ISOENZYMES OF GLUCOSE-6-PHOSPHATE-DEHYDROGENASE FROM THE PLANT FRACTION OF SOYBEAN NODULES

Two isoenzymes of glucose 6-phosphate dehydrogenase (EC 1.1.1.49) have been separated f rom the plant fraction of soybean (Glycine max L. Merr. cv Williams) nodules by a procedure involving (NH4)2SO4 gradient fractionation, gel chromatography, chromatofocusing, and affinity chromatography. The isoenzymes, which have been termed glucose 6-phosphate dehydrogenases I and II, were specific for NADP+ and glucose 6-phosphate and had optimum activity at pH 8.5 and pH 8.1, respectively. Both isoenzymes were labile in the absence of NADP+. The apparent molecular weight of glucose 6-phosphate dehydrogenases I and II at pH 8.3 was estimated by gel chromatography to be approximately 110,000 in the absence of NADP+ and double this size in the presence of NADP+. The apparent molecular weight did not increase when glucose 6-phosphate was added with NADP+ at pH 8.3. Both isoenzymes had very similar kinetic properties, displaying positive cooperativity in their interaction with NADP+ and negative cooperativity with glucose 6-phosphate. The isoenzymes had half-maximal activity at approximately 10 micromolar NADP+ and 70 to 100 micromolar glucose 6-phosphate. NADPH was a potent inhibitor of both of the soybean nodule glucose 6-phosphate dehydrogenases.