Fig. 1. Sulfide metabolite formation and fate.

a | Various chemical metabolite fate pathways for sulfide and its related species are shown. The basal level of production of hydrogen sulfide (H₂S) is determined by the activity of three main enzymes: cystathionine γ-lyase (CTH), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (MPST). In addition, bacterial enzymes (such as sulfate-reducing bacteria (SRB), sulfite reductase [NADPH] flavoprotein α-component (CysJ) and anaerobic sulfite reductase subunit A (AsrA)) can reduce terminal sulfide oxidation end products (such as thiosulfate, sulfate and sulfite) back to H₂S. H₂S can undergo a myriad of reactions leading to the formation of small oxoacids of sulfur, sulfane sulfur species and acid-labile sulfur species. b | Various enzymatic and non-enzymatic biochemical pathways are involved in sulfide metabolite formation. Sulfide catabolism through the mitochondrial H₂S oxidation pathway leads to the metabolic end products of sulfate and thiosulfate. CARS, cysteinyl–tRNA synthetase (also known as cytoplasmic cysteine–tRNA ligase); CAT, cysteine aminotransferase; CysSH, cysteine; CysSSH, cysteine hydropersulfide; ETHE1, persulfide dioxygenase; GSH, glutathione; GSSG, glutathione disulfide; MP, mercaptopyruvate; PP_i, inorganic pyrophosphate; SQR, sulfide–quinone oxidoreductase; SQR-SSH, sulfide–quinone oxidoreductase hydropersulfide.