Development of monoclonal antibodies to human microsomal epoxide hydrolase and analysis of "preneoplastic...

Microsomal epoxide hydrolase (mEH) is a drug metabolizing enzyme which resides on the endoplasmic reticulum (ER) membrane and catalyzes the hydration of reactive epoxide intermediates that are formed by cytochrome P450s. mEH is also thought to have a role in bile acid transport on the plasma membrane of hepatocytes. It is speculated that efficient execution of such multiple functions is secured by its orientation and association with cytochrome P450 enzymes on the ER membrane and formation of a multiple transport system on the plasma membrane. In certain disease status, mEH loses its association with the membrane and can be detected as distinct antigens in the cytosol of preneoplastic foci of liver (preneoplastic antigen), in the serum in association with hepatitis C virus infection (AN antigen), or in some brain tumors. To analyze the antigenic structures of mEH in physiological and pathological conditions, we developed monoclonal antibodies against different portions of mEH. Five different kinds of antibodies were obtained: three, anti-N-terminal portions; one anti-C-terminal; and one, anti-conformational epitope. By combining these antibodies, we developed antigen detection methods which are specific to either the membrane-bound form or the linearized form of mEH. These methods detected mEH in the culture medium released from a hepatocellular carcinoma cell line and a glioblastoma cell line, which was found to be a multimolecular complex with a unique antigenic structure different from that of the membrane-bound form of mEH. These antibodies and antigen detection methods may be useful to study pathological changes of mEH in various human diseases.