A study of glucose and xylose oxidation catalyzed by the glucose-6-phosphate dehydrogenase of rat liver cytosol

Abstract

Purified glucose-6- P dehydrogenase from the rat liver soluble fraction catalyzes a slow oxidation of d-glucose and d-xylose when high concentrations of both enzyme protein and of substrate are present. The glucose and xylose dehydrogenase activity could not be separated from the glucose-6- P dehydrogenase activity by ion-exchange chromatography, polyacrylamide disc-gel electrophoresis, or isoelectric focusing. The three activities were affected identically during thermal denaturation studies and by use of the steroid inhibitors dehydroepiandrosterone and progesterone. Both NAD and NADP served as coenzyme with the glucose and xylose substrates, but NADP had the lower K m value. The addition of bicarbonate activated the NADP-dependent glucose and xylose oxidations about 10- and 3-fold, respectively. Phosphate stimulated NADP-dependent glucose oxidation about 2.5-fold but had little effect upon xylose oxidation. Bicarbonate activated NAD-dependent glucose oxidation about 2-fold while NAD-dependent xylose oxidation was unaffected under our assay conditions. Both bicarbonate and phosphate are competitive inhibitors of the glucose-6- P oxidation. Bicarbonate inhibition of rat hepatic cytosol glucose-6- P dehydrogenase activity appears to be insufficient in magnitude at physiological bicarbonate levels to represent a significant metabolic control of this activity in rat liver.