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Cytochrome P450 3A4 regiospecific oxygenation of raloxifene: Evidence for the formation of the di‐quinone methide
Author(s) -
Moore Chad Douglas,
Yost Garold S
Publication year - 2008
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.22.1_supplement.1131.5
Subject(s) - quinone methide , chemistry , raloxifene , cyp3a4 , glutathione , cytochrome p450 , reactive intermediate , quinone , biochemistry , stereochemistry , metabolism , estrogen receptor , medicine , catalysis , enzyme , cancer , breast cancer
While the bioactivation of xenobiotics to toxic intermediates through cytochrome P450 (CYP)‐catalyzed oxygenation reactions is a well recognized process, the production of reactive electrophilic intermediates by P450s through dehydrogenation reactions is not well characterized. Raloxifene is a second generation selective estrogen receptor modulator used for the treatment and prevention of osteoporosis in postmenopausal women. Previous studies have shown CYP3A4‐mediated metabolism of raloxifene produces two hydroxylated metabolites and several glutathione (GSH) adducts. The hydroxylated metabolites and one of the GSH adducts could theoretically be derived from the same dehydrogenated intermediate. It is postulated that raloxifene is dehydrogenated to a reactive di‐quinone methide. To evaluate the formation of the di‐quinone methide, raloxifene was incubated with CYP3A4 in the presence of H 2 18 O or 18 O 2 . LC/MS analysis showed that 3′‐hydroxyraloxifene incorporated oxygen only from 18 O 2 , while the source of the oxygen for the 7‐hydroxyraloxifene was inconclusive. These results demonstrate CYP3A4 selectively oxygenates raloxifene from 18 O 2 at the 3′‐position, but not at the 7‐position. Furthermore, the lack of CYP3A4‐mediated oxygenation at the 7‐position suggests that the di‐quinone methide is hydrated by active‐site H 2 O, but labeled H 2 18 O could not serve as the source of hydration. Supported by NIH grant # GM074249 form the National Institute of General Medical Sciences .