Induction, activation and inhibition of epoxide hydrase: an anomalous prevention of chlorobenzene-induced hepatotoxicity by an inhibitor of epoxide hydrase

Epoxide hydrase activity, measured with ³H]styrene oxide as substrate, is present in mammalian liver, kidney, lung, intestine and skin. The hepatic level of the enzyme, measured in vitro with ³H]styrene oxide, benzene oxide or naphthalene-1,2-oxide, is elevated substantially by pretreatment of rats with phenobarbital and to a lesser extent by pretreatment with 3-methylcholanthrene. Metyrapone and 1-(2-isopropylphenyl)-imidazole, two monooxygenase inhibitors, activate epoxide hydrase in vitro, but have no demonstrable effect on the enzyme in vivo. 3,3,3-Trichloropropene oxide, a potent in vitro inhibitor of epoxide hydrase, has no effect on monooxygenase activity measured in vitro with ³H]benzenesulfonanilide. Trichloropropene oxide is extremely toxic. In sub-lethal dosages, it does not significantly inhibit epoxide hydrase activity in vivo, although it and several other epoxides do react with and thereby reduce hepatic levels of glutathione. Cyclohexane oxide, another potent in vitro inhibitor of epoxide hydrase, reduces hepatic glutathione levels to 10% of control values. This relatively non-toxic substance should potentiate the hepatotoxicity of chlorobenzene by inhibiting further metabolism of the toxic chlorobenzene oxide intermediate through either hydration or conjugation with glutathione. Instead, co-administration of cyclohexene oxide and chlorobenzene significantly reduces the rate of metabolism of ¹⁴C]chlorobenzene and prevents the hepatic centrilobular necrosis caused by chlorobenzene in rats. Arene oxide-mediated hepatotoxicity apparently is dependent upon a variety of factors including both rates of formation and degradation of arene oxides in tissue. The presently known hydrase inhibitors are not sufficiently selective in their effects on liver cells to permit a quantitative assessment of the relative importance of these factors.