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Effect of an excess intake of leucine, with and without additions of vitamin B6 and/or niacin, on tryptophan and niacin metabolism in rats.

Three experiments were carried out on a total of 78 female rats to examine the effect of an excess intake of leucine, with and without additions of vitamin B6 and/or niacin, on the excretion of tryptophan and niacin metabolites, and on branched-chain amino acid transaminase activity in some organs and on serum amino acid levels. An excess in take of leucine (10% leucine) retarded the growth of rats, and vitamin B6 and niacin deficiencies did not enhance this effect. Urinary excretion of N-methyl-2-pyridone-5-carboxamide (pyridone) and niacin was scarcely affected by the excess intake of leucine, but the excretion of N•L-methylnicotinamide (N-MNA) seemed to decrease. Urinary excre tion of quinolinic acid decreased during the experimental period in all the rats. These facts suggest that the conversion of quinolinic acid to nicotinamide adenine dinucleotide phosphate (NADP) may not be in hibited by an excess intake of leucine. Urinary excretion of 5-hydroxy indoleacetic acid increased in all the rats, and there was no significant difference among them. Branched-chain amino acid transaminase ac tivity decreased in vitamin B6 deficiency, but rather increased in excess leucine. The serum leucine level did not increase in humans in our previous experiment, but in the present experiment in rats, it increased on the 17th and the 21st days of administration of 10% leucine. Serum valine and isoleucine levels decreased. On the contrary, serum tryptophan level increased. An excess intake of leucine, even with a large amount of it, did not seem to be related to the incidence of pellagra, so far as the urinary excretions of tryptophan and niacin metabolites are concerned.