Quantitative Studies on the Urinary Excretion of Tryptophan Metabolites by Humans Ingesting a Constant Diet

The urinary excretion of N-methyl-2-pyridone-5-carboxamide (pyridone), kynurenic acid, xanthurenic acid, kynurenine, Na-acetylkynurenine, o-aminohippuric acid, anthranilic acid glucuronide and an unknown diazotizable aromatic amine was determined on 4 normal males ingesting a constant diet containing an estimated 15 mg of nicotinic acid and 900 mg of tryptophan. After 6 days on the costant diet 2.0 gm of L-tryptophan was given as a single oral dose and the excretion of the metabolites was determined for 6 more days. On the constant diet about 16 mg of pyridone were excreted per day. The other metabolites of tryptophan accounted for 2.5% of the amino acid. Following the ingestion of a single 2.0 gm supplement of tryptophan the pyridone excretion increased to an extent which would account for about 0.8% of the dose. The increase in the excretion of the other metabolites accounted for 1.3% of the supplemental tryptophan. Kynurenic acid, o-aminohippuric acid, and kynurenine were important urinary metabolites of the oral supplement of amino acid. Xanthurenic acid, acetylkynurenine, and anthranilic acid glucuronide were minor metabolites by comparison. A comparison of these results with data of a similar nature obtained with human subjects on a self-selected diet suggested that a constant diet was not necessary for quantitative studies on the metabolism of supplemental doses of tryptophan, unless the conversion to nicotinic acid and its metabolites was of particular interest.