Glucose Dehydrogenase (Hexose 6-Phosphate Dehydrogenase) and the Microsomal Electron Transport System

Evidence Supporting Their Possible Functional Relationship

Abstract

The ability of a microsomal enzyme, glucose dehydrogenase (hexose 6-phosphate dehydro-genease) to supply NADPH to the microsomal electron transport system, was investigated. Microsomes could perform oxidative demethylation of aminopyrine using microsomal glucose dehydrogenase in situ as an NADPH generator. This demethylation reaction had apparent K_m values of 2.61×10^-5M for NADP⁺, 4.93×10^-5M for glucose 6-phosphate, and 2.14×10^-4M for 2-deoxyglucose 6-phosphate, a synthetic substrate for glucose dehydrogenase. Phenobarbital treatment enhanced this demethylation activity more markedly than glucose dehydrogenase activity itself. Latent activity of glucose dehydrogenase in intact microsomes could be detected by using inhibitors of microsomal electron transport, i.e. carbon monoxide and p-chloromercuribenzoate (PCMB), and under anaerobic conditions. These observations indicate that in microsomes the NADPH generated by glucose dehydrogenase is immediately oxidized by NADPH-cytochrome c reductase, and that glucose dehydrogenase may be functioning to supply NADPH.