Rat and human liver microsomal epoxide hydratase. Purification and evidence for the existence of multiple forms.

Liver microsomal epoxide hydratase (EC 4.2.1.63) was purified from untreated rats, from rats treated with phenobarbital, 3-methylcholanthrene, or trcms- stilbene oxide, and from three different human autopsy samples. More than one form of epoxide hydratase could be separated from each liver source. Most of the rat and human preparations were apparently homo- geneous as judged by polyacrylamide gel electropho- resis in the presence of sodium dodecyl sulfate; all of the preparations examined had apparent M,s of 50,000 as judged by this technique. Different epoxide hydra- tase fractions showed differences upon rechromatog- raphy or agarose gel electrophoresis. Amino acid com- positions were determined for all of the apparently homogeneous enzyme preparations; analysis of the data indicated differences between some of the isolated proteins. Differences were also found between some of the epoxide hydratase fractions when their activities toward the substrates styrene-7,8-oxide, benzo(a)py- rene-4,5-oxide, benzo(a)pyrene-7,8-oxide, benzo(a)- anthracene-5,6-oxide, dibenzo(a,h)anthracene-5,6-0x- ide, 3-methylcholanthrene-11,12-oxide, and f-chloro- ethylene oxide were examined under conditions of fixed time and enzyme and substrate concentrations. Some apparent differences were also observed in the effects of fixed concentrations of metyrapone, cyclohexene ox- ide, and 3,3,3&ichloropropylene oxide on enzymatic activities of the various epoxide hydratase fractions. These observations, coupled with immunological studies presented in the accompanying paper (Guen- gerich, F. P., Wang, P., Mason, P. S., and Mitchell, M. B. (1979) J. Biol. Chem. 254, 12255-12259), provide evi- dence that 1) rat and human liver microsomal epoxide hydratases are similar to each other in many respects but are distinguishable by several criteria, 2) multiple forms of epoxide hydratase exist in livers of individual rats or humans, and 3) the natures of these forms and the levels of each can be altered by treatment with various xenobiotics. Epoxide hydratase (EC 4.2.1.63) is a microsomal enzyme which catalyzes the formation of trans-dihydrodiols from a variety of epoxides, many of which are known carcinogens (3). Such hydration is usually, although not always (4), associated