Effect of fasting on enzymes of carbohydrate metabolism and brush border membrane in rat intestine

The effect of 1-, 3-, and 5-day fasting on enzymes of brush border membrane and carbohydrate metabolism in the rat small intestine has been studied. Fasting led to significant decrease in the specific activities of brush border marker enzymes alkaline phosphatase and sucrase, as compared to levels in unfasted controls, both in the homogenates and in brush border membrane preparations. Kinetic studies revealed that these changes were due to changes in the maximal velocity (Vmax) only of alkaline phosphatase, but both the Michaelis constant (Km) and V a, were altered for sucrase. Fasting decreased the activities of lactate dehydrogenase, malate dehydrogenase, and isocitrate dehydrogenase but increased the activities of glucose-6-phosphatase and fructose 1,6-bisphosphatase in mucosal homogenates. The activities of glucose-6-phosphate dehydrogenase and malic enzyme were also decreased by fasting. Thus fasting caused changes in the activities of various enzymes involved in the metabolism of glucose. The results of these enzyme studies suggest that the degradation of glucose is decreased but its production by gluconeogenesis is enhanced upon fasting. (C) 2004 Elsevier Inc. All rights reserved.