Microsomal epoxide hydrolase in different rat strains.

Epoxide hydrolase activity was determined in hepatic microsomes of adult males of 22 rat strains. The specific activity varied between 4.3 and 12.7 nmole styrene glycol/mg protein per min. The enzyme in F344, DA and Sprague--Dawley rats, strains with low, high and intermediate activity, respectively, was studied in more detail. No differences in substrate specificity and pH-dependence of the activity were observed between the strains with high and low activity, and immunoprecipitation by antibodies raised against microsomal epoxide hydrolase purified from Sprague--Dawley rats showed that the amounts of enzyme protein in microsomes from DA and F344 rats correlated with the activities. These results indicate quantitative rather than qualitative differences in epoxide hydrolase. The enzyme activity was inherited in an autosomal and codominant manner. The hepatic activity in females (about 78% of that in males) and, with the limitation that only few situations were studied, the trans-stilbene oxide-induced activity were under the same genetic control as the basal hepatic activity in males. In contrast, some extrahepatic tissues showed strain differences in epoxide hydrolase activity which contrasted with those found in liver. Hence, the enzyme activity in one tissue cannot serve as a reliable guide to the relative activity in another tissue, unless a specific correlation between the two tissues has been established. Although the strain differences in activity were not very large in themselves, in combination with inter-individual variation, sex differences and effects of the enzyme inducer transstilbene oxide they led to a 20-fold variation in hepatic epoxide hydrolase activity among the rats investigated in the present study.