Abstract

The activity of liver microsomal CYP2E1 is commonly measured as the rate of 5-chloro-2-benzoxazolone (chlorzoxazone) 6-hydroxylation, which requires separation of 6-hydroxychlorzoxazone and chlorzoxazone by high pressure liquid chromatography (HPLC). In the present study, we describe a solvent extraction (non-HPLC) assay for measuring CYP2E1 activity, based on the 6-hydroxylation of [¹⁴C]chlorzoxazone. When [¹⁴C]chlorzoxazone was incubated with human or rat liver microsomes in the presence of NADPH, the major product formed was 6-[¹⁴C]hydroxychlorzoxazone. Unreacted [¹⁴C]chlorzoxazone was quantitatively extracted from the incubation mixture with dichloromethane under conditions that resulted in ∼45% extraction of 6-[¹⁴C]hydroxychlorzoxazone. The amount of 6-[¹⁴C]hydroxychlorzoxazone remaining in the aqueous incubation mixture (∼55% of the total amount formed) was quantified by liquid scintillation spectrometry. The limit of detection for this assay was 100 pmol of 6-[¹⁴C]hydroxychlorzoxazone. The solvent extraction procedure was validated by comparing the rates of formation of 6-[¹⁴C]hydroxychlorzoxazone with those determined by HPLC under a variety of experimental conditions. The close correspondence between the two analytical methods suggests that the extraction procedure for measuring 6-[¹⁴C]hydroxychlorzoxazone provides a simple, sensitive, and rapid alternative to the HPLC procedure for measuring CYP2E1 activity. In rats, the assay is not specific for CYP2E1 because CYP1A1 also catalyzes the 6-hydroxylation of chlorzoxazone. Recombinant human CYP1A1 also catalyzed the 6-hydroxylation of chlorzoxazone (at 15 the rate of CYP2E1), although CYP1A1 is not expressed in human liver microsomes. The non-HPLC assay was used to investigate the postulated role of CYP1A2 in the 6-hydroxylation of chlorzoxazone by human liver microsomes. Recombinant CYP1A2 did not catalyze the 6-hydroxylation of chlorzoxazone, and studies with 1-[(3,4-dimethoxyphenyl)methyl]-6,7-dimethoxyisoquinoline, which inhibits CYP1A2 but not CYP2E1, indicated that, in human liver microsomes, the 6-hydroxylation of chlorzoxazone is catalyzed by CYP2E1 with little or no contribution from CYP1A2 enzymes over a wide range of substrate concentrations.