Further studies on the properties of human placental microsomal cytochrome P-450

Abstract

Spectral studies of human placental microsomal cytochromes provided evidence that androstenedione will bind either to two separate cytochromes or to two sites on the same cytochrome. Scatchard and Lineweaver-Burk plots indicated the presence of a binding site with high affinity and low capacity and a second site with a lower affinity but higher capacity. Both sites were discernible in the presence of high concentrations of NADPH, and similar binding constants were obtained. The absorption maximum (450 nm) of the carbon monoxide complex of NADPH-reduced placental cytochrome P-450 could be abolished completely by additions of low concentrations of androstenedione. 19-hydroxyandrostenedione or 19-oxoandrostenedione, but not by high concentrations of 19-norandrostenedione. 19-nortestosterone, pregnenolone or benzo[a]pyrene. Compounds capable of binding significantly to placental microsomal cytochrome P-450 appeared to fall into three categories: C-19 or C-18 steroids, with structures similar to that of androstenedione or 19-norandrostenedione: C-18 steroids with structures very similar to that of β-estradiol; or substances capable of forming ferrihemochromes. A large number of foreign organic compounds which produced type-I binding spectra in rat liver microsomes exhibited either no or very minimal binding to placental cytochrome P-450. Most compounds that exhibited significant binding to placental cytochrome P-450 also were inhibitors of rates of androstenedione aromatizalion (aromatase activity). Important exceptions were carbon monoxide, metyrapone and nicotinamide. Diethylstilbestrol acted as a relatively potent inhibitor of aromatase activity but displayed no discernible binding. Results of the study strongly supported a functional role for placental cytochrome P-450 in the aromatization reaction.