A population approach to enzyme characterization and identification: application to phenacetin O-deethylation

Pharm Res. 2000 Dec;17(12):1531-6. doi: 10.1023/a:1007665310830.


Purpose: To determine the enzyme kinetics (EK) and identify the human cytochrome(s) P450 (CYP) involved in the deethylation of phenacetin to acetaminophen using a population-based method.

Methods: A sparse data set was generated from incubations containing human liver microsomes (n = 19) with phenacetin. Estimates of the EK parameters were obtained by fitting the concentration-velocity data to Michaelis-Menten models by using nonlinear mixed effects modeling. Relationships between the EK parameters and the CYP activities determined for these liver microsomes were examined.

Results: A two-enzyme kinetic model with a saturated, low KM enzyme and an unsaturated, high KM enzyme capable of forming acetaminophen best fit the data. The population estimates of the EK parameters were Vmax1, 911 pmol/min/mg protein; KM1, 11.3 microM; and Cl(int2), 0.4 microl/min/mg. The coefficients of variation for interliver variability in Vmax1 and residual error of the model were 39% and 15%, respectively. When the selective catalytic activities were examined as potential covariates, 7-ethoxyresorufin O-deethylation (CYP1A2) activity was found to be associated with the low KM enzyme, however, the high KM enzyme(s) could not be identified.

Conclusions: The population approach characterized the EK parameters and identified the low KM enzyme responsible for phenacetin O-deethylation as CYP1A2. Population modeling of EK provides valuable information on inter- and intraliver variability in CYP dependent activities.

MeSH terms

  • Algorithms
  • Analgesics, Non-Narcotic / metabolism*
  • Biotransformation
  • Cytochrome P-450 Enzyme System / metabolism
  • Dealkylation
  • Humans
  • In Vitro Techniques
  • Microsomes, Liver / metabolism
  • Models, Biological
  • Phenacetin / metabolism*
  • Population


  • Analgesics, Non-Narcotic
  • Cytochrome P-450 Enzyme System
  • Phenacetin