 Until the discovery of TPH2 in 2003, serotonin levels in the central nervous system were believed to be regulated by serotonin synthesis in peripheral tissues, in which tryptophan hydroxylase is the dominant form.Tryptophan hydroxylase (TPH; EC 1.14.16.4) is the rate-limiting enzyme in the synthesis of serotonin (5-hydroxytryptamine, or 5HT).5HT is causally involved in numerous central nervous activities, and it has several functions in peripheral tissues, including the maintenance of vascular tone and gut motility.