Summary: glucose-6-phosphate dehydrogenase (EC was purified to homogeneity. The subunit molecular weight was 55 300 and the molecular weight of the native protein was 138000 by gel filtration and 131 800 by rate zonal ultracentrifugation on sucrose gradients. There was some evidence for a small population of higher molecular weight oligomers during rate zonal centrifugation. A sedimentation coefficient of 8·5S was determined by analytical ultracentrifugation (suggesting the presence of the tetramer). Three bands of enzyme activity were separated on polyacrylamide gels. Digests of the proteins in the three bands gave peptide patterns with no detectable difference after SDS electrophoresis. Linear double reciprocal plots were obtained with glucose 6-phosphate as a variable substrate. The apparent K for glucose 6-phosphate was 214 μ and the was 520 μol (mg protein)min. A sigmoidal saturation curve and a non-linear double reciprocal plot were observed for NADP, giving a Hill constant of 1·5 and [S] for NADP of 89 μ. The enzyme was inhibited by NADPH, with 50% inhibition at a NADP: NADPH ratio of 0·4 when total NADP(H) concentration was 0·2 m. ATP, ADP, AMP and cAMP had no effect on enzyme activity. Palmitoyl CoA inhibited the initial velocity of the reaction by 95% at a concentration of 1·5 μ when incubated with enzyme prior to NADP addition.


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