Characterization of human ovarian oestradiol-17 beta oxidoreductase activity.

Oestradiol-17 beta oxidoreductase activity, which catalyzes the interconversion of oestrone and oestradiol, was investigated in preparations of human ovaries. The enzyme activities were localized primarily in the 105,000 X g supernatant fraction; dialyzed supernatant preparations were used in subsequent studies. The pH optima were 6.9 for reduction and 8.1 for 17 beta-dehydrogenation. The apparent Michaelis constants for oestrone and oestradiol were 1 X 10(-7) M and 5 X 10(-7) M, respectively. The enzyme activity was present with either NADP(H) or NAD(H), though (NADP(H) were the preferred cofactors. Non-aromatic steroids androstenedione, dehydroepiandrosterone, testosterone and 5-androstene-3beta,17beta-diol were poor substrates for the enzyme preparation. Methylation of the phenolic hydroxyl of oestrone and oestradiol resulted in slightly enhanced activities. The sulfhydryl reagent, N-ethylmaleimide, inhibited the reduction of oestrone. A dialyzed supernatant preparation retained approximately 79% of the original enzyme activity when stored at -20 degrees C for 6 weeks.