Effects of high fat and high carbohydrate diets on liver pyruvate dehydrogenase and its activation by a chemical mediator released from insulin-treated liver particulate fraction: effect of neuraminidase treatment on the chemical mediator activity.

Abstract

Rats were fed a high fat diet or a high glucose diet for 5-7 days. Basal pyruvate dehydrogenase activity (both the active form and the total enzyme activity) was decreased in liver homogenates from fat diet-adapted rats as compared to those fed the glucose diet. Supernatants from insulin-exposed liver particulate fractions from fat-fed rats showed decreased stimulation of pyruvate dehydrogenase activity as compared to those from glucose-fed rats. There was no difference in the response of the mitochondria from the two groups when they were stimulated by supernatants from insulin-treated liver particulate fractions from stock diet-fed rats. Liver particulate fractions from fat-fed rats showed decreased generation of the chemical activator in response to Concanavalin A and trypsin stimulation. This suggests that fat feeding results in a decrease in membrane protease substrate availability. Treatment of the insulin mediator with neuraminidase and beta-D-galactosidase resulted in inactivation of the mediator. Presence of exogenous enzyme substrates during enzyme digestion protected the mediator from inactivation, suggesting that carbohydrate residues are important in the action of the insulin mediator. This fat diet-induced decrease in the generation of a chemical mediator of insulin action may result from 1) a decrease in insulin binding, shown earlier; 2) a decrease in the amount of protease substrate; and 3) an alteration in its carbohydrate composition, which is important in its ability to activate pyruvate dehydrogenase.