The Effect of Leucine on the Activity of Three Enzymes in the Kynurenine Pathway of Tryptophan Metabolism in the Rat

The effect of adding a mixture of indispensable amino acids simulating zein, with or without leucine, to a basal 6 percent casein, niacin-free diet on weight gain, feed consumption, and activity of kynurenine hydroxylase, 3-hydroxykynureninase, and 3-hydroxyanthranilic oxidase, three enzymes involved in the tryptophan to niacin pathway, was investigated. Three groups of Sprague-Dawley-Long Evans rats were fed one of the experimental diets for two weeks. Livers from each rat were removed and analyzed for the activity of the three enzymes under investigation.

The data obtained indicated that there were no significant differences in weight gain and feed consumption between rats fed each of the three experimental diets. In addition, the values obtained for activity of kynurenine hydroxylase also were not significantly different between rats fed each of the three diets. Although there were no significant differences in the activity of 3-hydroxykynureninase between rats fed each of the diets, the difference between the activity of 3-hydroxykynereninase in the liver of rats fed the indispensable amino acid mixture without leucine and the activity obtained from rats fed the basal diet approached significance (P > 0.10). The activity of 3-hydroxyanthranilic oxidase was significantly higher (0.10 < P < 0.05) in rats fed either of the diets containing the amino acid mixtures than in rats fed the basal diet. The activity of 3-hydroxyanthranilic oxidase in the liver of rats fed the diet containing the amino acid mixture without leucine was significantly lower (P < 0.05) than the activity in rats fed the indispensable amino acid mixture containing leucine.