Differential time-dependent inactivation of P450 3A4 and P450 3A5 by raloxifene: a key role for C239 in quenching reactive intermediates

Chem Res Toxicol. 2007 Dec;20(12):1778-86. doi: 10.1021/tx700207u. Epub 2007 Nov 15.


The role of C239 as the active-site residue responsible for forming the covalent linkage with raloxifene during P450 3A4 time-dependent inactivation (TDI) was recently identified. The corresponding residue in CYP3A5 is S239, and when the potential for TDI in P450 3A5 was investigated, only reversible inhibition was observed against midazolam and testosterone, with median inhibitory concentration (IC50) values of 2.4 and 2.9 microM, respectively. In a similar fashion, when C239 was replaced with alanine in P450 3A4, TDI was successfully engineered out, and the reversible inhibition was characterized by IC50 values of 3.7 and 3.5 microM against midazolam and testosterone, respectively. Metabolism studies confirmed that the reactive diquinone methide intermediate required for P450 3A4 inactivation formed in all of the P450 3A enzymes investigated. Furthermore, the absence of TDI in P450 3A5 led to an increase in the formation of GSH-related adducts of raloxifene compared with that for P450 3A4. Consequently, the absence of the nucleophilic cysteine leads to differential TDI and generation of reactive metabolites in the P450 3A enzyme, providing the foundation for pharmacogenetics that contributes to individual differences in susceptibility to adverse drug reactions.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Cloning, Molecular
  • Cysteine / chemistry*
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme Inhibitors*
  • Cytochrome P-450 Enzyme System / chemistry
  • Cytochrome P-450 Enzyme System / genetics
  • Escherichia coli / genetics
  • Humans
  • Models, Molecular
  • Mutation
  • Raloxifene Hydrochloride / metabolism
  • Raloxifene Hydrochloride / pharmacology*
  • Selective Estrogen Receptor Modulators / metabolism
  • Selective Estrogen Receptor Modulators / pharmacology*
  • Time Factors


  • Cytochrome P-450 Enzyme Inhibitors
  • Selective Estrogen Receptor Modulators
  • Raloxifene Hydrochloride
  • Cytochrome P-450 Enzyme System
  • CYP3A5 protein, human
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human
  • Cysteine