Epoxide hydrolase: a marker for experimental hepatocarcinogenesis.

Rat liver chemical hepatocarcinogenesis, induced by interrupted feeding of 2-acetylaminofluorene, results in various cellular preneoplastic stages and finally in a hepatoma in about 70 to 90 percent of the rats. The putative precursors of hepatomas, called hyperplastic nodules, appear after 12 weeks of feeding and, after 16 weeks of feeding carcinogen, most of them are persistent. Epoxide hydrolase is a tightly bound endoplasmic reticulum enzyme which is strongly induced in hyperplastic nodules and hepatomas. This enzyme has been purified, high-titre rabbit antiserum prepared to it, and this antiserum used to search for epoxide hydrolase immunodeterminants in the sera from chemically induced nodule or hepatoma bearing rats. An enzyme linked immunosorbent assay (ELISA) assay using this antiserum showed significant titres of circulating microsomal epoxide hydrolase antigen, range 0.01 to 2.50 (mean 1.18 +/- 0.30) micrograms per ml, in all 24 hepatoma bearing rats tested. Eight sera from animals with large hyperplastic nodules were also significantly positive for this antigen, while sera from six normal controls were negative (less than 0.004 microgram per ml serum). A passive hemagglutination inhibition assay with sheep or normal rat red blood cells sensitized with pure rat microsomal epoxide hydrolase was capable of detecting 0.2 microgram hydrolase per ml serum. With this assay, sera from four rats with hepatomas were found to contain 0.8 to 1.6 micrograms epoxide hydrolase immunodeterminants per ml. Control rat sera had no detectable immunodeterminants. Thus epoxide hydrolase, a marker induced during experimental chemical hepatocarcinogenesis and called the preneoplastic antigen, has been shown to be circulating in the tumor bearing host.