Enantioselectivity of microsomal epoxide hydrolase toward arene oxide substrates

The enantioselectivity of native and purified hepatic microsomal epoxide hydrolase toward one symmetrical arene oxide, phenanthrene 9,10-oxide, and the resolved enantiomers of the three chiral arene oxide substrates, benzo[a]pyrene 4,5- and 7,8-oxide and benzo[a]anthracene 5,6-oxide was examined. A model lipid system of nonionic detergent micelles was used to solubilize both substrates and products. The enantioselectivity of purified epoxide hydrolase in micellar solution was essentially the same as the microsomal enzyme in the absence of detergent. Regioselectivity of the enzyme was determined by a gas chromatographic-mass spectrometric procedure in which bis-trifluoroacetates of the dihydrodiols were pyrolyzed to trifluoroacetylphenols to locate the position of the enzyme-catalyzed incorporation of 18O from H218O into the dihydrodiol. Correlation of the enantio- and regioselectivity data with the known absolute configurations of the dihydrodiols permitted the assignment of the (4S,5R) and (5S,6R) absolute configurations to (+)-benzo[a]pyrene 4,5- and (+)-benzo[a]anthracene 5,6-oxide, respectively. A kinetic analysis of the hydration of arene oxide substrates in micellar solution is presented. The kinetic discrimination of the enzyme toward substrate enantiomers and the product enantioselectivity of the enzyme are discussed in terms of possible molecular deformations occurring in the transition state and the ability of the enzyme to bind and stabilize these transition states.