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.
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.
Authors: S Ekins; M Vandenbranden; B J Ring; J S Gillespie; T J Yang; H V Gelboin; S A Wrighton Journal: J Pharmacol Exp Ther Date: 1998-09 Impact factor: 4.030
Authors: S A Wrighton; M Vandenbranden; J C Stevens; L A Shipley; B J Ring; A E Rettie; J R Cashman Journal: Drug Metab Rev Date: 1993 Impact factor: 4.518
Authors: L L von Moltke; D J Greenblatt; S X Duan; J Schmider; L Kudchadker; S M Fogelman; J S Harmatz; R I Shader Journal: Psychopharmacology (Berl) Date: 1996-12 Impact factor: 4.530