Studies of human placental aromatase

Kinetic analysis of aromatization of androstenedione (Δ⁴) indicates that 3 mol of oxygen and NADPH are consumed in the formation of estrone (El). During the steady state, the 3d oxidation appears to be the rate limiting step since both 19-hydroxy and 19-oxo-androstenedione accumulate prior to El. Cytochrome P-450 participation in each hydroxylation step is suggested by (1) inhibition by known P-450 inhibitors other than CO, (2) inhibition by an antibody to NADPH-cytochrome c reductase, (3) spectral studies of P-450 binding of substrate and intermediates, and (4) solubilization and partial resolution of enzyme components using digitonin and DEAE cellulose. A survey of steroidal compounds has revealed that 5α-reduced androstenedione is a potent competitive inhibitor which may be of physiologic importance in control of ovarian estrogen synthesis. Finally, many steroidal drugs used in treatment of breast cancer are competitive inhibitors. The non androgenic compound 1-ene-testololactone is effective both in vitro and in vivo suggesting that antitumor activity of steroidal drugs may be due to inhibition of estrogen synthesis rather than direct androgen action.