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Hydrogen sulfide is produced in small amounts by some cells of the mammalian body and has a number of biological signaling functions. Only two other such gases are currently known: nitric oxide and carbon monoxide.
The gas is produced from cysteine by the enzymes cystathionine beta-synthase and cystathionine gamma-lyase. It acts as a relaxant of smooth muscle and as a vasodilator and is also active in the brain, where it increases the response of the NMDA receptor and facilitates long term potentiation, which is involved in the formation of memory.
Eventually the gas is converted to sulfite in the mitochondria by thiosulfate reductase, and the sulfite is further oxidized to thiosulfate and sulfate by sulfite oxidase. The sulfates are excreted in the urine.
Due to its effects similar to nitric oxide , hydrogen sulfide is now recognized as potentially protecting against cardiovascular disease. The cardioprotective role effect of garlic is caused by catabolism of the polysulfide group in allicin to H2S, a reaction that could depend on reduction mediated by glutathione.