STUDIES WITH GLUCOSE 6-PHOSPHATE DEHYDROGENASE FROM HUMAN PLACENTA

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Abstract
Glucose 6-phosphate dehydrogenase (EC 1.1.1.49) has been purified from the supernatant fraction of human placenta. Pregnenolone and dehydroepiandrosterone are inhibitors of a non-competitive type, with respect to NADP+. 17[beta]-estradiol was an inhibitor of the reaction but only at high concentrations (2.10-5 M). 17[alpha]-estradiol at the same concentration was a more potent inhibitor. Several other steroids, both natural and synthetic, were inactive as inhibitors even at 10-5 M. The stability of the enzyme was increased by concentration and by the presence of NADP+ and NADPH at physiologic concentrations. Removal of bound NADP+ caused in-activation which could be almost completely reversed by exposure to physiologic levels of NADP+ (10-5 M). This cofactor induced reactivation was not retarded by 17[beta]-estradiol, 17 [alpha]-estradiol, or dehydroepiandro-sterone at 10-5 M. These observations militate against inactivation of placental glucose 6-phosphate dehydrogenase as an important mechanism of steroid action in vivo.