Cytochrome p450 architecture and cysteine nucleophile placement impact raloxifene-mediated mechanism-based inactivation.

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VandenBrink BM, Davis JA, Pearson JT, Foti RS, Wienkers LC, Rock DA

Cytochrome p450 architecture and cysteine nucleophile placement impact raloxifene-mediated mechanism-based inactivation.

Mol Pharmacol. 2012 Nov;82(5):835-42. doi: 10.1124/mol.112.080739. Epub 2012 Aug 2.

PubMed ID

22859722 [ View in PubMed

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Abstract

The propensity for cytochrome P450 (P450) enzymes to bioactivate xenobiotics is governed by the inherent chemistry of the xenobiotic itself and the active site architecture of the P450 enzyme(s). Accessible nucleophiles in the active site or egress channels of the P450 enzyme have the potential of sequestering reactive metabolites through covalent modification, thereby limiting their exposure to other proteins. Raloxifene, a drug known to undergo CYP3A-mediated reactive metabolite formation and time-dependent inhibition in vitro, was used to explore the potential for bioactivation and enzyme inactivation of additional P450 enzymes (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A5). Every P450 tested except CYP2E1 was capable of raloxifene bioactivation, based on glutathione adduct formation. However, raloxifene-mediated time-dependent inhibition only occurred in CYP2C8 and CYP3A4. Comparable inactivation kinetics were achieved with K(I) and k(inact) values of 0.26 muM and 0.10 min(-1) and 0.81 muM and 0.20 min(-1) for CYP2C8 and CYP3A4, respectively. Proteolytic digests of CYP2C8 and CYP3A4 Supersomes revealed adducts to Cys225 and Cys239 for CYP2C8 and CYP3A4, respectively. For each P450 enzyme, proposed substrate/metabolite access channels were mapped and active site cysteines were identified, which revealed that only CYP2C8 and CYP3A4 possess accessible cysteine residues near the active site cavities, a result consistent with the observed kinetics. The combined data suggest that the extent of bioactivation across P450 enzymes does not correlate with P450 inactivation. In addition, multiple factors contribute to the ability of reactive metabolites to form apo-adducts with P450 enzymes.

DrugBank Data that Cites this Article

Drug Enzymes

Drug	Enzyme	Kind	Organism	Pharmacological Action	Actions
Raloxifene	Cytochrome P450 2C8	Protein	Humans	Unknown	Inhibitor	Details
Raloxifene	Cytochrome P450 3A4	Protein	Humans	Unknown	Inhibitor	Details