Tryptophan Hydroxylase

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of the biogenic monoamine serotonin (5-hydroxytryptamine, 5-HT). TPH belongs to the family of the aromatic amino acid hydroxylases, including phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH), which all have a strict requirement for dioxygen, non-heme iron (II) and tetrahydrobiopterin (BH4).Tryptophan hydroxylase is activated by phosphorylating conditions (ATP-Mg²⁺) in a calcium-dependent, cyclic AMP-independent manner. Two existing TPH isoforms (TPH-1 and TPH-2) are responsible for the generation of two distinct serotonergic systems in vertebrates.

TPH1, predominantly expressed in the gastrointestinal tract and pineal gland, mediates 5-HT biosynthesis in non-neuronal tissues, while TPH2, mainly found in the raphe nuclei of the brain stem, is accountable for the production of 5-HT in the brain. TPH-1, the principal enzyme for peripheral serotonin biosynthesis, provides a novel target to design anabolic agents for osteoporosis. TPH2 is a phenotypic marker for serotonin neurons and is known to be extremely labile to oxidation. As a major modulator of 5-HT neurotransmission and the stress response, TPH2 is of both pathophysiological and pharmacological significance, and is emerging as a new therapeutic target for the treatment of stress disorders.