Urinary excretory ratio of anthranilic acid/kynurenic acid as an index of the tolerable amount of tryptophan.

Some people may take excessive tryptophan as a supplement in the expectation that the tryptophan metabolite, melatonine, will help to induce sufficient sleep. We investigated the basis for a useful index to assess the risk of a tryptophan excess. Young rats were fed on a 20% casein diet with 0, 0.5, 1.0, 2.0 or 5.0% added tryptophan for 30 d the apparent toxicity and growth retardation was observed in the 5.0% tryptophan-added group. Metabolites of the Tryptophan-nicotinamide pathway and such intermediates as kynurenic acid (KA), anthranilic acid (AnA), xanthurenic acid, 3-hydroxyanthranilic acid and quinolinic acid in 24-h urine increased in a dose-dependent manner. Of those metabolites and intermediates, the urinary excretion of KA progressively increased, and that of AnA dramatically increased in the 2.0 and 5.0% tryptophan-added groups. The urinary excretory ratio of AnA/KA was a high value for both the groups. These results suggest that the urinary ratio of AnA/KA could be a useful index to monitoran excessive tryptophan intake.