Inhibition and inactivation of estrogen synthetase (aromatase) by fluorinated substrate analogues

19,19-Difluoroandrost-4-ene-3,17-dione (1) and 19-fluorcandrost-4-ene-3,17-dione (2) have been synthesized, and the interaction of these compounds with the estrogen synthetase (aromatase) activity of human placental microsomes has been studied. 1 has been found to cause time-dependent, irreversible inactivation of this enzyme (Ki = 1 micron, kinact = 0.023 min-1). A possible mechanism of this process is enzymatic generation of an acyl fluoride through oxidation of 1. Compound 2 does not cause inactivation, and this substrate analogue has been shown to be converted to estrone in high yield by this enzyme system.