Characterization of tryptophan-niacin metabolism in rats fed with an excessive tryptophan diet

We investigated tryptophan-niacin metabolism in rats when fed with an excessive tryptophan diet. Male rat of the Wistar strain (3 weeks old) were divided into the four groups of five rats each, and one of the group was fed with a 20% casein diet added with 0, 0.5, 1, 2, and 5% tryptophan for 30 days. The last day urine samples (24-h urine) were collected for analyses for the metabolites of tryptophan such as kynurenic acid (KA), xanthurenic acid (XA), 3-hydroxyanthranilic acid (3-HA), quinolinic acid (QA), nicotinamide (Nam), N1-methylnicotinamide (MNA), N1-methyl-4-pyridone-3-carboxamide(4-Py), and N1-methyl-2-pyridone-5-carboxamide (2-Py). The urinary excretion of KA, XA, and 3-HA were increased according to the intake of tryptophan. However, the excretion of QA was almost the same in the groups between the groups of 2 and 5% tryptophan diets. The sum metabolites of Nam + MNA + 2-Py + 4-Py were almost the same in the 1-5% tryptophan diets. The value of (2-Py + 4-Py)/MNA decreased sharply in the 0.5 and 1% Trp diets. Therefore the adverse effects of dietary Trp were observed from the diet containing 0.5% Trp, which was calculated as 0.45 g/kg of rat body weight.