Figure 1. Hydrogen sulfide and nitric oxide biosynthetic pathways.
Hydrogen sulfide (H2S) and nitric oxide (NO) are produced by enzymatic and non-enzymatic pathways. Non-enzymatic production of H2S is mediated through reducing elemental sulfur or organic polysulfides. Enzymatic production of H2S is mediated by cystathionine γ-lyase (CSE), cystathionine-beta synthase (CBS), and 3-mercaptopyruvate sulfuretransferase (3-MST). NO is produced by nitrate/nitrite pathway which can be enzymatic or non-enzymatic. NO is also produced from L8 arginine by neuronal NO synthase (nNOS), inducible NO synthase (iNOS), and endothelial NO 9 synthase (eNOS).