Effects of Thyroid Hormones on Metabolism. Effect of Thyroxine and Iodinated Casein on Liver Enzyme Activity

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Abstract
The effect of thyroxine injection and feeding iodinated casein upon the various liver components in rats was investigated. Both treatments decreased the liver to body weight ratio, total liver protein and caused a disappearance of glyco-gen. There was a marked increase in glucose-6-phosphatase, a decrease in phosphorylase, and relatively smaller changes in other glycolytic enzyme activities after treatment. The enzymes of the pentose phosphate pathway increased as did malic enzyme activity, although a fourth TPN-linked dehydrogenase, isocitric, decreased. L-[alpha]-Glycerolphosphate dehydrogenase decreased in the hyperthyroid animals. All 3 of the tricarboxylic acid cycle enzymes measured increased in activity after thyroxine injection. The results were not as clear cut after feeding iodinated casein, and differences between the 2 treatments were observed. The enzymes concerned with aliphatic amino acid metabolism were increased in activity after treatment, with the exception of glutamicpyruvic transaminase, which was unaltered. The enzymes measured which are associated with aromatic amino acid metabolism were decreased or unchanged in activity after treatment. Four enzymes which occur in both the particulate free fraction and the mitochondria were subjected to a precursory examination for the relative distribution in normal and treated rats. The distribution was identical for all of these enzymes with the exception of glutamic dehyrogenase. Possible physiological significances of the observed alterations are discussed.