The relation between urinary 5-hydroxyindoleacetic acid levels and the ratio of tryptophan to other large neutral amino acids placed in the stomach.
We assayed 5-hydroxyindoleacetic acid (5-HIAA) in urine samples (3- or 6 1/2-hr collection) after individual rats received 6-8 ml of water, of amino acids in solution, or of glucose by stomach tube. Tryptophan (Trp) solutions caused dose-related increases in urinary 5-HIAA; these were blocked when animals received carbidopa, an inhibitor of peripheral aromatic amino acid decarboxylase. The concurrent administration of a large neutral amino acid (LNAA; valine or isoleucine) with oral Trp, in high doses probably sufficient to compete with Trp for transport into gut cells, blocked the Trp-induced rise in urinary 5-HIAA. Concurrent administration of glycine (not a LNAA) in equivalent doses did not. Pretreatment with pyridoxine blocked the Trp-induced rise in urinary 5-HIAA but not that in brain serotonin (5-hydroxytryptamine, 5HT). These observations confirm the previous suggestion that while brain serotonin synthesis depends on the plasma Trp/LNAA ratio (which varies inversely with the proportion of protein to total calories in the diet), gut serotonin synthesis depends largely on the Trp/LNAA ratio in the dietary protein itself (and, probably, within the gut lumen postprandially). The range of molar Trp/LNAA ratios at which free LNAAs significantly diminish the effects of ingested Trp on gut serotonin synthesis (as reflected by urinary 5-HIAA) is similar to the range found in dietary proteins.
D000284:Administration, Oral; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D002230:Carbidopa / Q000494:pharmacology; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism / Q000652:urine*; D007421:Intestine, Small / Q000378:metabolism; D007532:Isoleucine / Q000494:pharmacology*; D008297:Male; D011736:Pyridoxine / Q000494:pharmacology; D051381:Rats; D012701:Serotonin / Q000378:metabolism; D014364:Tryptophan / Q000008:administration & dosage / Q000494:pharmacology*; D014633:Valine / Q000494:pharmacology*
D007532:Isoleucine; D012701:Serotonin; D006897:Hydroxyindoleacetic Acid; D014364:Tryptophan; D014633:Valine; D011736:Pyridoxine; D002230:Carbidopa