The ontogeny of cytochrome P450 (CYP450) activity, expression and volatile organic compound (VOC) metabolism in Sprague-Dawley (S-D) rats
McPhail, Brooks Tia'
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CYP450 levels are altered by development and minimum metabolism occurs in the young due to an under developed hepatic system. A minimum amount of data is available for developmental toxicological studies. Rodents are often used as surrogates for toxicological and pharmaceutical studies. In the current study, Sprague-Dawley (S-D) rats were used to assess the enzymatic activity, protein expression and metabolic capabilities of developing post-natal day (PND) rodents. Cytochrome P450 (CYP450) enzymes are members of Phase I metabolism which are involved in the oxidation, reduction and hydrolysis of compounds. CYP2E1, CYP1A1/2 and CYP2B1/2 are the predominate isozymes responsible for the metabolism of xenobiotics. PNDs 1, 5, 10, 15, 21, 30, 40, 50, 60 and 90 hepatic microsomes were used to measure the ontogeny of each isozyme enzymatic activity and protein expression. CYP2E1 activity and expression showed a decrease with an increase in age, with peak levels occurring at PND 21; CYP1A1/2 showed slightly lower activity and expression in younger animals with peak levels occurring between PND 21 through 40; CYP2B1/2 activity and expression was also slightly lower in younger animals with peak levels occurring at PND 21. After the enzymatic activity and protein expression was established, the metabolic capacity of PND 10, 15, 21, 60 and 90 were determined using the Michaelis-Menten parameters Km and Vmax in hepatic microsomal samples. The volatile organic compounds (VOCs) dichloromethane (DCM), trichloroethylene (TCE) and perchloroethylene (PERC) were used to measure the hepatic microsomal metabolism. Each VOC is commonly used in industrial settings and are metabolized by Phase I CYP450s. DCM, TCE and PERC showed an increase in metabolism with an increase in time and protein concentration. Although TCE and PERC are high and low metabolizing compounds, respectively, which form similar metabolites, DCM and TCE metabolism showed a similar pattern throughout development while PERC remained relatively constant. In spite of these differences, each compound showed peak Vmax levels at PND 21. The current study has shown that rodents have the highest enzymatic activity, protein expression and metabolic ability prior to adulthood; therefore, this data could prove useful in the risk assessment of VOCs for developing humans.