Human cytochromes P450 mediating phenacetin O‐deethylation in vitro: Validation of the high affinity component as an index of CYP1A2 activity

Abstract

Phenacetin O‐deethylation, widely used as an index reaction for cytochrome P450 1A2 (CYP1A2) activity, displays biphasic kinetics in human liver microsomes. CYP1A2 has been identified as contributing to the high affinity component, but is not verified as the sole contributor to the high affinity phase. In addition, the human CYP isoforms accounting for the low affinity phase have not been identified. We have used heterologously expressed human CYP isoforms to identify, kinetically characterize, and predict the relative contribution of the major human liver CYP isoforms mediating phenacetin O‐deethylation. CYP1A2 (Km 31 µM) is the only high affinity phenacetin O‐deethylase in human liver microsomes, while CYPs 2A6 (Km 4098 µM), 2C9 (Km 566 µM), 2C19 (Km 656 µM), 2D6 (Km 1021 µM), and 2E1 (Km 1257 µM) all contribute to the low affinity phase of the reaction. Considering the relative abundance of the various CYPs in human liver, CYP1A2 accounts for 86% of net reaction velocity at a substrate concentration of 100 µM, while CYP2C9 becomes the primary phenacetin O‐deethylase at substrate concentrations of 865 µM and higher and accounts for 31% of the net Vmax of the reaction. Predictions from kinetic studies on heterologously expressed CYPs are consistent with chemical inhibition studies on human liver microsomes with sulfaphenazole and α‐naphthoflavone that suggest a greater role for CYP2C9, and a smaller role for CYP1A2, at higher substrate concentrations. Thus CYP1A2 is the only high affinity human liver phenacetin O‐deethylase, thereby validating the use of the high affinity component as an index of CYP1A2 activity in human liver microsomes.