Cold-adaptation II

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Abstract
Taking the diurnal variation of enzyme activity into account, the action of thyroid hormone on rat liver phosphoenolpyruvate carboxykinase [EC 4.1.1.32] was investigated. The results obtained were as follows; 1) Thyroidectomy diminished the enzyme activity and L-thyroxine significantly enhanced it either in intact rats or in thyroidectomized rats. The dose of L-thyroxine required for significant increase in the enzyme activity 24 hr after a single injection was as little as 10 μg per 100 g body weight, and significant activation was observed 6 hr after injection of 50 μg per 100 g body weight into thyroidectomized rats. 2) The activation of the enzyme by thyroxine was almost completely blocked by cycloheximide. 3) The adrenal gland was not essential for the increase in enzyme activity caused by thyroxine. 4) Thyroid hormone was not the sole essential factor for induction of the enzyme after cold-exposure.

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