Intramolecular deuterium isotope effect and enantiotopic differentiation in oxidative demethylation of chiral [monomethyl-d3]methoxychlor in rat liver microsomes.

Intramolecular deuterium kinetic isotope effects on the O-demethylation of methoxychlor [2,2-bis-(4-methoxyphenyl)-1,1,1-trichloroethane] were measured in liver microsomes taken from rats treated with phenobarbital or beta-naphthoflavone and from untreated rats. The substrates were (R)-, (S)- and racemic [monomethyl-d3]methoxychlor, and the ratio of [d3]- to [d0]-mono-O-demethylated metabolites was measured by GC-MS selected-ion monitoring. The magnitude of the observed ratio of [d3]- to [d0]-metabolites in each microsomal preparation was largest on the reaction of the (S)-substrate, followed by racemic substrate, and then (R). Each value is a composite of the intramolecular kinetic isotope effect and enantiotopic differentiation during the reaction. Each intramolecular isotope effect value estimated from these values was smaller than the reported intrinsic value. A relatively slow intramolecular interchange of two methoxyl groups in the methoxychlor molecule in the enzyme-substrate complex was indicated during the reaction. There also was evidence of high enantiotopic differentiation.