Hepatic Tyrosine Trans aminase Activity and Plasma Tyrosine Concentration in Rats with Altered Thyroid Function

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Abstract
The activity of hepatic tyrosine- a-ketoglutarate transaminase, the first enzyme in the major degradative pathway of tyrosine metabolism, was measured in hyperthyroid, athyrotic and euthyroid male rats. Treatment of 15 rats with L-thyroxine for 4–6 weeks increased enzyme activity from 89.7 Mmoles p-hydroxyphenylpyruvic acid formed/g dry wt/hr to 126, whereas radiothyroidectomy of 12 rats decreased enzyme activity to 44 μmoles. Tyrosine transaminase activity of the athyrotic rats was not changed by adrenalectomy. To test the specificity of the changes in tyrosine transaminase activity, a related enzyme, phenylalanine-pyruvate transaminase, was simultaneously assayed and was found to be unaltered by any changes in thyroid function. Plasma tyrosine concentration increased from 14.1 μg/ml to 17.1 in rats chronically treated with thyroxine and decreased to 11.5 μg/ml after radiothyroidectomy. L-Thyroxine treatment for only 1–3 days increased plasma tyrosine levels, but hepatic tyrosine transaminase activity was unchanged. Since chronic thyroxine treatment resulted in higher levels of plasma tyrosine, as well as in a simultaneous increase in hepatic tyrosine transaminase activity, it is suggested that separate mechanisms may control the activity of the liver enzyme and the level of tyrosine in the plasma.