A new hypothesis for the mechanism for cytochrome P-450 dependent aerobic conversion of hexahalogenated benzenes to pentahalogenated phenols

Abstract

The mechanism for cytochrome P-450 dependent conversion of hexahalogenated benzenes was investigated. This was done mainly by studying the in vitro and in vivo biotransformation of pentafluorochlorobenzene using 19F NMR. Identification of the metabolites formed in vivo from pentafluorochlorobenzene demonstrated that the fluorine atom para with respect to the chlorine substituent was preferentially eliminated during biotransformation. 19F NMR data also demonstrated that this fluorine atom is eliminated as a fluoride anion. In vitro studies on the biotransformation of pentafluorochlorobenzene demonstrated that defluorination was oxygen dependent and resulted in formation neither of 2,3,5,6-tetrafluoro-1-chlorobenzene nor of an amount of hydroxylated metabolites that could account for the fluoride anion formation. This formation of a high amount of fluoride anion, which was not accompanied by formation of a similar amount of other 19F NMR detectable products, demonstrates that the mechanism involved should also explain formation of a highly reactive intermediate that ends up bound to cellular macromolecules. On the basis of the results obtained, three of the four mechanisms proposed in the literature for the mechanism of the cytochrome P-450 dependent conversion of hexahalogenated benzenes to pentahalogenated phenols could be eliminated. Finally, results from molecular orbital computer calculations were compared to results from in vivo biotransformation studies for polyhalogenated benzenes. On the basis of the results obtained a new hypothesis for the mechanism of cytochrome P-450 dependent conversion of hexahalogenated benzenes to pentahalogenated phenols was proposed. This mechanism suggests that the reaction might proceed by elimination of a halogen anion and formation of a benzohaloquinone with a positive charge on a halogen substituent para with respect to the dehalogenated position. In a subsequent chemical reduction this benzohaloquinone could be reduced by, for instance, NADPH to give the final pentahalogenated phenol, although due to its high reactivity it might also end up bound to cellular macromolecules.