Quantitative correlation of glucose uptake and phosphorylation with the activities of glucose-phosphorylating enzymes in perfused livers of fasted and fed rats.

The purposes of these studies were (a) to examine the kinetics of glucose uptake by isolated perfused livers over a wide range of glucose concentrations (i.e. 2 to 80 mu), and (6) to compare observed rates of net hepatic glucose uptake and hepatic glucose phosphorylation calculated therefrom with levels of the liver glucose-phosphorylating enzymes glucokinase and hexokinase which were measured in biopsy samples from perfused livers. Levels of glucose-&phosphate phosphohydrolase and carbamyl phosphate:glucose phosphotransferase activity also were assayed in these biopsy samples. Preparations from 48-h fasted rats were compared with those from normal fed rats. Observed rates of net Dglucose uptake by perfused livers from either fasted or fed rats were nonlinear functions of varying perfusate glucose loads. Further, rates of glucose utilization by livers of fasted rats were essentially equal to rates with those of fed rats, despite glucokinase being lower by about 60% in the former. At higher tested perfusate glucose loads (i.e. >ll mn with fasted rats; >30 mn with fed rats), rates of net glucose utilization were greater than could be accounted for on the basis of observed, combined glucokinase plus hexokinase activities. Estimated levels of carbamyl phosphate:glucose phosphotransferase activity were relatively small with fed animals, somewhat higher with livers from fasted rats with higher glucose levels, but in no situation sufficient to rectify the observed discrepancies between net glucose uptake rates and those of glucokinase plus hexokinase. D-Glucose uptake rates were inhibited by 30 mu Lornithine and by 30 mn 3-o-methyl-o-glucose, but not by 30 mn L-glucose. It is suggested that glucose phosphorylation mechanisms in addition to glucokinase and hexokinase, possibly certain phosphotransferase activities of glucose-6 phosphatase or other, uncharacterized high K, kinases,