Epoxide hydrolase activity in human blood mononuclear leukocytes: individual differences in native and...

Epoxide hydrolase (EH; EC 3.3.2.3) activity was measured in whole-cell sonicates of native and cultured peripheral blood mononuclear cells (PBMCs) from 19 healthy unrelated Caucasian donors (age, 28-55 years). We used 1,2-epoxy-3-(p-nitrophenoxy)propane (0. 34 mM) as the substrate and, for the diol assay, a quantitative HPLC method with spectrophotometric detection. One portion of the PBMCs was frozen immediately, while the other portion was PHA-stimulated and cultivated for 36 h. In native leukocytes, the EH activity varied from 2.2 to 8.2 pmol/min per 10(6) cells (3.8-fold), the mean+/-SD was 5.6+/-1.4 pmol/min per 10(6) cells. In most of the samples from different donors, the specific activity increased in cultivation, varying from 2.4 to 15.4 pmol/min per 10(6) cells (6. 3-fold), the mean+/-SD being 8.5+/-3.8 pmol/min per 10(6) cells. From a methodological point of view, enzyme measurement in native cells is simple to perform and may provide a better index of the specific activity, as the accuracy of electronic cell counting is better for cell samples taken before than after cultivation. The differences in the EH activity of PBMCs indicate that significant interindividual variation may occur in the detoxification of epoxides produced in the human lymphocyte test systems commonly used for genotoxicity screening of chemicals in vitro. Further studies are needed to determine the extent to which the reproducibility and thus also the sensitivity of such assays could be improved by analyses carried out to control the donors for their EH phenotype.