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Major Pathways of H₂S synthesis in vivo. H₂S is synthesized from the amino acids L-cysteine and L-cystathionine by one of two enzymes (depending on cell type), cystathion-ine-β-synthase (CBS) and cystathionine-γ-lyase (CSE). Aminooxyacetate and propargyl-glycine (PAG) are commonly used inhibitors of CBS and CSE activity, respectively. The most prominent source of H₂S in the vasculature is CSE. Removal of H₂S is thought to occur via oxidation to sulfate (SO₄²⁻), sulfite (SO₃²–) and thiosulfate (S₂O₃²⁻), scavenging by methemoglobin (MetHb) or methylation by enzymes such as thiolmethyltransferase (TMT) and rhodanese to form methanethiol (CH4S) and dimethylsulfide (CH₃SCH₃).