STUDIES WITH GLUCOSE 6-PHOSPHATE DEHYDROGENASE FROM HUMAN PLACENTA

Abstract

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+. 17β-oestradiol was an inhibitor of the reaction but only at high concentrations (2 · 10−5 m). 17α-oestradiol 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 inactivation which could be almost completely reversed by exposure to physiologic levels of NADP+ (10−5 m). This cofactor induced reactivation was not retarded by 17β-oestradiol, 17α-oestradiol, or dehydroepiandrosterone at 10−5 m. These observations militate against inactivation of placental glucose 6-phosphate dehydrogenase as an important mechanism of steroid action in vivo.