Oxidation of butadiene monoxide to meso- and (+/-)-diepoxybutane by cDNA-expressed human cytochrome P450s and by mouse, rat, and human liver microsomes: evidence for preferential hydration of meso-diepoxybutane in rat and human liver microsomes

Abstract

Butadiene monoxide (BM) can be oxidized by cDNA-expressed human cytochrome P450 enzymes and by mouse, rat, and human liver microsomes to yield meso- and (+/-)-diepoxybutane (DEB). The DEB diastereomers were separated by gas chromatography (GC) and characterized by GC-mass spectrometry. Of eight cDNA-expressed human P450 enzymes examined, only incubations with 2E1, 2A6, and 2C9 led to DEB detection; total DEB formation rate by 2E1 was nearly four- and sixfold higher than the rates observed with 2C9 and 2A6, respectively, while incubations with 1A1, 1A2, 2B6, 2D6, or 3A4 did not lead to DEB detection. meso-DEB was detected preferentially in 2A6 and 2E1 incubations (ratios nearly 2:1), whereas incubations with 2C9 led to detection of both isomers in approximately equal amounts. Incubations of mouse, rat, or human liver microsomes with BM and chlorzoxazone provided further evidence for the involvement of 2E1 in BM oxidation; meso-DEB was the major DEB form detected at high BM concentrations. The Vmax/Km ratio for total DEB formation obtained with mouse liver microsomes was higher than the ratios obtained with rat and human liver microsomes. However, DEB hydrolysis in human liver microsomes was greater than that observed with rat liver microsomes, while no hydrolysis was detectable in mouse liver microsomes. When the DEB diastereomers were added in a 1:1 ratio to human or rat liver microsomes, selective hydrolysis of meso-DEB was observed. These results, which characterize new metabolic reactions for BM and DEB, may be of toxicological importance, as significant species differences reflecting both the stereoselective oxidation of BM by P450 enzymes and the subsequent preferential meso-DEB hydration by epoxide hydrolase have been demonstrated.