an enzyme which catalyzes the initial step in the biosynthesis of serotonin. It utilizes tetrahydrobiopterin as a component enzyme to modify the dietary amino acid i-tryptophan to 5-hydroxytryptophan. This response is the rate-restricting step in serotonin synthesis, inhibited by amounts of tryptophan within the brain in addition to by amount of activity of neurons which utilize serotonin as a neurotransmitter.
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