Differential stereoselectivity of cytochromes P-450b and P-450c in the formation of naphthalene and anthracene 1,2-oxides. The role of epoxide hydrolase in determining the enantiomer composition of the 1,2-dihydrodiols formed.

As is the case for cytochrome P-450c, arene 1,2-oxides have been identified as initial metabolites when naphthalene and anthracene are oxidized by cytochrome P-450b in a highly purified, reconstituted system. Overall rates of metabolism by cytochrome P-450b are greater than 3-fold and greater than 50-fold lower than the respective rates of metabolism by cytochrome P-450c. For both hydrocarbons, the (-)-(1S,2R)-oxide predominates (74%) with cytochrome P-450b as the terminal oxidant, based on trapping the labile arene oxides as N-acetyl-L-cysteine S-conjugates of known absolute configuration. This result is in marked contrast to data obtained with cytochrome P-450c where the (+)-(1R,2S)-oxides predominate (73-greater than 95%). In the absence of added epoxide hydrolase, the metabolically formed arene oxides rapidly isomerize to phenols. Addition of increasing amounts of epoxide hydrolase to the incubation medium results in the formation of trans-1,2-dihydrodiols at the expense of phenols from the common arene oxide intermediates. Evaluation of the kinetic parameters (Km and kcat) for the hydration of the (+)- and (-)-enantiomers of both arene oxides by epoxide hydrolase has indicated that the (+)-(1R,2S)-enantiomers exhibit lower values of Km (approximately 1 microM) whereas the values of kcat are similar for both enantiomers of a given arene oxide. These parameters have allowed construction of a mathematical model which predicts the enantiomer composition of the dihydrodiols formed from naphthalene in reconstituted systems containing specific epoxide hydrolase concentrations. The data reported argue against a selective functional coupling mechanism between cytochrome P-450c and epoxide hydrolase in the metabolism of naphthalene and anthracene to the 1,2-dihydrodiols.