Mechanism-Based Inhibition of Human Cytochrome
Tressler, Michael C.
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Human cytochrome P450 IA2 (CYPIA2) is involved in the oxygenation of both exogenous and endogenous substrates. Aflatoxin B1, a mycotoxin produced by the mold Aspergillus flavus found on corn and peanuts, is a known substrate of CYP1A2. Amongst other metabolites, CYP1A2 converts Aflatoxin B1 into a 8,9-exo epoxide, a reactive electrophile and carcinogen. The drug Oltipraz (OPZ) is a known inhibitor of CYP1A2, hypothesized to be mechanism-based. In this paper we discuss how using microsomes OPZ was tested as a mechanism-based inhibitor of CYP1A2. CYP1A2 catalyzes the O-deethylation of 7-ethoxyresorufin to resorufin, a fluorescent product. Resorufin formation was used as a measure of CYP1A2 activity, excitation at 510 nm, emission at 586 nm using a fluorometer. CYP1A2 O-deethylation of 7-ethoxyresorufin drops to 30% relative activity following ten minutes of incubation in the initial inactivation assay not containing substrate. Activity was not recovered following dialysis and reconstitution of the system. Therefore, OPZ is an irreversible inhibitor of human cytochrome P450 1A2 supporting previous studies (Langouet, S., Furge, L.L., Kerriguy, N., Nakamura, K., Guillouzo, A., Guengerich, F.P "Inhibition of human Cytochrome P450 Enzymes by 1,2-Dithiole-3-thione, Oltipraz and Its Derivatives, and Sulforaphane." Chemical Research in Toxicology. 13 (1999): 245-252.).