Biphasic O-deethylation of phenacetin and 7-ethoxycoumarin by human and rat liver microsomal fractions

Biochem Pharmacol. 1981 Sep 1;30(17):2451-6. doi: 10.1016/0006-2952(81)90340-3.


Human and rat liver microsomal fractions exhibit non-linear Michaelis-Menten kinetics in the O-deethylation of both phenacetin and 7-ethoxycoumarin. Comparison of various models indicated that the data were best described by a biphasic plot, which could be interpreted in terms of two populations of cytochrome P-450. The K(m)'s of the high affinity phase of 7-ethoxycoumarin O-deethylase activity were 1.8 +/- 0.4 microM and 2.3 +/- 0.4 microM for human and rat respectively while the K(m)'s of the low affinity phase were 205 +/- 20 microM and 237 +/- 59 microM in the two species respectively. V(max) of the high affinity phase of human 7-ethoxycoumarin O-deethylase activity was 96.9 +/- 19.0 pmol mg(-1) min(-1) and the activity of the corresponding phase in the rat was 2.7 times greater. The activities of the low affinity phase were 10-15 times greater than the respective activity of the high affinity phase. Rat and human also had similar values for the K(m)'s of the two phases of phenacetin O-deethylase activity, around 5 microM for the high affinity phase and 300 microM for the low affinity phase. Total activity was very similar in the two species, 1500-1750 pmol mg(-1) min(-1) and the difference between the two phases of activity was only 2.5-fold in man and 10-fold in rat. Studies on the effects of the in vitro modifiers of monooxygenase activity alpha-naphthoflavone and metyrapone further supported the hypothesis that the two phases of O-deethylase activity represent two different forms or populations of cytochrome P-450.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 7-Alkoxycoumarin O-Dealkylase / metabolism
  • Animals
  • Benzoflavones / metabolism
  • Coumarins / metabolism*
  • Cytochrome P-450 Enzyme System / metabolism
  • Humans
  • Kinetics
  • Male
  • Metyrapone / metabolism
  • Microsomes, Liver / enzymology
  • Microsomes, Liver / metabolism*
  • Phenacetin / metabolism*
  • Rats
  • Rats, Wistar
  • Subcellular Fractions
  • Substrate Specificity


  • Benzoflavones
  • Coumarins
  • 7-ethoxycoumarin
  • alpha-naphthoflavone
  • Cytochrome P-450 Enzyme System
  • 7-Alkoxycoumarin O-Dealkylase
  • Phenacetin
  • Metyrapone