Thyroid Hormones on Amino Acid and Protein Metabolism. I. Concentration and Composition of Free Amino Acids in Blood Plasma of the Rat

Abstract

Free amino acid concentrations and compositions of blood plasma of rats in various thyroid hormone states were determined. The plasma amino acid concentration was increased significantly after feeding desiccated thyroid (2-fold on the 10th day) and was significantly decreased after thyroidectomy. Doses of L-triiodothyronine from 0.1 to 1.0 μmole/200 g of body weight, given to thyroidectomized rats, caused a return of plasma amino acid levels to near-normal values within 12 hr. The thyroid hormone status of the animal does not appear to alter the diurnal changes in plasma amino acids. Amino acid analysis showed that most plasma amino aicds increased significantly after feeding desiccated thyroid for 8 days, while proline and citrulline remained at the normal level, and glycine decreased. Likewise, most amino acids were decreased after thyroidectomy, with the exception of glycine and citrulline, which were increased. The concentrations of alanine and glutamic acid were increased, while the levels of proline, citrulline and glycine were decreased after a single injectionof L-triiodothyronine into thyroidectomized rats. The concentration of aamino- rc-butyric acidwas greatly elevated (approximately 20-fold) by feeding desiccated thyroid or by a single injection of L-triiodothyronine. There is a good correlation between the plasma concentrations of αamino-n-butyric acid, glutamic acid, glycine and alanine and the thyroid hormone status of the rat. (Endocrinology85: 1166, 1969)