Dehydrogenation of glucose and xylose catalyzed by rat and sheep liver microsomal and soluble fractions

Abstract

The soluble fraction (homogenates from which nuclei, mitochondria, and the microsomal fraction have been removed) of rat and sheep liver catalyze the formation of NADH and NADPH in the presence of a glucose substrate. The activity is obtained reproducibly from sheep liver preparations, but in rat liver soluble fractions it is extremely variable (0–3.8 μmoles NAD(P)H formed per hour per gram liver). The activity of the rat liver soluble fraction is stimulated by the presence of the bicarbonate anion. A comparison of the substrate specificity of purified microsomal glucose dehydrogenase and that of the rat liver soluble fraction, together with a study of the effect of glucose-6-P and the bicarbonate anion on the activity of the two preparations, shows that the glucose-dependent NAD(P)H formation catalyzed by the rat liver soluble fraction is not due to the presence of glucose dehydrogenase. Sheep liver soluble fractions may contain some glucose dehydrogenase activity, but 10-fold more activity is found in the sheep hepatic microsomal fraction. Glucose-6-P dehydrogenase, partially purified from the rat liver soluble fraction, catalyzes a slow oxidation of D-glucose. This activity is stimulated by the bicarbonate anion. However, the observed glucose oxidation catalyzed by glucose-6-P dehydrogenase is probably too low to account for the occurrence of glucose oxidative activity in the soluble fraction. A D-xylose dehydrogenase activity has been found in the soluble fraction of rate liver. This activity was formerly believed to be associated with microsomal glucose dehydrogenase alone. Data are reported which suggest that microsomal glucose dehydrogenase may play a metabolic role under conditions where glucokinase activity is low or absent. A role for the activity of the soluble fraction catalysis of glucose oxidation, at least in rat liver, is less likely.