Growth and Metabolism in Rats Fed a Low Protein Diet Containing Various Levels of Tyrosine and p-Hydroxyphenylpyruvic Acid

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Rats fed a low protein diet containing 3% or more of tyrosine developed external pathological lesions, accompanied by marked depression in growth and food intake. Their liver tyrosine (Tyr) transaminase activity increased in proportion to dietary tyrosine level, but p-hydroxyphenylpyruvic acid (pHPP) hydroxylase activity was not elevated. The excreted amounts of tyrosine and pHPP in urine were increased markedly, and the tyrosine concentration in plasma and tissues was markedly elevated. There was a correlation between free tyrosine concentration in body fluid and the development of pathological lesions. When rats were fed a low protein diet containing graded levels of pHPP up to 5%, they did not develop external lesions, but their growth and food intake were depressed more severely than in rats fed the same levels of tyrosine. Neither Tyr transaminase nor pHPP hydroxylase activity in liver was increased, and tyrosine and pHPP concentrations in plasma and tissues were not elevated by excessive intake of pHPP. From these results, it is concluded that pHPP was not responsible for the development of eye and paw lesions characteristic of tyrosine toxicity.
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