Modification of the endocrine milieu intérieur by administration or withdrawal of hormones considerably influenced the concentration in liver of lactic dehydrogenase, glucose-6-phosphate dehydrogenase, and 6-phosphogluconic dehydrogenase. Hormones effective in this regard are thyroxine, estradiol-17ß, dihydrotestosterone, and 3α-hydroxysteroids in the androstane series. These hormonal effects are both organ-selective and, in liver, enzyme-selective.

Thyroxine and steroids are not equivalent in their influence on the 3 hepatic dehydrogenases. Thyroxine was preeminent in this regard among hormones investigated in the present experiments.

Each of the pyridine nucleotide-linked dehydrogenases in liver had an individually characteristic response to the administration of various hormones which was reflected in its hepatic level. The concentration in liver of lactic dehydrogenase was profoundly raised or lowered, respectively, by the administration of thyroxine or by thyroidectomy, and injection of steroids had small influence on its level.

The levels of glucose-6-phosphate dehydrogenase and of 6-phosphogluconic dehydrogenase were considerably influenced by injection of thyroxine, estradiol-17ß, or testosterone. Sex steroids did not increase the levels of these enzymes in hypophysectomized or thyroidectomized rats unless thyroxine was administered concurrently.

Under designated conditions, it was found that the concentration of 6-phosphogluconic dehydrogenase was increased by hormonal methods which did not elevate the concentration of glucose-6-phosphate dehydrogenase. But high levels of glucose-6-phosphate dehydrogenase were not achieved without a considerable elevation of 6-phosphogluconic dehydrogenase as well.

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