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

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Abstract
Three experiments were performed on 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 intake 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''-methylnicotinamide (N-MNA) seemed to decrease. Urinary excretion of quinolinic acid decreased during the experimental period in all rats. These facts suggest that the conversion of quinolinic acid to NADP may not be inhibited by an excess intake of leucine. Urinary excretion of 5-hydroxyindoleacetic acid increased in all rats, and there was no significant difference among them. Branched-chain amino acid transaminase activity decreasedin vitamin B6 deficiency, but rather increased in excess leucine. The serum leucine level did not increase in humans in a previous experiment, but in the present experiment in rats, it increased on the 17th and 21st days of administration of 10% leucine. Serum valine and isoleucine levels decreased; the 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 were concerned.

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