RT Journal Article
SR Electronic
T1 Trans-stilbene Oxide: A Selective Inducer of Rat Liver Epoxide Hydratase
JF Molecular Pharmacology
JO Mol Pharmacol
FD American Society for Pharmacology and Experimental Therapeutics
SP 834
OP 847
VO 14
IS 5
A1 HANSUELI SCHMASSMANN
A1 FRANZ OESCH
YR 1978
UL http://molpharm.aspetjournals.org/content/14/5/834.abstract
AB Trans-stilbene oxide (TSO) was shown to be a potent rat liver epoxide hydratase inducer which up to doses leading to maximal epoxide hydratase induction did not increase monooxygenase activities. The selectivity of rat liver epoxide hydratase induction by TSO was established using the cytochrome P-450 content, the NADPH-cytochrome c reductase, the aminopyrine N-demethylase and two benzo(a)pyrene monooxygenase activities as indicators of the monooxygenase system. TSO treatment caused a dose-dependent increase of epoxide hydratase activity of up to 300% the control value, whereas apart from a slight increase of the NADPH-cytochrome c reductase no significant changes in the five monooxygenase parameters were observed. Due to the broad and overlapping specificities of the various monooxygenases it is unlikely that an increase in any of them would not be reflected by an increase of activity towards aminopyrine or benzo(a)pyrene. Since this still cannot fully be excluded, we do not term this induction specific but selective. The limits of the selectivity of epoxide hydratase induction by TSO were investigated by using excessive doses of TSO, which produced slightly less induction of the epoxide hydratase than optimal doses. At such doses of TSO the monooxygenase activities were also induced. Studies on the kinetics of the epoxide hydratase induction showed that two days after a single injection of 2.5 mmoles TSO per kg body weight, maximal induction of epoxide hydratase was reached and thereafter the enzyme level declined slowly reaching control values after 12 days. Over the whole induction period no significant increase of the five monooxygenase parameters was found with the exception of a slight (30%) shortlived increase in NADPH-cytochrome c reductase activity. Twelve hours after administration of the inducer the aminopyrine N-demethylase and the two benzo(a)pyrene monooxygenase activities were reduced to about 60% of controls, probably because of the presence of relatively high concentrations of TSO in the microsomes at this early time point. Addition of TSO in vitro (10-1000 µM) did not influence the determination of the cytochrome P-450 content and had no effect on the epoxide hydratase activity, but inhibited the aminopyrine N-demethylase and the benzo(a)pyrene monooxygenase activities. However, no inhibition of the monooxygenase activities was seen when control liver microsomes were incubated together with various amounts of liver microsomes from TSO treated rats (under conditions of maximal epoxide hydratase induction), indicating that no induction of monooxygenase activities occurred which was overshadowed by the presence of TSO or its metabolites in the microsomes. The TSO-induced increase in epoxide hydratase activity toward four structurally very different substrates was virtually identical indicating that the induced enzyme or at least its active site is very similar if not identical to the basal enzyme. Concomitant treatment with actinomycin D or cycloheximide and TSO showed that the epoxide hydratase induction was dependent on RNA and protein synthesis. Moreover, SDS-gel electrophoresis performed with liver microsomes from TSO-treated rats clearly showed an increased intensity of the band, which co-migrated with a standard of epoxide hydratase purified to homogeneity. Thus, the increased epoxide hydratase activity after TSO treatment is due to the presence of higher amounts of enzyme protein and not to an activation of the enzyme by TSO or its metabolites. ACKNOWLEDGMENT We thank Mr. A. J. Sparrow for preparation of [3H]benzo(a)pyrene 4,5-oxide and [7-3H]styrene oxide; Dr. P. Sims, Chester Beatty Research Institute, London, for the gift of [3H]3-methylcholanthrene 11,12oxide; Mr. C. H. Walker for determination of HEOM hydratase activity; Dr. E. Pfeiffer from the Institute of Hygiene at Mainz for pathological examination of the animals; and Dr. P. Bentley in this laboratory for the epoxide hydratase standard.