Fig. 1
From: Emerging pharmacological tools to control hydrogen sulfide signaling in critical illness
Pathways of H2S production. Cysteine is produced from homocysteine via transsulfuration pathway mediated by cystathionine β-synthase (CBS) and cystathionine gamma-lyase (CSE, CGL or CTH). H2S is produced from homocysteine and cysteine by CBS and CSE. 3-Mercaptopyruvate sulfurtransferase (3-MST) generates 3-MST-cysteine persulfide (MST-SSH) utilizing mercaptopyruvate which is produced from cysteine by cysteine aminotransferase (CAT). H2S is released from MST-SSH via non-enzymatic reaction or catalytic activity of thioredoxin (Trx). H2S is oxidized by sulfide oxidation unit (SOU) to produce thiosulfate and sulfate utilizing O2 as described in the “H2S catabolism” section. H2S is generated from thiosulfate by non-enzymatic reaction using reductants in hypoxia or catalytic activity of thiosulfate sulfurtransferase (TST) or 3-MST. H2S is non-enzymatically produced from cysteine in an iron (Fe3+)- and vitamin B6-dependent manner