Figure 2.

Source of H₂S. There are four enzymatic pathways (dotted green rectangle) for the biosynthesis of H₂S, including CBS, CSE, 3MST coupled with CAT, and 3MST that utilize L-cysteine, L-Homocysteine, and 3-mercaptopyruvate (3MP) as substrates. CBS and CSE may generate H₂S in the cytosol whereas 3MST mainly resides and synthesizes H₂S in mitochondria. A small portion of endogenous H₂S is derived via nonenzymatic reduction (dotted blue rectangle). In the presence of reducing equivalents such as NADPH and NADH, reactive sulfur species in persulfides, thiosulfate, and polysulfides are reduced into H₂S and other metabolites. Other sources of H2S are represented by diet, bacterial activities, inhalation, irrigation, and drinking of sulfurous waters. 3MP, 3-mercaptopyruvate; 3MST, 3-mercaptopyruvate sulfurtransferase; CAT, cysteine aminotransferase; CBS, cystathionine β-synthase; CSE, cystathionine γ-lyase; H₂S, hydrogen sulfide; NADH, nicotinamide adenine dinucleotide; NADPH, nicotinamide adenine dinucleotide phosphate.