Dietary tryptophan is the source of the formation of serotonin. Enzymes and cofactors necessary for serotonin synthesis are present in both the enterochromaf-fin cells of the gastrointestinal tract and neurons in the brain. Tryptophan is initially hydroxylated to form 5-hydroxytryptophan. Decarboxylation of the latter compound results in the formation of serotonin (Fig. 24.2).
The enzymes responsible for the metabolism of serotonin are present in all of the cells containing this amine and in the liver. Serotonin is initially oxidatively deami-nated to form 5-hydroxyindoleacetaldehyde; this compound is subsequently rapidly oxidized to the major metabolite 5-hydroxyindoleacetic acid, which is excreted in the urine. Much of the serotonin released in the brain at synapses is taken back into the initial neuron by an active reuptake mechanism to be released again.
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