Effects of Three Kinds of Dietary Nitrogen Sources on the Metabolic Fate of Tryptophan

抄録

We have recently reported that the conversion ratio of tryptophan to nicotinamide [urinary excretion of (nicotinamide + N¹-methylnicotinamide (MNA)+ N¹-methyl-l-pyridone-S-carboxamide (2-Py) + N¹-methyM-pyridone-S-carboxamide (4-Py) (μmol/day)/tryptophan intake during urine collection (μmol/day)] was changeable according to the kind of dietary nitrogen sources. In these experiments, we investigated whether the fate of tryprophan is changeable according to nitrogen sources or not. Weanling rats were fed with a diet low in nitrogen (1.1311% nitrogen) containing a suitable amount of nicotinic acid (6mg/100g of diet) for 17 days. Egg white (EW), egg white proteolysate-5 (EWP-5), and mixture of amino acids simulating the amino acid pattern of EWP-5 were used as dietary nitrogen sources. The urinary excretion of 5-hydroxyindole-3-acetic acid and the sum of nicotinamide and its metabolites in the group fed with the EWP-5 diet was the lowest, but, the excretion ratio of (2-Py + 4-Py)/MNA, which has been reported to be an index of amino acid adequacy, in the group fed with the EWP-5 diet was the highest. From these findings, it was suggested that the tryptophan in EWP-5 (mixtures of small peptides) entered the degradation pathway of tryptophan such as serotonin and nicotinamide biosynthesis with difficulty, but that it was easy to enter the protein biosynthesis, compared with tryptophan in EW (protein and mixtures of amino acids).