Dietary and Hormonal Regulation of Enzymes of Fructose Metabolism in Rat Liver

Fructokinase (ketohexokinase, EC 2.7.1.3), aldolase (ketose-1-aldehyde lyase, EC 4.1.2.7), and triokinase (ATP:D-glyceraldehyde 3-phosphotransferase, EC 2.7.1.28) activities of rat liver were measured under various dietary and hormonal conditions. Specific activities in terms of liver weight did not change greatly, but marked changes were observed in total liver activity. All three activities decreased to about one-half or less of their normal total activity on fasting for 48 to 72 hours, and were restored completely to normal in 24 hours by fructose feeding. With glucose feeding, recovery was complete for aldolase but only partial for fructokinase and triokinase. There was no recovery of activity on feeding a high protein diet. Recovery of activity on fructose feeding occurred within a 4- to 8-hour period following a lag of about 16 hours, and roughly paralleled increases in liver weight. Long term feeding of fructose resulted in the maintenance of a considerably higher level of all three enzymes over that on a high fat or high protein diet. Levels of all three enzymes were unchanged in alloxan-diabetic rats. Fructokinase activity of fed adrenalectomized rats was at the fasting level of normal rats and was neither lowered further on fasting nor increased by subsequent feeding of fructose or glucose. In contrast, aldolase and triokinase activities were normal and decreased sharply on fasting but did not recover on subsequent glucose or fructose feeding. Essentially the same pattern was found for hypophysectomized as for adrenalectomized rats. Throughout all of the alterations of aldolase activity there was an invariably constant ratio of 1 for activity toward the two substrates, fructose 1,6-diphosphate and fructose 1-phosphate, suggesting the presence of a single aldolase species in rat liver.