The information available on the biological activity of hydrogen sulfide has been examined for present status of critical results pertaining to the toxicity of hydrogen sulfide. This review of the literature is intended as an evaluative report rather than an annotated bibliography of all the source material examined on hydrogen sulfide. The information was selected as it might relate to potential toxic effects of hydrogen sulfide to man and summarized, noting information gaps that may require further investigation. Several recommendations are listed for possible consideration for either toxicological research or additional short- and long-term tests. Two bibliographies have been provided to assist in locating references considered in this report: (1) literature examined but not cited and (2) reference citations. The majority of the references in the first bibliography were considered peripheral information and less appropriate for inclusion in this report.

A Critical Review of the Literature on Hydrogen Sulfide Toxicity

Reactive oxygen, nitrogen, and sulfur species, referred to as ROS, RNS, and RSS, respectively, are produced during normal cell function and in response to various stimuli. An imbalance in the metabolism of these reactive intermediates results in the phenomenon known as oxidative stress. If left unchecked, oxidative molecules can inflict damage on all classes of biological macromolecules and eventually lead to cell death. Indeed, sustained elevated levels of reactive species have been implicated in the etiology (e.g., atherosclerosis, hypertension, diabetes) or the progression (e.g., stroke, cancer, and neurodegenerative disorders) of a number of human diseases.1 Over the past several decades, however, a new paradigm has emerged in which the aforementioned species have also been shown to function as targeted, intracellular second messengers with regulatory roles in an array of physiological processes.2 Against this backdrop, it is not surprising that considerable ongoing efforts are aimed at elucidating the role that these reactive intermediates play in health and disease.

Site-specific, covalent modification of proteins represents a prominent molecular mechanism for transforming an oxidant signal into a biological response. Amino acids that are candidates for reversible modification include cysteines whose thiol (i.e., sulfhydryl) side chain is deprotonated at physiological pH, which is an important attribute for enhancing reactivity. While reactive species can modify other amino acids (e.g., histidine, methionine, tryptophan, and tyrosine), this Review will focus exclusively on cysteine, whose identity as cellular target or “sensor” of reactive intermediates is most prevalent and established.3 Oxidation of thiols results in a range of sulfur-containing products, not just disulfide bridges, as typically presented in biochemistry textbooks. An overview of the most relevant forms of oxidized sulfur species found in vivo is presented in Chart 1.






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Chart 1

Biologically Relevant Cysteine Chemotypesa

aRed, irreversible modifications. Green, unique enzyme intermediates. Note: Additional modifications can form as enzyme intermediates including thiyl radicals, disulfides, and persulfides.

Cysteine-Mediated Redox Signaling: Chemistry, Biology, and Tools for Discovery

Hydrogen sulfide (H2S), a gaseous species produced by both bacteria and higher eukaryotic organisms, including mammalian vertebrates, has attracted attention in recent years for its contributions to human health and disease. H2S has been proposed as a cytoprotectant and gasotransmitter in many tissue types, including mediating vascular tone in blood vessels as well as neuromodulation in the brain. The molecular mechanisms dictating how H2S affects cellular signaling and other physiological events remain insufficiently understood. Furthermore, the involvement of H2S in metal-binding interactions and formation of related RSS such as sulfane sulfur may contribute to other distinct signaling pathways. Owing to its widespread biological roles and unique chemical properties, H2S is an appealing target for chemical biology approaches to elucidate its production, trafficking, and downstream function. In this context, reaction-based fluorescent probes offer a versatile set of screening tools to visualize H2S pools in living systems. Three main strategies used in molecular probe development for H2S detection include azide and nitro group reduction, nucleophilic attack, and CuS precipitation. Each of these approaches exploits the strong nucleophilicity and reducing potency of H2S to achieve selectivity over other biothiols. In addition, a variety of methods have been developed for the detection of other reactive sulfur species (RSS), including sulfite and bisulfite, as well as sulfane sulfur species and related modifications such as S-nitrosothiols. Access to this growing chemical toolbox of new molecular probes for H2S and related RSS sets the stage for applying these developing technologies to probe reactive sulfur biology in living systems.

/pdf/chemical-probes-for-molecular-imaging-and-detection-of-34y1mdog9w.pdf

Chemical probes for molecular imaging and detection of hydrogen sulfide and reactive sulfur species in biological systems

A search for derivatized amino acids in proteins has shown that the extent of posttranslational modification of proteins is quite substantial. While only 20 primary amino acids are specified in the genetic code and are involved as monomer building blocks in the assembly of the polypeptide chain, about 140 amino acids and amino acid derivatives have been identified as constituents of different proteins in different organisms. A brief consideration of the questions about where and when the derivatization reactions occur, how the specificity of the reactions is established, and how the posttranslational modifications can facilitate biological processes, reveal a need for more information on all these points. Answers to these questions should represent significant contributions to our understanding of biochemistry and cell biology.

http://science.sciencemag.org/content/sci/198/4320/890.full.pdf

Posttranslational covalent modification of proteins

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

Chemical Biology of H2S Signaling through Persulfidation.

The addition of sodium sulfide to disulfide-containing proteins dissolved in 0.01 N NaOH produces an absorption in the range of 320–350 nm similar to that exhibited by cystine or cystamine treated under the same conditions. Proteins free of cystine and cysteine and proteins containing disulfide bonds which have been reduced with sodium borohydride do not produce this absorbance. Urea and guanidine at 6 M concentration increase the rate of production of the absorbance while sodium dodecyl sulfate at concentration of 0.02 M has an inhibitory effect both on the rate and the extent of the absorbance. The effect of pH, concentration of reagents and of the additions of cyanide and hypotaurine has been studied. The 320–350 nm absorbance as been related to the production of persulfide groups (R-SSH) as a result of the cleavage of disulfide bonds by sulfide. Attempts to prepare a purified model protein containing persulfide groups have failed owing to the instability of the persulfide group in conditions far from those used for its preparation.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1970.tb00275.x

Interaction of Proteins with Sulfide

Pantothenoylcysteine-4′-phosphate decarboxylase has been purified 700-fold from horse liver by by an improved method. Enzyme activity was determined by measuring the amount of labeled CO2 produced from the labeled cysteine moiety of the substrate molecule. The enzyme is not bound to any cellular structure since more than 80% of the total activity of the homogenate is recovered in the supernatant fraction. The enzyme does not seem to contain pyridoxal phosphate, is strongly inhibited by this latter and by carbonyl reagents, such as sodium borohydride, hydroxylamine and phenylhydrazine. These findings strongly support the presence in the enzyme molecule of a reactive carbonyl group. The enzymic reaction rate is increased over 20-fold by sulfhydryl compounds, has an absolute specificity for pantothenoylcysteine 4′-phosphate and is not inhibited either by pantothenoylcysteine or by the reaction product, pantetheine 4′-phosphate.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1974.tb03805.x

Erminia Barboni

Papers

Interaction of Proteins with Sulfide

Pantothenoylcysteine-4′-phosphate Decarboxylase from Horse Liver