The glucose/glucose-6-phosphate cycle in the periportal and perivenous zone of rat liver

Periportal and perivenous hepatocytes contain different activities (V) of antagonistic key enzymes such as glucokinase and glucose-6-phosphatase. In order to get an insight into the metabolism of the periportal and perivenous area the flux rates (v) of the glucose/glucose-6-phosphate cycle were calculated on the basis of the Michaelis-Menten equation using the measured zonal concentrations of glucose and glucose 6-phosphate, the zonal activities of glucokinase and glucose-6-phosphatase previously reported and the half-saturating substrate concentrations (Km) of the two enzymes found in the literature. The concentrations of glucose were obtained as a first approximation by measuring the concentrations in portal (= periportal) and hepatovenous (= perivenous) blood; those of glucose 6-phosphate were calculated from the levels determined in microdissected periportal and perivenous liver tissue. The calculations showed (a) that the overall cycling rates agreed remarkably well with those reported for intact animals and (b) that during a normal feeding rhythm the periportal zone should catalyze net glucose output and the perivenous zone should mediate net glucose uptake, as proposed by the model of 'metabolic zonation'.