Whole-body autoradiographic studies on the distribution of 14C-labeled D- and L-5-hydroxytryptophan, 5-hydroxytryptamine and 5-hydroxyindole-3-acetic acid in rats.

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Abstract
The distribution of D- and L-isomers of 14C-labeled 5-hydroxytryptophan (5-HTP) was comparatively investigated by means of whole-body autoradiography following i.v. and oral administration to rats. The distribution of 14C-labeled 5-hydroxytryptamine (5-HT, serotonin) and 5-hydroxyindole-3-acetic acid (5-HIAA), the main end product of 5-HTP metabolism, was also studied for comparison purposes. The following differences were found in the distribution pattern of radioactivity between D- and L-isomers of 5-HTP: a rapid and appreciable uptake of radioactivity in the brain only by L-5-HTP, but none by D-5-HTP and 5-HT; a high uptake and accumulation of radioactivity in the adrenal medulla by L-5-HTP and 5-HT, but none by D-5-HTP; a high distribution of radioactivity in the skeletal muscle by L-5-HTP, but none by D-5-HTP and 5-HT; a gradual accumulation of radioactivity in the spleen by L-5-HTP and 5-HT, but none by D-5-HTP; a high accumulation and retention in the pancreas by both D- and L-5-HTP, but none by 5-HT; a high absorbability of L-5-HTP from the intestine in contrast to a very limited absorbability of the D-isomer. The distribution pattern, particularly in regard to the brain uptake, of L-5-HTP-14C did not change substantially on increasing the oral dosage, in contrast to a significant change observed for L-dopa. 5-HIAA, when injected i.v., was eliminated from the body extremely rapidly through the urinary route. These results were discussed with respect to the possible use of L-5-HTP orally as a brain serotonin precursor.

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