Regulation of Glucose-6-Phosphate Dehydrogenase Activity in Red Blood Cells From Hemolytic and Nonhemolytic Variant Subjects

Because of the strong inhibition of glucose-6-phosphate dehydrogenase by physiologic concentrations of NADPH and ATP, only 0.1%-0.2% of its potential activity (V_max) is estimated to be expressed in human red cells that contain low concentrations of NADP and glucose 6-phosphate. 6-Phosphogluconate dehydrogenase is also strongly inhibited by NADPH, ATP, and 2,3-diphosphoglycerate under the physiologic conditions, and only about 0.2% of its potential activity is estimated to be expressed in red cells. The very low shunt pathway activity can be explained by these findings. Some human glucose-6-phosphate dehydrogenase variants that are associated with severe enzyme deficiency, such as Gd Union, Gd Markham, and Gd Mediterranean, have no chronic hemolytic problem. On the other hand, several variants with less severe enzyme deficiency, such as Gd Manchester, Gd Alhambra, and Gd Tripler, are associated with chronic hemolytic anemia. Examination of the enzymes from these variant subjects under simulated physiologic conditions revealed that: (1) Enzymes from the hemolytic variant subjects are strongly inhibited by a physiologic concentration of NADPH due to their high K_m for NADP or low K_i for NADPH. Thus, these variant enzymes presumably cannot generate NADPH in red cells to maintain an adequate level of reduced glutathione. (2) The nonhemolytic variant enzymes are far less sensitive to the inhibition by NADPH, because of their low K_m for NADP and high K_i for NADPH. These variants are also more resistant to the inhibition by ATP. The activities of these variant enzymes in red cells are estimated to be more than 30% of the normal level. In black people, the common variant A—, associated with enzyme deficiency, is less sensitive to the inhibition by NADPH, and it is probably as active as normal in red cells, under physiologic conditions.