Activation by bicarbonate, orthophosphate, and sulfate of rat liver microsomal glucose dehydrogenase

Abstract

1. 1. The effects of the anions bicarbonate, orthophosphate, and sulfate on rat liver microsomal glucose dehydrogenase (β- d-glucose:NAD(P) + oxidoreductase, EC 1.1.1.47) have been studied in detail. 2. 2. Significant activations of glucose dehydrogenase activity were produced by all these anions in the range from 2.5 to 50 mM. 30 mM sulfate, orthophosphate, and bicarbonate, respectively, effected increases in activity levels to approximately 550, 400, and 300% of control values. 3. 3. Activations were noted at all of a variety of H + concentrations between pH 7 and 10. The pH optimum, 9.2 in the absence of effector, was shifted slightly towards neutrality in the presence of increasing concentrations of the various anions. 4. 4. Kinetic studies revealed that K m values for glucose and for nucleotide substrate were independent of second substrate concentrations. The various anions all effected increases in the v max value and decreases in K m values for both glucose and NAD. 5. 5. Relatively small but significant inhibitions of glucose 6-P dehydrogenase activity also catalyzed by this multifunctional enzyme were produced by the various anions, while ATP was a potent inhibitor. 6. 6. It is suggested on the basis of similarities of the effects noted with the two multifunctional enzymes that the mechanism of anion action previously proposed by H orne et al. 3 ( Biochemistry, 9 (1970) 610) for yeast glucose-6-P dehydrogenase also obtains, with a slight modification, for the mammalian microsomal glucose dehydrogenase. 7. 7. Although the high K m value for glucose (0.33–1.25 M) may contraindicate a physiological role for glucose dehydrogenase activity, the observation that in the presence of physiological levels of anionic activators glucose dehydrogenase activity with 0.5 M d-glucose may approach or exceed the level of total hepatic cellular glucose-6-P dehydrogenase activity (microsomal plus cytosolic) raises the possibility that, as previously has been suggested by M etzger et al. 38 a specialized, compartmentalized biological role for this hepatic activity is possible.