Human liver cytosolic epoxide hydrolases

Human liver epoxide hydrolases were characterized by several criteria and a cytosolic cis ‐stilbene oxide hydrolase (cEH cso ) was purified to apparent homogeneity. Styrene oxide and five phenylmethyloxiranes were tested as substrates for human liver epoxide hydrolases. With microsomes activity was highest with trans ‐2‐methylstyrene oxide, followed by styrene 7, 8‐oxide, cis ‐2‐With methylstyrene oxide, cis ‐1,2‐dimethylstyrene oxide, trans ‐1, 2‐dimethylstyrene oxide and 2, 2‐dimethylstyrene oxide. With cytosol the same order was obtained for the first three substrates, whereas activity with 2, 2‐dimethylstyrene oxide was higher than with cis ‐1,2‐dimethylstyrene oxide and no hydrolysis occurred with trans ‐1, 2‐dimethylstyrene oxide. Generally, activities were lower with cytosol than with microsomes. The isoelectric point for both microsomal styrene 7, 8‐oxide and cis ‐stilbene oxide hydrolyzing activity was 7.0, whereas cEH cso had an isoelectric point of 9.2 and cytosolic trans ‐stilbene oxide hydrolase (cEH TSO ) of 5.7. The cytosolic epoxide hydrolases could be separated by anion‐exchange chromatography and gel filtration. The latter technique revealed a higher molecular mass for cEH cso than for cEH TSO . Both cytosolic epoxide hydrolases showed higher activities at pH 7.4 than at pH 9.0, whereas the opposite was true for microsomal epoxide hydrolase. The effects of ethanol, methanol, tetrahydrofuran, acetonitrile, acetone and dimethylsulfoxide on microsomal epoxide hydrolase depended on the substrate tested, whereas both cytosolic enzymes were not at all, or only slightly, affected by these solvents. Effects of different enzyme modulators on microsomal epoxide hydrolase also depended on the substrates used. Trichloropropene oxide and styrene 7, 8‐oxide strongly inhibited cEH cso whereas cEH TSO was moderately affected by these compounds. Immunochemical investigations revealed a close relationship between cEH cso and rat liver microsomal, but not cytosolic, epoxide hydrolase. Interestingly, cEH TSO has no immunological relationship to rat microsomal, nor to rat cytosolic epoxide hydrolase. cEH TSO from human liver differed also from its counterpart in the rat in that it was only moderately affected by tetrahydrofuran, acetonitrile and trichloropropene oxide. Five steps were necessary to purify cEH cso . The enzyme has a molecular mass (49 kDa) identical to that of rat liver microsomal epoxide hydrolase.