Stereoselective hydroxylation of 4-methyl-2-cyclohexenone in rats: its relevance to R-(+)-pulegone-mediated hepatotoxicity.

R-(+)-Pulegone, a monoterpene ketone, is a potent hepatotoxin. One of the major metabolites of pulegone has been shown to be p-cresol, a glutathione depletor and a known toxin. Allylic hydroxylation of 4-methyl-2-cyclohexenone results in the formation of p-cresol. The present study documents for the first time the involvement of cytochrome P-450 system and the stereochemical preference in this hydroxylation reaction. Incubation of PB-induced rat liver microsomes as well as reconstituted PB-induced cytochrome P-450 system with +/-4-methyl-2-cyclohexenone in the presence of NADPH and O(2) resulted in the formation of 4-hydroxy-4-methyl-2-cyclohexenone and p-cresol. From the assay mixture, the unreacted substrate, viz., 4-methyl-2-cyclohexenone was isolated and purified and its optical rotation was found to be 2.2 (in CHCl(3)). The observed enantiomeric excess in the recovered substrate was further confirmed by circular dichroism (CD) studies. The CD spectrum has a peak at 292nm and a trough at 270nm. The enantiomeric excess in the recovered substrate indicates that the hydroxylation at C-4 position is stereoselective. The significance of these results with respect to pulegone-mediated hepatotoxicity is discussed.