Abstract

The enantioselective sulfoxidation of a series of alkyl p-tolyl sulfides was compared using purified rabbit lung and mini-pig liver flavin-containing monooxygenase (FMO). Analysis was performed by chiral-phase high pressure liquid chromatography, which afforded baseline resolution of each pair of enantiomers. The extent of enantioselective sulfoxidation was found to be a function of (a) the isozyme employed, (b) the steric bulk of the alkyl substituent, and (c) pH. At pH 8.5, rabbit lung FMO catalyzed the oxidation of methyl, ethyl, propyl, and isopropyl sulfides to products with greater than 99, 91, 85, and 63% (R)-(+)-stereochemistry, respectively. Corresponding values for the mini-pig liver form were 91, 82, 72, and 41% (R)-(+)-sulfoxide. The stereochemical profile obtained with the isolated rabbit lung form could be duplicated exactly in microsomal preparations if precautions were taken to abolish the contribution that P-450 makes to net stereochemistry. It was noted that increasing the reaction mixture pH from 8.5 to 10 led to a decrease in the stereochemical purity of products obtained from the lung form. In contrast, the stereochemical profile obtained with the isolated mini-pig liver form could not be exactly duplicated in suitably treated microsomal preparations. No evidence for multiple forms of mini-pig liver FMO was obtained, and it was concluded that discrepancies between microsomal and purified FMO metabolic profiles were most consistent with a minor modification to active site geometry occurring during purification of the mini-pig form. These data show that the active site chirality of rabbit lung and mini-pig liver FMO is largely retained following removal from microsomal membranes. Qualitative similarities in the structure-activity relationships exhibited by microsomal or purified FMO from rabbit lung and mini-pig liver suggest some conservation of active site geometry between these two otherwise distinct FMOs. Quantitative differences in the structure-activity relationships exhibited by the two FMO forms indicate that analysis of product stereochemistry may be a useful method for the discrimination of catalytically distinct FMO isozymes.