Some Effects of Amino Acids and Cortisol on Tyrosine Toxicity in the Rat

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Abstract
An excessive tyrosine intake causes growth depression and paw and eye lesions in young rats fed a low protein diet. The objective of this investigation was to determine the effects of various amino acid supplements and of cortisol injections on the development of tyrosine toxicity syndrome in rats fed a high tyrosine diet. Plasma tyrosine concentration, liver tyrosine transaminase activity and excretion of tyrosine metabolites were measured in rats receiving the amino acid supplements. Supplements of glycine, methionine, tryptophan or a mixture of leucine, isoleucine and valine as well as injections of cortisol and Celite prevented the development of the pathological lesions. Threonine supplementation prevented the growth-depressing effect of high dietary tyrosine and also the development of lesions. High tyrosine diets increased adrenal weight. Cortisol and Celite injections as well as various dietary supplements of amino acids decreased plasma tyrosine concentration. A high tyrosine intake increased liver tyrosine transaminase activity and some amino acid supplements caused further increases in the activity of the enzyme. However, the changes in tyrosine transaminase were not consistent and did not appear to be related to changes in plasma tyrosine concentration. A high tyrosine intake also resulted in increased total phenol excretion. The various amino acid supplements did not increase it further. After 3 weeks of feeding threonine and glycine supplements there was a substantial increase in homogentisic acid excretion in urine which suggests that the rate of catabolism of tyrosine is increased by such treatments.