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Fate of Threonine and Leucine in Rats Fed Threonine-deficient Diets

Laboratory of Nutritional Biochemistry, Department of Agricultural Chemistry, Nagoya University, Chikusa, Nagoya, Japan

The incorporation of threonine-¹⁴C and leucine-¹⁴C into liver and muscle protein of young rats was determined after 3 hours and 2 weeks of feeding two different threonine-deficient diets. The rate of incorporation of two amino acids into the tissue protein was compared by calculating the relative specific activities; the ratio of specific radioactivity of the protein to those of the acid-soluble fraction in both liver and muscle. It was found that the incorporation of leucine-¹⁴C into protein decreased in both liver and muscle of rats fed the threonine-deficient diets, whereas incorporation of threonine-¹⁴C into protein was increased by feeding the same diets. Because a close parallel was found between body weight gain and relative specific activities of the tissue protein when leucine-¹⁴C was injected, it was observed that tissue protein values in rats injected with leucine-¹⁴C are a better index of the rate of protein synthesis in rats fed the threonine-deficient diets, than the rate of incorporation of threonine-¹⁴C into tissue protein. The high value of the relative specific activities of the protein observed when the threonine-¹⁴C was injected appeared to indicate the increased utilization of threonine in the animals fed the threonine-deficient diets. Metabolic adaptation by the rats fed the threonine-deficient diets has been discussed based on the experimental findings.