H 2 S: A Simple Molecule with Multifaceted Biochemistry and Functions

Hydrogen sulfide (H₂ S) is a colorless gas whose characteristic smell of rotten eggs can be perceived with concentrations lower than 1 ppm. It is a natural component of volcanic gases but also it is generated by some microorganisms during the anaerobic decomposition of organic compounds, usually present in airless waters with low oxygen content, such as wetlands, swamps, and landfills. Historically, the first reference regarding the toxicity of H₂S gas in biological systems came from the 18th century. Accordingly, the Italian medical doctor Bernadino Ramazzini (1700), in his book entitled “Dissertation on Workers' Diseases,” described some dangerous consequences of H₂S on the human body such as “the painful irritation and inflammation of sewer gas on the eye in sewer workers.” However, the physiological relevance of H₂S was reported in 1996 in a pioneer study showing that H₂S can be endogenously generated in animal cells and functions as a neuromodulator (Abe and Kimura, 1996). Thus, a new area of research was opened whose expectations have been changing with each new finding that has spread both in the plant and animal kingdoms (Kimura, 2015; Mishra et al., 2021; Yamasaki and Cohen, 2016).

Regardless of the generation of H₂S in microorganisms, it is remarkable that the enzymatic biosynthesis of H₂S is clearly different in plant and animal cells. In higher plants, the generation of H₂S occurs through the metabolism of sulfur that starts from the absorption of sulfate, and the direct generation of H₂S requires another group of enzymes that are located in different subcellular compartments including cytosol, chloroplast, and mitochondrion. H₂S is a byproduct in several cytosolic reactions including the conversion of cysteine (Cys) to ammonia and pyruvate by the pyridoxal 5′-phosphate (PLP)-dependent L-cysteine desulfhydrase (LCD). Moreover, the Cys synthases catalyze the synthesis of L-Cys from O-acetyl-L-serine and H₂S. In chloroplast, the sulfite reductase and in the mitochondrion, the PLP-dependent D-cysteine desulfhydrase (DCD) and cyano alanine synthase are the enzymes responsible for H₂S generation. However, most of the information about the enzymes involved in H₂S biosynthesis comes from the model plant Arabidopsis thaliana.

In contrast, in animal cells, H₂S is produced in the transsulfuration pathway by the action of the PLP-dependent enzymes cystathionine β-synthase and cystathionine γ-lyase (also known as CTH), but it can also be generated during the Cys catabolism pathway by cysteine aminotransferase/mercaptopyruvate sulfur transferase. There is an additional enzyme, methanethiol oxidase that converts methanethiol into formaldehyde, hydrogen peroxide, and H₂S.

This Forum entitled “H₂S: from plants to mammals” is constituted of four review articles and a research article and all together will provide the most updated information on H₂S from different but complementary perspectives including basic biochemical aspects of H₂S, plus persulfide metabolism and protein persulfidation until their implications in plant development and human pathologies. The review article by Ogata et al. (2023) on “Persulfide biosynthesis pathways well conserved evolutionally among all organisms” provides a new perspective on the biological and physiological aspect of persulfides (RSSH) and polysulfides that involved sulfur-catenated molecular species with the general composition R-Sn-R′ (n > 2) or R-Sn-H (n > 1) where R can correspond to either Cys, glutathione, or a protein. In addition, a new methodological approach is discussed for the reliable detection and quantification of low molecular weight and protein persulfide/polysulfides based on liquid chromatography-electrospray ionization-tandem mass spectrometry analysis using β-(4-hydroxyphenyl)ethyl iodoacetamide as a trapping agent. Furthermore, it highlighted the relevance of a new persulfide synthase named cysteinyl-tRNA synthetase (CARS). By using a rigorously quantitative approach, it has been identified that CARS is a novel persulfide synthase whose activity is highly conserved from the Bacteria to Eukarya.

Complementary, the article “Emerging chemical biology of protein persulfidation” by Vignane and Filipovic (2023) offers an updated overview of the relevance of sulfur-based chemistry in the process of protein persulfidation that is an oxidative post-translational modification where thiol (RSH) groups in cysteine residues are transformed into RSSH affecting either positive or negative the function of the target protein. However, the process of transpersulfidation is also described between a persulfidated protein and a unpersulfidated protein that is mediated by the activity of a cysteine desulfurase. Furthermore, the protective effect of protein persulfidation against the oxidation of the cysteine residues is evaluated during cellular oxidative stress processes. In higher plants, H₂S exerts multiple functions that range from physiological processes such as seed germination, plant growth, stomatal movement, senescence, plant cell death, and fruit ripening among others, as well as in the mechanisms of response against environmental stresses.

The review by Huang and Xie (2023) entitled “Hydrogen sulfide signaling in plants” provides a well-organized overview of diverse aspects such as enzymatic components involved in H₂S biosynthesis, the use of different types of exogenous H₂S donors to study its physiological functions, new strategies for in vivo detection of H₂S, plant protein persulfidation and detection, and how this post-translational modification can regulate the function of key proteins involved in the stomatal closure mediated by abscisic acid under different environmental conditions.

Iciek et al. (2023) provide an assessment on “Reactive sulfur species in human diseases” where the relevance of the cellular homeostasis of H₂S and derived molecules from a clinical perspective of neurological diseases including Parkinson, Alzheimer, Down syndrome, Huntington, and schizophrenia as well as diabetes mellitus, cancer, cardiovascular, and respiratory diseases is highlighted. Thus, it has been found that a decrease in the content of H₂S and other derived molecules designated as reactive sulfur species (RSS) is associated with some of these human diseases. Thus, the search for technical approaches to quantify the content of RSS in biological samples, the examination of different H₂S donors, and their mechanism of action could be useful tools in the treatment of these disorders providing new complementary therapeutic strategies. Along with these four reviews that cover different aspects of H₂S metabolism and function at different levels in different organisms, there is also a research article by Muñoz-Vargas et al. (2023) entitled “H₂S-generating cytosolic L-cysteine desulfhydrase and mitochondrial D-cysteine desulfhydrase from sweet pepper (Capsicum annuum L.) are regulated during fruit ripening and by nitric oxide” focused on the identification and biochemical characterization of two H₂S-generating enzymes from L-Cys and D-Cys designated LCD and DCD in the nonclimacteric sweet pepper fruits.

The provided data showed that the activity of both enzymes is downregulated during the ripening. Furthermore, it is demonstrated that the exogenous application of nitric oxide (NO) gas reverted this effect of both LCD and DCD in ripe fruits, suggesting that NO functions as an upstream signal of the H₂S metabolism. Likewise, it is analyzed at the molecular level how NO can modulate the activity of these enzymes by nitration of tyrosine residues causing their inhibition, as well as by S-nitrosation that exerts a protective mechanism for RSH groups.

Thus, the data support that both enzymes are downregulated during ripening, but this effect was reverted after the exogenous treatment of the fruits with NO, suggesting that NO functions as an upstream signal regulating the H₂S generation. Taken together, the data demonstrate the functional crosstalk between both molecules.

Although the relevance of H₂S in plants and animals is currently unquestionable, what is clear is that there are still many unresolved questions, and likewise, there are challenges in the development of new methodological techniques that allow progress in the mechanism of action of H₂S. However, I am confident that this Forum will serve to stimulate future research and advancement in the respective individual subfields within the H₂S area throughout the different organisms. Figure 1 illustrates the main functions of H₂S in physiological and stress conditions in plant and animal organisms, some of which have been revised here.

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FIG. 1.

Overview of the physiological functions of H₂S in plants as well as in animals. An imbalance in the production of H₂S can lead to pathological effects. Exogenous amplification of H₂S may exert beneficial effects. Future challenges will address the improvement of the methodological methods for the correct identification and detection and quantification of H₂S and derived molecules such as polysulfides or persulfidated proteins. H₂S, hydrogen sulfide.