Kinetic Analysis of Reversible Inhibition of 16α-Hydroxyandrostenedione Aromatization in Human Placental Microsomes by Suicide Substrates of Androstenedione Aromatization

To gain insight into the catalytic function of aromatase and its substrate specificity, we studied reversible inhibition of 16α-hydroxyandrostenedione (16α-OHAD) aromatization in human placental microsomes by several suicide substrates of androstenedione (AD) aromatization, including 4-hydroxyAD (1), 6-oxoAD (2) and its 19-hydroxy analogue 3, androst-5-ene-4,7,17-trione (4), and 10β-acetoxyandrost-5-en-7,17-dione (5) that, in contrast, do not cause a suicide inactivation of 16α-OHAD aromatization. All inhibitors examined blocked 16α-OHAD aromatization in a competitive manner with apparent Ki values ranging from 0.50 to 980 nM. The relative Ki values between inhibitors 1—5 obtained in the 16α-OHAD aromatization experiments were markedly different from those obtained in the AD aromatization experiments. The results predict that all inhibitors examined bind to the 16α-OHAD binding site in a manner that does not cause suicide inactivation of 16α-OHAD aromatization. These findings would be useful for understanding the active (binding) site structure as well as the catalytic function of aromatase.