Time-Dependent Inactivation of Human Placental Aromatase by Bromoacetoxy 4-Androsten-3-ones in the Presence of Reduced Nicotinamide Adenine Dinucleotide Phosphate (NADPH)

Abstract

6-Bromoacetoxyandrostenediones (1 and 2) and 17β-bromoacetoxy-4-androsten-3-one having a 6β-bromo (3), 6-keto (4), 6β-methoxyl (5) substituent were evaluated as mechanism-based inactivators of human placental aromatase. All of these compounds except the 6α-bromoacetate 1 showed a time-dependent, pseudo-first-order inactivation of aromatase in the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH) with apparent K_i's of 40, 50, 30, and 34 μM and k_inact's of 0.048, 0.364, 0.267, and 0.040 min^-1, respectively, for compounds 2, 3, 4, and 5. The enzyme inactivation with compounds 3 and 4 was blocked by the addition of the substrate androstenedione to the incubates, and NADPH and oxygen were required for their effective time-dependent inactivation.