Species differences in the covalent binding of [14C]tamoxifen to liver microsomes and the forms of cytochrome P450 involved

Ian N.H. White (Lead / Corresponding author), Francesco De Matteis, Anthony H. Gibbs, Chang Kee Lim, C. Roland Wolf, Colin Henderson, Lewis L. Smith

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63 Citations (Scopus)


Species differences in the NADPH-dependent covalent binding of [14C]tamoxifen to liver microsomes have been studied using preparations from humans, female F344 rats and DBA/2 mice. Protein binding has been used as an index of metabolic activation and as a surrogate for DNA binding in order to establish which forms of cytochrome P450 are responsible for genotoxicity. A panel of 12 human liver microsomes has been characterized and immunoquantified for nine cytochrome P450 isoenzymes. Binding of tamoxifen (45 μM) (25 ± 2.5 pmol/15 min/mg protein, mean ± SE) correlated (P < 0.05) with CYP3A4 and CYP2B6 content. Covalent binding of [14C]tamoxifen to microsomal preparations from human breast tumour tissue could also be detected but at levels 7-fold lower than in liver. The covalent binding of tamoxifen to mice, rat or human liver microsomal preparations increased with increasing substrate concentration. Covalent binding of [14C]tamoxifen (45 μM) in rats was 3.8-fold and mice 17-fold higher than in human liver microsomal preparations. In mice, the apparent Km (9.6 ± 1.9 μM) was very much lower than for rats (119 ± 41 μM). Pretreatment of female rats with phenobarbitone or dexamethasone resulted in a 4- to 5-fold increase in [14C]tamoxifen binding, relative to controls, consistent with the involvement of CYP2B1 and CYP3A1 in the metabolic activation. It cannot be distinguished at present if the same reactive metabolites are involved in protein and DNA binding. The greater potential of mouse liver microsomes to activate tamoxifen, relative to rats, does not reflect DNA damage or hepatocarcinogenicity seen following dosing with tamoxifen in vivo. It is concluded that covalent binding of tamoxifen to protein in vitro cannot be directly related to the carcinogenic potential of this compound. However, in the three species investigated, results suggest that the rat is a better model than the mouse for human liver microsomal activation of tamoxifen both with respect to kinetic parameters and the pattern of metabolic products.

Original languageEnglish
Pages (from-to)1035-1042
Number of pages8
JournalBiochemical Pharmacology
Issue number8
Publication statusPublished - 18 Apr 1995


  • CYP-isoenzymes
  • induction
  • metabolism
  • protein binding
  • species
  • tamoxifen

ASJC Scopus subject areas

  • Biochemistry
  • Pharmacology


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