Yasutomi Nishizuka

Tryptophan-5-Hydroxylase in Mammalian Brain*

Publisher Summary This chapter describes a procedure to explore the properties and function of tryptophan-5-hydroxylase in mammalian brain. The biosynthesis of 5-HT in the brain was reported recently following direct administration of radioactive tryptophan into the brain of rats and pigeons. The animals were treated with a monoamine oxidase (MAO) inhibitor prior to injection to prevent the breakdown of 5-HT. Subsequent reports from several laboratories, described the hydroxylation of tryptophan in vitro with extracts of various mammalian tissues such as carcinoid tumors, brain, and neoplastic mouse mast cells. The detailed properties of tryptophan-5-hydroxylases as well as the reaction mechanism involved are still largely unexplored, mainly because of the relatively low activity of this enzyme and the lack of a sensitive and reliable assay for its activity. Assay methods so far available are based on the determination of radioactive 5-HT separated from a large excess of unreacted tryptophan by paper chromatography, paper electrophoresis, or ion-exchange resins. Mast cell tryptophan 5-hydroxylase is an exception, being active enough to accumulate 5-HTP.