Tryptophan hydroxylase (TPH) is an enzyme that catalyzes the 5-hydroxylation of tryptophan, which is the first step in the biosynthesis of indoleamines (serotonin and melatonin).
Two isoforms TPH1 and TPH2 have been described. TPH1 occurs mainly in tissues that express serotonin in the periphery (skin, gut, pineal gland). TPH2 is exclusively expressed in neuronal cell types and is the predominant isoform in the central nervous system.
In mammals, serotonin biosynthesis occurs predominantly in neurons which originate in the Raphe nuclei of the brain.