration. Here we review determinants of protein and low-molecular-weight thiol S-sulfhydration/desulfhydration, similarities with S-nitrosylation/denitrosylation, and methods that are being employed to investigate and quantify these gasotransmitter-mediated cell signaling systems.