The effect of insulin on enzyme adaptations to diets and hormones.

1. Insulin prevented increases in liver glucose 6-phosphatase and fructose 1,6-diphosphatase activities in response to cortisol administration, but it did not prevent increases in response to diets high in protein or in fructose. 2. Insulin increased pyruvate kinase activity in animals treated with cortisol or fed a high protein diet; both treatments decrease pyruvate kinase activity. Insulin did not increase the pyruvate kinase activity of animals receiving a high glucose diet. 3. Insulin administration had a more marked effect on glucose 6-phosphate dehydrogenase activity than on malic dehydrogenase (decarboxylating) activity. 4. The increases in serine dehydrase, glutamic-oxalacetic transaminase, and malic dehydrogenase in response to cortisol treatment were partially or completely prevented by insulin. The increases in glutamic-pyruvic transaminase and lactic dehydrogenase in response to cortisol administration were not affected by insulin. Insulin either had no effect on increases observed in these enzymes after high protein or high fructose feeding or caused even greater increases after dietary treatment. Two enzymes (glutamicpyruvic transaminase and lactic dehydrogenase) not affected by insulin in the glucose-fed group were increased by insulin in groups receiving high protein or high fructose diets.