Metabolism of 19-Methyl-Substituted Steroids by Human Placental Aromatase

Denise D. Beusen, Douglas F. Covey, H. L. Carrell

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44 Scopus citations

Abstract

The 19-methyl analogues of androstenedione and its aromatization intermediates (19-hydroxyandrostenedione and 19-oxoandrostenedione) were evaluated as substrates of microsomal aromatase in order to determine the effect of a 19-alkyl substituent on the enzyme's regiospecificity. Neither the androstenedione analogue [10-ethylestr-4-ene-3,17-dione (lc)] nor the 19-oxoandrostenedione analogue [10-acetylestr-4-ene-3,17-dione (3c)] was converted to estrogens or oxygenated metabolites by placental microsomes. In contrast, both analogues of 19-hydroxyandrostenedione [10-[(1S)-1-hydroxyethyl]estr-4-ene-3,17-dione (2c) and 10-[(1R)-l-hydroxyethyl]estr-4-ene-3,17-dione (2e)] were converted to the intermediate analogue 3c in a process requiring 02 and either NADH or NADPH. No change in enzyme regiospecificity was detected. The absolute configuration of 2e was determined by X-ray crystallography. Experiments with 1802 established that 3c generated from 2c retained little 180 (<3%), while 3c arising from 2e retained a significant amount of 180 (≈70%). All four 19-methyl steroids elicited type I difference spectra from placental microsomes in addition to acting as competitive inhibitors of aromatase (K1= 81 nM, 11 μM, 9.9 μM, and 150 nM for lc, 2c, 2e, and 3c, respectively). Pretreatment of microsomes with 4-hydroxyandrostenedione (a suicide inactivator of aromatase) abolished the metabolism of 2c and 2e to 3c, as well as the type I difference spectrum elicited by 2c and 2e. The failure of 2c, 2e, and 3c to undergo aromatization was rationalized in the context of a mechanistic proposal for the third oxygenation of aromatase requiring hydrogen abstraction at C1 of 19,19-dihydroxyandrostenedione, homolytic cleavage of the C10-Cl9bond, and oxygen rebound at C19.

Original languageEnglish
Pages (from-to)7833-7841
Number of pages9
JournalBiochemistry
Volume26
Issue number24
DOIs
StatePublished - 1987

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