Mechanism-based inactivation of cytochrome P450 3A4 by 4-ipomeanol

Chem Res Toxicol. 2004 Feb;17(2):150-7. doi: 10.1021/tx034143l.


Earlier phase I and II clinical studies showed that 4-ipomeanol produced selective hepatotoxicity. To investigate the mechanism of bioactivation of 4-ipomeanol, we thoroughly studied the interaction of 4-ipomeanol with human cytochrome P450 3A4 (EC 4-Ipomeanol produced a time- and concentration-dependent inactivation of P450 3A4. More than 80% of the P450 3A4 activity was lost after its incubation with 4-ipomeanol at the concentration of 75 microM in 12 min. The inactivation was characterized by a rate of inactivation (kinact) of 0.15 min(-1) and by an inactivation potency (KI) of 20 microM. In addition, the inhibition of P450 3A4 by 4-ipomeanol was NADPH-dependent and irreversible. Glutathione, catalase, and superoxide dismutase failed to protect P450 3A4 from inactivation by 4-ipomeanol. The presence of testosterone, a substrate of P450 3A4, protected the enzyme from inactivation. The estimated partition ratio of the inactivation was approximately 257. Covalent binding studies demonstrated that reactive metabolites of 4-ipomeanol modified P450 3A4 but not P450 reductase (EC The stoichiometry of binding between reactive metabolites of radiolabeled 4-ipomeanol and P450 3A4 was approximately 1.5:1. In addition to P450 3A4, reactive metabolites of 4-ipomeanol were found to covalently bind to other proteins. 4-Ipomeanol failed to inactivate P450 1A2 in human liver microsomes. In conclusion, 4-ipomeanol irreversibly inhibited P450 3A4, and it was characterized as a mechanism-based inactivator of P450 3A4. This finding facilitates the understanding of the mechanism of bioactivation of 4-ipomeanol by human hepatic enzymes.

MeSH terms

  • Cytochrome P-450 CYP1A2 Inhibitors
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme Inhibitors*
  • Dose-Response Relationship, Drug
  • Enzyme Inhibitors / pharmacology*
  • Humans
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology
  • NADP / pharmacology
  • Substrate Specificity
  • Terpenes / pharmacology*


  • Cytochrome P-450 CYP1A2 Inhibitors
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Terpenes
  • NADP
  • CYP3A protein, human
  • Cytochrome P-450 CYP3A
  • 4-ipomeanol