Assessment of rat liver microsomal epoxide hydrolase as a marker of hepatocarcinogenesis

The influence of eleven xenobiotics on the activity and amount of hepatic microsomal epoxide hydrolase was determined. Activity was assayed using three different substrates after rats were fed, throughout 3 weeks, diets containing one of six hepatocarcinogens, viz. 2-acetylaminofluorene, 3'-methyl-4-dimethylaminoazobenzene, 4'-fluoro-4-dimethylaminoazobenzene, thioacetamide, aflatoxin B1 and ethionine. Five hepatocarcinogens induced activity 4- to 10-fold; ethionine was relatively ineffective as an inducer. Two non-carcinogenic analogues of hepatocarcinogens, viz. fluorene and p-aminoazobenzene, caused no appreciable increase in enzyme activity, but phenobarbital, barbital and 1-naphthylisothiocyanate induced activity 2- to 3-fold. All eleven xenobiotics increased the amount of microsomal epoxide hydrolase 2- to 9-fold when examined immunochemically using either a radial diffusion assay or an enzyme-linked immunosorbent assay (ELISA). Serum glutamic oxaloacetic acid transaminase activity was not appreciably elevated by feeding ten of the xenobiotics, suggesting that inductions were not owing to toxicity. Using ELISA, microsomal epoxide hydrolase was detected in post-microsomal (PM) supernatant fractions from control rat liver, thus confirming an earlier report by Gill et al. [Carcinogenesis 3, 1307 (1982)]. The eleven xenobiotics induced the amount of ELISA-detectable antigen in PM supernatant fractions by 3- to 34-fold. Longer centrifugation of PM supernatant fractions yielded a pellet fraction that contained 92 +/- 1.2% of the ELISA-detectable antigen irrespective of the xenobiotic regimen. Relationships between xenobiotic induction of microsomal epoxide hydrolase activity and amount and hepatocarcinogenesis are discussed.