Molecular probes of the mechanism of cytochrome P450. Oxygen traps a substrate radical intermediate

Harriet L.R. Cooper, John Taylor Groves

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42 Scopus citations


The diagnostic substrate tetramethylcyclopropane (TMCP) has been reexamined as a substrate with three drug- and xenobiotic-metabolizing cytochrome P450 enzymes, human CYP2E1, CYP3A4 and rat CYP2B1. The major hydroxylation product in all cases was the unrearranged primary alcohol along with smaller amounts of a rearranged tertiary alcohol. Significantly, another ring-opened product, diacetone alcohol, was also observed. With CYP2E1 this product accounted for 20% of the total turnover. Diacetone alcohol also was detected as a product from TMCP with a biomimetic model catalyst, FeTMPyP, but not with a ruthenium porphyrin catalyst. Lifetimes of the intermediate radicals were determined from the ratios of rearranged and unrearranged products to be 120, 13 and 1 ps for CYP2E1, CYP3A4 and CYP2B1, respectively, corresponding to rebound rates of 0.9 × 1010 s-1, 7.2 × 1010 s -1 and 1.0 × 1012 s-1. For the model iron porphyrin, FeTMPyP, a radical lifetime of 81 ps and a rebound rate of 1.2 × 1010 s-1 were determined. These apparent radical lifetimes are consistent with earlier reports with a variety of CYP enzymes and radical clock substrates, however, the large amounts of diacetone alcohol with CYP2E1 and the iron porphyrin suggest that for these systems a considerable amount of the intermediate carbon radical is trapped by molecular oxygen. These results add to the view that cage escape of the intermediate carbon radical in [FeIV-OH -R] can compete with cage collapse to form a C-O bond. The results could be significant with regard to our understanding of iron-catalyzed C-H hydroxylation, the observation of P450-dependent peroxidation and the development of oxidative stress, especially for CYP2E1.

Original languageEnglish (US)
Pages (from-to)111-118
Number of pages8
JournalArchives of Biochemistry and Biophysics
Issue number1
StatePublished - Mar 1 2011

All Science Journal Classification (ASJC) codes

  • Molecular Biology
  • Biophysics
  • Biochemistry


  • Hydroxylation
  • Mechanism
  • Oxidation
  • Radical
  • Rebound


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