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To gain insight into the nature of the substrate binding site and the catalytic function of aromatase, we studied the inhibition of androstenedione aromatization by 4beta,5beta-epoxy-16alpha-hydroxyandrostenedione (4) and its 19-hydroxy and 19-oxo derivatives, 5 and 6, as well as the biochemical aromatization of these steroids in human placental microsomes. The 19-methyl and 19-oxo compounds, 4 and 6, were weak competitive inhibitors of aromatase, with apparent K(i) values of 246 microM and 270 microM, respectively, whereas the 19-hydroxy compound 5 inhibited aromatase in a non-competitive manner with the K(i) of 135 microM. The 19-methyl compound 4 inactivated aromatase in a time-dependent manner with k(inact) of 0.213 min(-1) in the presence of NADPH in air, but the other two did not cause it. The conversion of the three epoxides into estrogen, as well as 19-oxygenation of 19-methyl steroid 4 with human placental microsomes in the presence of NADPH in air, were not detected by gas chromatography-mass spectrometry. The present results are consistent with the two binding sites theory in the active site of aromatase.

biological and pharmaceutical bulletin

Aromatase Inhibition by 4.BETA.,5.BETA.-Epoxides of 16.ALPHA.-Hydroxyandrostenedione and Its 19-Oxygenated Analogs, Potential Precursors of Estriol Production in the Feto-Placental Unit.