Determination of epoxide hydrolase activity in whole cells (human lymphocytes) and activation by benzoflavones

Epoxide hydrolase (epoxide hydratase, epoxide hydrase, E.C. 3.3.2.3) activity so far has only been measured in subcellular preparations. We show here that, with the highly lipophilic substrate (3H)-benzo(a)pyrene 4,5-oxide, the activity can be determined in intact cells. Whole human lymphocytes hydrolyze it at a similar rate to that in lymphocyte homogenate. We have previously reported that cultivation of lymphocytes in a medium containing 5,6-benzoflavone leads to an increase in epoxide hydrolase activity. We now demonstrate that this stimulation is due to enzyme activation and that enzyme induction does not contribute to this increase to any measurable extent. Moreover, both 5,6-benzoflavone and 7,8-benzoflavone activate epoxide hydrolase. This activation occurs not only in cell homogenate, but also - with a similar concentration-response relationship - in whole lymphocytes. Hence measurement of epoxide hydrolase activity in subcellular preparations reflects the activity in these intact cells. Furthermore, insofar as a concentration of 1 microM of the benzoflavones is sufficient to cause a measurable (10 to 20%) activation, it appears likely that foreign compounds can activate epoxide hydrolase in man.