title: 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.
abstract: To gain further insight into the mechanism for inactivation of aromatase by androst-5-ene-7,17-dione (1) and its 19-nor analog 4, 10 beta-oxygenated 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 to inhibit aromatase in human placental microsomes. All of the steroids studied inhibited the enzyme in a competitive manner with apparent Ki values ranging from 1.1 to 35 microM. The delta 1(10)-compound 7 was the most potent inhibitor among them. All of the inhibitors caused a time-dependent inactivation of aromatase in the presence of NADPH in air with the kinact values ranging from 0.036 to 0.190 min-1. The substrate androstenedione protected the inactivation, but a nucleophile, L-cysteine, did not, in each case. In contrast, each inhibitor did not cause the time-dependent inactivation in the absence of NADPH. These 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 to the active site of aromatase during the mechanism-based inactivation caused by the suicide substrates 1 and/or 4.
authors.affiliation: Tohoku College of Pharmacy, Aobaku, Sendai, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Numazawa
authors.forename: M
authors.initials: M
authors.lastname: Tachibana
mesh_terms: D000731:Androstanes / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology*; D000735:Androstenedione / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D001141:Aromatase / Q000378:metabolism; D047072:Aromatase Inhibitors; D001667:Binding, Competitive; D004791:Enzyme Inhibitors / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology*; D005260:Female; D006801:Humans; D007700:Kinetics; D009249:NADP / Q000494:pharmacology; D010920:Placenta / Q000201:enzymology; D011247:Pregnancy; D013329:Structure-Activity Relationship; D013997:Time Factors
chemical_list: C108250:5,6-epoxyandrosta-7,17,19-trione; D000731:Androstanes; D047072:Aromatase Inhibitors; D004791:Enzyme Inhibitors; C108249:androst-5-ene-7,17-dione; D000735:Androstenedione; D009249:NADP; D001141:Aromatase