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Failure of Dietary Leucine to Influence the Tryptophan-Niacin Pathway in the Chicken

A. M. Penz, Jr.¹, A. J. Clifford^*, Q. R. Rogers and F. H. Kratzer²

Department of Avian Sciences ^* Department of Nutrition Department of Physiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616

Three experiments were conducted with young chicks to determine whether dietary leucine affects the conversion of tryptophan to niacin. Either tryptophan or niacin improved growth and reduced perosis when chicks were fed a purified diet marginal in tryptophan and deficient in niacin. Addition of 4.8% L-leucine to the diet did not alter the growth and perosis prevention response obtained with tryptophan. Liver weight was slightly increased by the addition of 5.4% L-leucine to the diet. Plasma insulin was slightly reduced by leucine and by isoleucine and valine. Picolinic carboxylase in the kidney was reduced in chicks fed 0.2% tryptophan with no niacin and was also reduced when isoleucine and valine were added to the diets. Liver picolinic carboxylase activity was not influenced by diet. Plasma isoleucine and valine were reduced by the addition of leucine to the diet and were increased again when isoleucine and valine were added to the diet. Plasma leucine was increased by the addition of leucine but was not altered by valine and isoleucine. Plasma tryptophan was not influenced by dietary supplements of leucine or isoleucine and valine. The results show that in the chick there is no evidence for an effect of leucine on the tryptophan to niacin pathway.

KEY WORDS: • tryptophan-niacin pathway • leucine excess • branched-chain amino acids • plasma amino acids

¹ Present address: Dept. de Zootecnia, Universidade Federal do Rio Grande do Sul, Porte Alegre, Brasil.

² To whom reprint requests should be addressed.

Manuscript received 28 March 1983.