Existence of multiple forms of microsomal epoxide hydrolases with radically different substrate specificities

Evidence for the existence in rat and rabbit liver of two microsomalepoxide hydrolases with radically different substrate specificitieswas obtained, one with a broad specificity (EH_b), whilst theother catalyzed the hydrolysis of cholesterol 5,6-oxide (EH_ch),a reaction taken as diagnostic since it was not observed withpure fractions of EH_b. The two enzymes were physically separatedby immunoprecipitation using antibodies which had been raisedagainst EH_b purified to apparent homogeneity. The substratespecificity of the two enzymes is radically different and mutuallycomplementary. Cholesterol 5, 6-oxide has a trisubstituted oxiranering. All epoxides of this nature tested to date were not, orvery poor, substrates of EH_b. The two enzymes can also effectivelybe discriminated by inhibitors, in that 5, 6-imino-5-cholestane-3ß-olpotently inhibits EH_ch but not EH_b whilst 1, 1, 1-trichloropropeneoxide has the opposite specificity. The cytosolic EH did notsignificantly contribute to the catalysis of the hydrolysisof cholesterol 5, 6-oxide.