title: Evaluation of the assumptions of an ontogeny model of rat hepatic cytochrome P450 activity.
abstract: We previously reported an ontogeny model of hepatic cytochrome P450 (P450) activity that predicts in vivo P450 elimination from in vitro intrinsic clearance. The purpose of this study was to conduct investigations into key assumptions of the P450 ontogeny model using the developing rat model system. We used two developmentally dissimilar enzymes, CYP2E1 and CYP1A2, and male rats (n = 4) at age groups representing critical developmental stages. Total body and liver weights and hepatic microsomal protein contents were measured. Following high-performance liquid chromatography analysis, apparent K(M) and V(max) estimates were calculated using nonlinear regression analysis for CYP2E1- and CYP1A2-mediated chlorzoxazone 6-hydroxylation and methoxyresorufin O-dealkylation, and V(max) estimates for p-nitrophenol and phenacetin hydroxylations, respectively. Hepatic scaling factors and V(max) values provided estimates for infant scaling factors (ISF). The data show microsomal protein contents increased with postnatal age and reached adult values after postnatal day (PD) 7. Apparent K(M) values were similar at all developmental stages except at < or =PD7. Developmental increases in probe substrate V(max) values did not correlate with the biphasic increase in immunoquantifiable P450. The activity of two different probe substrates for each P450 covaried as a function of age. A plot of observed ISF values as a function of age reflected the developmental pattern of rat hepatic P450. In summation, these observations diverge from several of the model's assumptions. Further investigations are required to explain these inconsistencies and to investigate whether the developing rat may provide a predictive paradigm for pediatric risk assessment for P450-mediated elimination processes.
authors: College of Pharmacy and Nutrition, University of Saskatchewan, Saskatoon, SK, Canada. jane.alcorn@usask.ca
authors: Jane
authors: J
authors: Alcorn
authors: Fawzy A
authors: FA
authors: Elbarbry
authors: Mohammed Z
authors: MZ
authors: Allouh
authors: Patrick J
authors: PJ
authors: McNamara
mesh_terms: D000367:Age Factors; D000375:Aging / Q000378:metabolism*; D000818:Animals; D001835:Body Weight; D002753:Chlorzoxazone / Q000378:metabolism; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism*; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism*; D003580:Cytochromes; D003640:Dealkylation; D006900:Hydroxylation; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000196:embryology / Q000201:enzymology* / Q000254:growth & development; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D008954:Models, Biological; D009596:Nitrophenols / Q000378:metabolism; D009929:Organ Size; D010078:Oxazines / Q000378:metabolism; D010615:Phenacetin / Q000378:metabolism; D011506:Proteins / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results; D018570:Risk Assessment; D013379:Substrate Specificity
chemical_list: D003580:Cytochromes; D007527:Isoenzymes; D009596:Nitrophenols; D010078:Oxazines; D011506:Proteins; C076580:7-methoxyresorufin; D019392:Cytochrome P-450 CYP2E1; C017170:Cyp1a2 protein, rat; D019388:Cytochrome P-450 CYP1A2; D010615:Phenacetin; D002753:Chlorzoxazone; C024836:4-nitrophenol