Lanosterol 14 alpha-methyl demethylase. Isolation and characterization of the third metabolically generated oxidative demethylation intermediate.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

“…Both 32 and 33 are metabolized to 24 in rats, although the metabolic pathway that has been proposed for this reaction is mechanistically unsatisfactory since it fails to account for the extrusion of a carbon atom of 33 in a way that is chemically feasible Scheme suggests an alternative mechanism that invokes a Baeyer–Villiger-type process to generate fluoromethyl 2-fluoroacetic acid ( 12′ ) as an intermediate, an apparently unknown compound that can function as a precursor to 5 and then 22 . − While the enzyme that catalyzes this reaction is also unknown, both flavin-containing monooxygenase 5 (FMO5) and the hydroperoxy form of P450 enzymes are known to catalyze similar oxidative rearrangements. − Hydrolytic cleavage of 12′ leads presumably to the release of formaldehyde and fluoride.…”

“…Notably, following oral administration of 40 at 100 mg/kg, the concentrations of 5 in these tissues reached toxic levels of 0.2 to 0.9 μg/g of tissue. While a mechanistic explanation for the release of 5 from 40 was not offered, a Baeyer–Villiger rearrangement catalyzed by an FMO or a P450 enzyme may be responsible, as suggested by the mechanism presented in Scheme . , The initial addition reaction is facilitated by the electron-withdrawing fluorine substituent and involves the addition of a peroxy moiety, which is a known intermediate in the P450 catalytic cycle . This moiety attacks the electrophilic carbonyl to afford an intermediate that rearranges to ester 41 .…”

Oxidizing species in the mechanism of cytochrome P450

“…A role for the nucleophilic ferric peroxy anion in the oxidation of highly electrophilic substrates such as aldehydes was first postulated by Akhtar and colleagues to explain the P450catalyzed 14α-demethylation of lanosterol (Scheme 3). 44 The probable mechanism involves homolytic cleavage of the oxygen-oxygen bond in a peroxy-aldehyde adduct to give an alkoxy free radical that decays to the olefin as the result of hydrogen abstraction by the concomitantly formed ferryl species (path a). However, in the specific case of lanosterol demethylase, it has been reported that the Baeyer-Villiger rearrangement product can be isolated from the reaction (path b), 44 although it has not been demonstrated that the Baeyer-Villiger product can be converted into the demethylated product by the enzyme.…”

“…44 The probable mechanism involves homolytic cleavage of the oxygen-oxygen bond in a peroxy-aldehyde adduct to give an alkoxy free radical that decays to the olefin as the result of hydrogen abstraction by the concomitantly formed ferryl species (path a). However, in the specific case of lanosterol demethylase, it has been reported that the Baeyer-Villiger rearrangement product can be isolated from the reaction (path b), 44 although it has not been demonstrated that the Baeyer-Villiger product can be converted into the demethylated product by the enzyme. Related mechanisms operate in two other sterol biosynthetic reactions, the demethylation of androgens to estrogens by aromatase (CYP19) (Scheme 4) and the CYP17catalyzed 17α,20-bond scission that converts 17α-hydroxyprogestogens into androgens (Scheme 5).…”

The biosynthesis of steroids and triterpenoids

“…First amongst these steps is removal of the 14 -methyl group catalysed by lanosterol 14 -demethylase (P-450 14DM ). The 14-methyl group is first converted to an aldehyde by two successive oxidations, followed by a third oxidation which is proposed to form a 14-formyloxy intermediate 458 (proceeding by a Baeyer-Villiger type rearrangement), from which formic acid is eliminated to form a conjugated 8,14-diene. (However, see mechanism proposed for fungal P-450 14DM in Scheme 9.)…”