Epoxide hydrolase activity in native and in mitogen-stimulated lymphocytes of various human donors

An assay for epoxide hydrolase activity using benzo(a)-pyrene 4,5-oxide as substrate was modified in such a manner that it allowed the estimation of activities in native and in cultivated mitogen-stimulated human lymphocytes. Their specific activities were about 1000-fold lower than those of human or rat liver microsomes. In native lymphocytes of 28 subjects, the specific activities of epoxide hydrolase varied from 2.0 to 7.2 pmol/min/mg protein. Repeated measurements in the same subjects usually showed reasonable consistency of the activity (variation less than 1.8-fold), but in 3 of 12 subjects the activity varied between 2.5- and 3-fold. This indicates that epigenetic factors can alter the activity. To circumvent effects due to variable exposure to such modulating factors, lymphocytes were cultivated and stimulated by mitogens. In such cells, the variation of enzyme activity clearly decreased. The specific activities in stimulated lymphocytes from 12 donors varied only from 4.4 to 7.0 pmol/min/mg protein. Phenobarbital, pregnenolone-16 alpha-carbonitrile, trans-stilbene oxide, 3-methylcholanthrene, and benzo(a)anthracene in the culture medium did not significantly induce or activate epoxide hydrolase, but beta-naphthoflavone increased the activity. The effect was similar in lymphocytes of 13 different donors, the highest and lowest increases being 1.8- and 2.6-fold. Thus, no genetic heterogeneity of epoxide hydrolase was observed in native or in cultivated and stimulated lymphocytes or in the response to beta-naphthoflavone. Between males and females or smokers and nonsmokers, no significant differences occurred. Inter- and intraindividual variation of epoxide hydrolase activity in native lymphocytes appears to be due almost exclusively to epigenetic factors, e.g., composition of the lymphocyte population or in vivo exposure to unknown modulators of epoxide hydrolase.