[Microsomal epoxide hydrolase (author's transl)].

Microsomal epoxide hydrolase (EC 3.3.2.3. ; formerly EC 4.2.1.63) catalyse the hydration of epoxides into trans dihydro-diols, easily excretable hydrophil compounds. It is an enzyme with very wide specificity for monosubstituted oxiranes, or cis disubstituted oxiranes. It is inactive on polysubstituted epoxides. The hydration is stereospecific with the attack of a carbon S and inversion of the configuration to give a dihydro-diol (R,R). Epoxide hydrolase was purified from microsomes of rat and human liver. It is a hydrophobic protein of molecular weight 49 000, the presence of which may be shown immunochemically or enzymatically in numerous organs. Its insertion in the microsomal membranes renders difficult precise kinetic studies. The enzyme concentration is high, which compensates an average catalytic power. The role of epoxide hydrolase is ambiguous : in general, it protects against reactive electrophilic spoxides, but it is also an agent of toxic activation in the case of polycyclic hydrocarbons with a bay region.