Characteristics and significance of the reverse glucose-6-phosphate dehydrogenase reaction

Glucose-6-phosphate dehydrogenase (G-6-PD) is the first enzyme of the hexose monophosphate pathway, and this important reaction is often considered to be irreversible. However, its apparent irreversibility is caused by the rapid removal of the immediate product, 6-phosphoglucono-delta-lactone. We have now investigated the reverse G-6-PD reaction, namely, the oxidation of reduced nicotinamide-adenine dinucleotide phosphate (NADPH) by 6-phosphoglucono-delta-lactone to form glucose-6-phosphate and nicotinamide-adenine dinucleotide phosphate (NADP). The substrate of the reaction, 6-phosphoglucono-delta-lactone, was rapidly generated from glucose-6-phosphate and NADP. The lactone was stabilized by addition of perchloric acid. A substrate analogue, 6-phosphoglucono-gamma-lactone, was synthesized by dehydrating 6-phosphogluconic acid. At pH 2.3 the t 1/2 of the delta-lactone was 2.4 hours; that of the gamma-lactone was 57 hours. The following kinetic parameters were established: Km delta-lactone 1027 +/- 183 mumol/L; Km gamma-lactone 266 +/- 71 mumol/L; Km NADPH less than 10 mumol/L; ratio of the Vmax G-6-PD forward/reverse reaction 2.0. Glucose-6-phosphate was found to be a competitive inhibitor with both lactones in the reverse G-6-PD reaction. Genetic mutants of humans in which the Km of G-6-PD for glucose-6-phosphate was diminished also had a diminished Km for 6-phosphoglucono-delta-lactone. Thus, it appears that the same active site on the enzyme binds glucose-6-phosphate in the forward reaction and 6-phosphogluconolactone in the reverse reaction.(ABSTRACT TRUNCATED AT 250 WORDS)