Effect of supplementation of limiting amino acids on tyrosine toxicity in rats fed wheat gluten diets.

Abstract

SummaryThe effect of supplementation of limiting amino acids on rats fed a 10 or 20% wheat gluten diet containing 5% tyrosine has been investigated. Growth-limiting amino acids in a 20% wheat gluten diet were lysine and threonine. When rats were fed a 20% wheat gluten diet containing excess tyrosine, the addition of 1.0% lysine• HC1 or 1.0% lysine• HC1 and 0.4% threonine completely prevented the development of tyrosine toxicity, and was accompanied by a lowering of free tyrosine concentrations in plasma, liver and muscle. In rats receiving a 10 % wheat gluten diet, lysine, threonine and methionine were limiting for growth. The addition of 1.0% lysine• HC1 to the 10% wheat gluten plus 5 tyrosine diet failed to alleviate the tyrosine toxicity, but the supplement of 1.0% lysine• HC1 and 0.4% threonine could partially alleviate the toxicity. The combined addition of 1.0% lysine• HC1, 0.4% threonine and 0.5% methionine was most effective. The free tyrosine concen trations in plasma, liver and muscle were lowered greatly by the supple mentation of three limiting amino acids, but amounts of free tyrosine and phenolic metabolites excreted in the urine did not decrease with ad dition of these limiting amino acids.It has been shown in young rats fed a low protein diet containing excess tyrosine, that the growth is retarded and eye and paw lesions develop (1-6). Recently we found that the growth depression and appearance of pathological lesions which developed in rats fed a 10% casein diet containing 5% tyrosine were alleviated by the supplement of methionine and threonine, the first and second limiting amino acids respectively, in the diet (7,8).On the other hand, it was also demonstrated in our laboratory (9) that the tyrosine toxicity produced by feeding an 11.6% wheat gluten diet containing 5 tyrosine (equivalent in N content to 10% casein) was not at all alleviated by the 115

Effect of additional protein or methionine and threonine on tyrosine catabolism in rats fed diets high in tyrosine.

“…That free tyrosine concentration in tissues increases greatly in rats fed a high tyrosine diet but decreases in animals fed a high tyrosine diet containing additional protein or some amino acids has been well demon strated. 5,8,14,18,19,24,25) Results of Experiment 2 showed that not only addition of extra casein but also methionine and threonine has a great effect on lowering free tyrosine concentrations. These data indicate clearly that there exists a high correlation between tyrosine concentra tion in body fluids and development of tyrosine toxicity, and also suggest that the main sub stance responsible for tyrosine toxicity is tyrosine itself.…”