OXIDATION OF ACETAMINOPHEN TO N-ACETYL-P-AMINOBENZOQUINONE IMINE BY HUMAN CYP3A4

We have investigated: (a) the formation of N-acetyl-p-aminobenzoquinon e imine (NAPQI) from acetaminophen (APAP) by reconstituted human liver CYP3A4, (b) the kinetics of NAPQI formation in microsomes prepared fr om four human livers varying in CYP1A2, 2E1 and 3A4 content determined by Western blot analysis, (c) the contribution of CYP3A4 to the total formation of NAPQI from 0.1 mM APAP in human liver microsomes using t roleandomycin as a specific inhibitor, and (d) the relationship betwee n the contribution of CYP3A4 to NAPQI formation and relative CYP3A4 co ntent. The K(m) of CYP3A4 for APAP was found to be approximately 0.15 mM, similar to concentrations observed in humans after therapeutic dos es of the drug. The kinetics of formation of NAPQI in human liver micr osomes were complex; the lower K(m) was similar to that found for reco nstituted CYP3A4. The contribution of CYP3A4 to total NAPQI formation varied from 1 to 20% among livers, and correlated with the relative CY P3A4 content, r2 = 0.88, P < 0.05. Our findings indicate that CYP3A4, the major P450 isoform in human liver and enterocytes, contributes app reciably to the formation of the cytotoxic metabolite NAPQI at therape utically relevant concentrations of APAP and suggest that APAP may be a previously unrecognized inhibitor of this enzyme.