The distribution and metabolism of L-tryptophan in healthy probands under dietary conditions.

Oral administration of 500 mg or 1,500 mg of L-tryptophan (TRP) respectively to healthy probands resulted in a significant increase in free and bound TRP in plasma 2 hours after consumption. Serotonin and 5-hydroxyindole-3-acetic acid (5-HIAA) also demonstrate a significant increase in plasma, while tyrosine shows no changes. However, a competitive reciprocity exists between the uptake of tryptophan and tyrosine because TRP increases significantly in the platelets after administration, and tyrosine is reduced. The concentration of serotonin also decreases, an effect which also indicates a competitive reciprocity between tryptophan and serotonin uptake. Hence, the platelet may be employed as a (neuronal) model for the study of mechanisms of amino acid transport through membranes and the factors by which they are influenced. Studies on metabolites of biogenic amines in urine, that is, indole-3-acetic acid (IAA), 5-HIAA, homovanillic acid (HVA), and vanillylmandelic acid (VMA) showed significant increase in IAA, 5-HIAA, and HVA after administration of 1,500 mg TRP. The increased concentration of HVA is traced to synthesis of serotonin in dopaminergic neurons, which leads to temporary secretion of dopamine, and subsequently, HVA.