Stereoselective and regioselective hydration of 7-methylbenz[c]acridine-5,6-oxide enantiomers by rodent and human microsomal epoxide hydrolases.

In the present study, we studied the regioselectivity and stereoselectivity of human microsomal epoxide hydrolase-catalyzed hydration of the enantiomers of the polycyclic aza-aromatic hydrocarbon K-region oxide, 7-methylbenz[c]acridine-5,6-oxide. We used a human microsomal epoxide hydrolase cDNA amplified from a liver cDNA library and expressed in COS-7 cells. Comparisons were made with the activities of rat and HLM preparations. The determination of the apparent Michaelis-Menten kinetic constants revealed that microsomal epoxide hydrolase, regardless of the source, exhibited enantioselectivity, with the 5S,6R-oxide being the preferred substrate. Regioselectivity of hydration for each stereoisomer was determined. Expressed human microsomal epoxide hydrolase and HLM catalyzed the attack of water predominantly (approximately 96%) at C5 of the 5R,6S-oxide, whereas 5S,6R-oxide was attacked less selectivity (approximately 60% at C5). These results are discussed in the context of available literature on the regioselectivity and stereoselectivity of rat and rabbit microsomal epoxide hydrolase and represents the first examination of human microsomal epoxide hydrolase regarding its regioselectivity and stereoselectivity of hydration.