Jjp. Bogaards et al., THE BIOTRANSFORMATION OF ISOPRENE AND THE 2 ISOPRENE MONOEPOXIDES BY HUMAN CYTOCHROME-P450 ENZYMES, COMPARED TO MOUSE AND RAT-LIVER MICROSOMES, Chemico-biological interactions, 102(3), 1996, pp. 169-182
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.