A time-dependent inactivation of aromatase by 19-substituted androst-4-ene-3,6,17-triones.

Diastereomeric (19S)- and (19R)-19-ethynyl-19-acetoxy derivatives of androst-4-ene-3,6,17-trione (AT) (9 and 10) and 19,19-difluoro AT (12) were synthesized. The 19,19-difluoro compound (12) was an effective competitive inhibitor of human placental aromatase with an inhibition constant (ki) of 1.8 microM but the acetylenic 9 and 10 were poor inhibitors of the enzyme with k(is) of 75 and 67 microM, respectively. Inhibitor 12 caused a time-dependent, biphasic loss of aromatase activity in the presence of reduced nicotinamide-adenine-dinucleotide phosphate (NADPH) in air, whereas the other two caused a time-dependent, pseudo-first-order inactivation of the activity with rate constants for inactivation of 0.250, 0.077, and 0.065 min-1 for steroids 12, 9, and 10. NADPH was required for the time-dependent inactivation, and the substrate androst-4-ene-3,17-dione prevented it. L-Cysteine did not protect aromatase from the inactivation.