Characterization of Microsomal Epoxide Hydrolase in Hyperplastic Liver Nodules of Rats1

Microsomal epoxide hydrolase, a major antigenic marker of putative preneoplastic hepatocytes was studied in hyperplastic liver nodules and in normal liver tissue. Hyperplastic nodules induced by two different protocols contained 2to 3-fold higher specific activities of microsomal epoxide hydrolase than did normal surrounding tissue. However, no increase in cytosolic epoxide hydrolase determined with typical substrates for the cytosolic enzyme was observed in either nodule or surrounding tissue. No activity for typical substrates of microsomal epoxide hydrolase nor material immunologically cross-reactive with mi crosomal epoxide hydrolase was observable in the cytosolic fraction. The increase in activity of microsomal epoxide hydrolase in hyperplastic nodules was similar for two endogenous steroid epoxides, 16a,17a-epoxy-androst-4-en-3-one and 16a,17aepoxy-estratrien-3-ol, and for two xenobiotic substrates, benzo(a)pyrene 4,5-oxide and styrà ̈ne 7,8-oxide. The effects of several diagnostic modulators, including activators as well as competitive, uncompetitive, and noncompetitive inhibitors of widely varying potencies were not significantly different for nodule microsomal epoxide hydrolase activity as compared to hydrolase from normal tissue. While no immunological cross-reactivity was observed be tween cytosolic and microsomal epoxide hydrolase, the micro somal epoxide hydrolase from nodule and normal tissue was immunologically indistinguishable by all criteria tested: Ouchterlony double diffusion experiments with solubilized microsomes of nodule and control tissue and anti-microsomal epox ide hydrolase immunoglobulin G yielded only one precipitin line which was fused with that formed by apparently homogeneous microsomal epoxide hydrolase and the antibody. Immunoprecipitation by adding increasing amounts of antibody to solubi lized microsomes precipitated the entire activity of microsomal epoxide hydrolase with dose-response curves not significantly different between nodule and control microsomes. The available evidence indicates that: (a) The cytosolic epox ide hydrolase is not the microsomal enzyme that has leaked into the cytosol thereby acquiring different properties. It is a distinct entity under different biosynthetic control, (b) The increased diffusibility of microsomal epoxide hydrolase in hy perplastic nodules observed previously is not reflected in a detectable appearance of the enzyme in the cytosol isolated from nodules, (c) No differences between the enzymic or immunological properties of microsomal epoxide hydrolase from nodule and normal tissue were detectable by any of the methods used. The increase in specific epoxide hydrolase activity in hyperplastic nodules appears to be due to a perma nent change in the control of the amount of microsomal epoxide hydrolase which is, however, with respect to all criteria tested, an enzyme protein very similar to that found in normal tissue.