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Effect of Dietary Protein on Tyrosine Toxicity in the Rat¹

Laboratory of Food and Nutrition, Department of Agricultural Chemistry, Faculty of Agriculture, Shizuoka University, Shizuoka, Japan

The objective of this study was to determine the effect of supplementation of different dietary proteins on tyrosine toxicity in rats fed a low protein diet containing excess tyrosine. The growth retardation and the development of eye and paw lesions that occur in rats ingesting a 10% casein plus 5% tyrosine diet could be alleviated by the extra addition of 15 or 40% casein to the diet. Similar effects were also observed after supplementation of wheat or corn gluten at levels equivalent to the nitrogen content of 15 or 40% casein. Liver tyrosine transaminase, p-hydroxyphenylpyruvate hydroxylase, and homogentisate oxidase activities were elevated by increasing the protein level in the diets. Further increase in tyrosine transaminase activity was produced in rats fed 10% casein diet containing excess tyrosine, but p-hydroxyphenylpyruvate hydroxylase and homogentisate oxidase activities failed to increase further. Urinary excretions of total phenols, volatile phenols, and ketonic acids (p-hydroxyphenylpyruvic acid), in general, increased in rats fed diets containing excessive tyrosine, but the level of total ketonic acids of rats fed a 10% casein plus 5% tyrosine diet was markedly decreased by the supplementation of 15 or 40% casein or wheat gluten (equivalent to 40% casein). Only trace amounts of homogentisic acid were excreted by rats consuming high tyrosine diets, except those fed a 10% casein diet supplemented with high levels of corn gluten. The liver ribosomes prepared from rats fed the 10% casein plus 5% tyrosine diet incorporated less L-[¹⁴C]leucine into protein in a cell-free system than those from animals fed excess tyrosine diets containing extra amounts of different proteins.

KEY WORDS: • tyrosine toxicity • tyrosine transaminase • p-hydroxyphenylpyruvate hydroxylase • homogentisate oxidase

¹ Parts of the data were presented at the annual meeting of Society of the Food and Nutrition in Japan, Tokyo, May 17, 1973.

Manuscript received 3 September 1974.