Mechanism-based inactivation of cytochrome P450 2B1 by 9-ethynylphenanthrene

Arch Biochem Biophys. 1995 Nov 10;323(2):295-302. doi: 10.1006/abbi.1995.9961.


The 7-ethoxycoumarin O-deethylase activity of rat cytochrome P450 (P450) 2B1 was inactivated by 9-ethynylphenanthrene (9EPh) in a time- and NADPH-dependent manner, and the loss of activity followed pseudo-first-order kinetics. At 20 degrees C, the extrapolated maximal rate constant of inactivation (kinactivation) was 0.45 min-1 and the inactivator concentration required for half-maximal inactivation (KI) was 138 nM. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and HPLC analysis demonstrated that [2'-3H]-9EPh was irreversibly bound to the protein moiety of P450 2B1 and the stoichiometry of binding was determined to be 0.82 mol of inactivator bound per mole of P450 2B1. A radiolabeled peptide of approximately 3.0 kDa was identified by autoradiography after Tricine SDS-PAGE analysis of the peptides generated from a cyanogen bromide cleavage of [2'-3H]9EPh-inactivated P450 2B1. After HPLC separation of these peptides, the fraction containing the most radioactivity was analyzed by matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) and peaks at m/z 2720.9 and 2939.9 were detected. The lower mass peak represents the molecular ion (MH+) for the peptide Ile290 to Met314 (theoretical 2722.2), while the higher mass peak corresponds to the MH+ of the modified peptide (theoretical 2940.5). The difference in mass (approximately 219) would correspond to the addition of a phenanthrylacetyl group to the peptide. When the fraction containing the modified and unmodified peptides was further digested with pepsin and reanalyzed by MALDI-MS, the site of attachment could be assigned to one of the amino acids contained in the peptide Phe297 to Leu307.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • 7-Alkoxycoumarin O-Dealkylase / antagonists & inhibitors*
  • Amino Acid Sequence
  • Animals
  • Aryl Hydrocarbon Hydroxylases*
  • Cytochrome P-450 Enzyme Inhibitors*
  • Cytochrome P-450 Enzyme System / chemistry
  • Enzyme Inhibitors / chemistry*
  • Kinetics
  • Male
  • Microsomes, Liver / enzymology
  • Molecular Sequence Data
  • NADP / metabolism
  • Peptide Mapping
  • Phenanthrenes / pharmacology*
  • Protein Binding
  • Rats
  • Steroid Hydroxylases / antagonists & inhibitors*
  • Steroid Hydroxylases / chemistry


  • 9-ethynylphenanthrene
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Phenanthrenes
  • NADP
  • Cytochrome P-450 Enzyme System
  • Steroid Hydroxylases
  • 7-Alkoxycoumarin O-Dealkylase
  • Aryl Hydrocarbon Hydroxylases
  • steroid 16-beta-hydroxylase