Regulation of glucose-6-phosphate dehydrogenase in human erythrocytes

Glucose-6-phosphate dehydrogenase catalyzes the initial and committed step of the pathway that is the principal source of NADPH in many cells. The intracellular rate of the enzyme in human erythrocytes was estimated from the rate at which the cells generated 14CO2 from 14C-labeled glucose in the presence of different amounts of methylene blue. This investigation differed from earlier studies in that: (a) accumulations of 6-phosphogluconate were considered in calculations of rate and (b) the cells were suspended in Krebs-Ringer bicarbonate buffer, which is the buffer system for erythrocytes in vivo. As with earlier studies, however, the intracellular enzyme was under unexplained inhibition or restraint relative to kinetic properties of the purified enzyme. Also, the intracellular enzyme exhibited sigmoid kinetics. In contrast, the isolated enzyme has been found to exhibit classical kinetics. In the course of dilution/ultrafiltration of the hemolysate a possible cause for the reduced activity of the enzyme was found: most of the NADP was bound to soluble macromolecules of the erythrocyte. The amount of NADP available to the enzyme was much less than the amount indicated by measurements of total (bound and unbound) NADP.