THE BIOTRANSFORMATION OF ISOPRENE AND THE 2 ISOPRENE MONOEPOXIDES BY HUMAN CYTOCHROME-P450 ENZYMES, COMPARED TO MOUSE AND RAT-LIVER MICROSOMES

The metabolism of isoprene was investigated with microsomes derived fr om cell. lines expressing human CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9 , CYP2D6, CYP2E1, or CYP3A4. The formation of epoxide metabolites was determined by gas chromatographic analysis. CYP2E1 showed the highest rates of formation of the isoprene monoepoxides 3,4-epoxy-3-methyl-1-b utene (EPOX-I) and 3,4-epoxy-2-methyl-1-butene (EPOX-II), followed by CYP2B6. CYP2E1 was the only enzyme showing detectable formation of the diepoxide of isoprene, 2-methyl-1,2:3,4-diepoxybutane, Both isoprene monoepoxides wire oxidized by CYP2E1 to the diepoxide at similar enzym atic rates. In order to determine the relative role of CYP2E1 in hepat ic metabolism, isoprene as well as the two monoepoxides were also incu bated with a series of ten human liver microsomal preparations in the presence of the epoxide hydrolase inhibitor cyclohexene oxide. The obt ained activities were correlated with activities towards specific subs trates for CYP1A2, CYP2A6, CYP2C9, CYP2D6, CYP2E1 and CYP3A. The resul ts were supportive for those obtained with single human P450 enzymes. Isoprene (monoepoxide) metabolism showed a significant correlation wit h CYP2E1 activity, determined as chlorzoxazone 6-hydroxylation. CYP2E1 is therefore the major enzyme involved in hepatic metabolism of isopr ene and the isoprene monoepoxides in vitro. To investigate species dif ferences with regard to the role of epoxide hydrolase in the metabolis m of isoprene monoepoxides, the epoxidation of isoprene by human liver microsomes was compared to that of mouse and rat liver microsomes. Th e amounts of monoepoxides farmed as a balance between epoxidation and hydrolysis, was measured in incubations with and without the epoxide h ydrolase inhibitor cyclohexene oxide. Inhibition of epoxide hydrolase resulted in similar rates of monoepoxide formation in mouse, rat and m an. Without inhibitor, however, the total amount of monoepoxides prese nt at the end of the incubation period was twice as high far mouse liv er microsomes than for rat and even 15 times as high as for human live r microsomes. Thus, differences in epoxide hydrolase activity between species may be of crucial importance for the toxicity of isoprene in t he various species. Copyright (C) 1996 Elsevier Science Ireland Ltd.