Dietary control of liver glucokinase activity in the normal rat

Abstract

Adult male rats were fasted 48 hours and then fed a diet containing either: (a) 60% glucose, (b) 60% fructose, or (c) no utilizable carbohydrate. The rats were killed at various times after the onset of the feeding, and glucokinase activity was measured in the particle-free supernatant fractions of their livers. An existing method for preparation of the particle-free, supernatant fraction of the liver and assay of its glucokinase activity was modified to yield activities 2 to 3 times higher than those previously reported. Glucokinase activity increased rapidly in response to the feeding of both the high-glucose and high-fructose diets. The response to glucose was consistently greater than that to fructose. No change from fasting levels was observed in the groups fed diets containing no utilizable carbohydrate. In rats fed the high glucose diet, liver glucokinase levels increased as early as 4 hours after the start of feeding. In the fructose-fed rats, the first change was noted at 12 hours. Glucokinase levels continued to increase over a 24-hour period in the glucose-fed rats but not in the fructose-fed rats. There was no apparent correlation between the increase in glucokinase activity and the level of protein in the supernatant fraction. Decreases in glucokinase activity observed in fasted rats seemed to follow similar changes in the protein content of the supernatant fraction.