Figure 1. Enzymatic and non-enzymatic synthesis of H₂S and its metabolism.

Enzymatic synthesis of H2S involves three enzymes in the mammalian systems, Cystathionine β-synthase (CBS), Cystathionine γ-lyase (CGL or CSE) and 3-mercaptopyruvate sulfurtransferase (MST) to form H2S. Non-enzymatic synthesis occurs through glucose, glutathione and polysulfides. H2S further metabolized into thiocyanate, methanethiol and thiosulfate catalyzed by rhodanese, bisulfide methyltransferase (BMT) and thiosulfate reductase (TSR) enzymes respectively. Thiosulfate can oxidize to sulfite through thiosulfate sulfurtransferase (TSST) and subsequently to sulfate. H2S reacts with hemoglobin to form sulfhemoglobin and with proteins in the tissues in the form of bound sulfur pool.