STUDIES DIRECTED TOWARD A MECHANISTIC EVALUATION OF AROMATASE INHIBITION BY ANDROST-5-ENE-7,17-DIONE - TIME-DEPENDENT INACTIVATION BY THE 19-NOR AND 5-BETA,6-BETA-EPOXY DERIVATIVES

To gain further insight into the mechanism for inactivation of aromata se by androst-5-ene-7,17-dione (1) and its 19-nor analog 4, 10 beta-ox ygenated steroids 5 and 6, Delta(1(10))-steroid 7, and 19-oxo-5 beta,6 beta-epoxy compound 8 were synthesized and tested for their ability t o inhibit aromatase in human placental microsomes. all of the steroids studied inhibited the enzyme in a competitive manner with apparent K- i values ranging from 1.1 to 35 mu M. The Delta(1(10))-compound 7 was the most potent inhibitor among them. All of the inhibitors caused a t ime-dependent inactivation of aromatase in the presence of NADPH in ai r with the K-inact values ranging from 0.036 to 0.190 min(-1). The sub strate and androstenedione protected the inactivation, but a nucleophi le, L-cystein, did not, in each case. In contrast, each inhibitor did not cause the time-dependent inactivation in the absence of NADPH. The se results show that the 5 beta,6 beta-epoxide 8 and/or the dienone 7 are not a reactive electrophile involved in the irreversible binding t o the active site of aromatase during the mechanism-based inactivation caused by the suicide substrates 1 and/or 4. (C) 1997 by Elsevier Sci ence Inc.