Physical Properties of Glucose 6-Phosphate Dehydrogenase from Neurospora crussa*

Neurospora glucose 6-phosphate dehydrogenase is a multimerit protein and occurs as tetramers and dimers with molecular weights of 206,000 and 104,000 to 114,000, respectively. The existence of the two types of molecules is supported by electron microscopic observations. The molecular weight of the subunits was estimated to be 57,000 by sodium dodecyl sulfate gel electrophoresis, and 63,000 by sedimentation equilibrium studies in guanidine.HCl. By both techniques the polypeptide chains appeared to be homogeneous, and reduction followed by alkylation with iodoacetamide has no effect on the apparent molecular weight. Since Neurospora glucose b-phosphate dehydrogenase is under the control of three unlinked genes, nonidentity of the polypeptide chains is indicated. Although the reduced and alkylated subunits are identical electrophoretically, two to three bands of protein were separable in preliminary electrofocusing studies. NADP influences the size of the native enzyme. Increasing concentrations of the cofactor convert mixtures of tetramers and dimers to tetramers with a concomitant increase in specific activity. The enzyme was estimated to consist of 72% tetramers at 2 X 10e5 M NADP, 92 % at 2 X 10e4 M NADP, and 100% at 2 mM NADP. It is suggested that NADP-mediated changes in the aggregation state and activity of glucose-6-phosphate dehydrogenase may be involved in the in vivo regulation of the enzyme activity, and in the pleiotropic effects of mutant glucose 6-phosphate dehydrogenases.