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Effect of Dietary Threonine Supplementation on Tyrosine Toxicity in the Rat

School of Theoretical and Environmental Sciences, Jawaharlal Nehru University, New Mehrauli Road, New Delhi-110057, India

The objective of this study was to determine the effect of threonine supplementation on tyrosine metabolism in rats fed a low protein diet with excess tyrosine. The growth retardation and the development of eye and paw lesions that occur in rats ingesting a basal plus 3% or 5% L-tyrosine diet could be alleviated partially by the addition of 0.5% or 1.0% L-threonine to the diet. An increased blood tyrosine level in rats fed excess tyrosine was also lowered by threonine supplementation. In tyrotoxic conditions, the activities of liver tyrosine transaminase (EC 2.6.1.5) and threonine dehydratase (EC 4.2.9.16) were elevated, but p-hydroxyphenyl pyruvic acid oxidase (EC 1.13.11.27) which is also intimately associated with tyrosine toxicity was found to be inactivated. Furthermore, biosynthesis of ascorbic acid in liver was significantly lowered in this condition. However, addition of L-threonine in the diet, not only could cure the signs developed due to excess tyrosine, but also could affect the levels of enzymes studied.

KEY WORDS: • tyrosine toxicity • threonine alleviation • tyrosine transaminase • pHPP oxidase • threonine dehydratase ascorbic acid biosynthesis

¹ Department of Biochemistry, University College of Science, Calcutta-19, India.

Manuscript received 10 May 1976.