Isotope effects in O- and N-demethylations mediated by rat liver microsomes: an application of direct insertion electron impact mass spectrometry

Isotope effects of ～2 have been found for the O-demethylation of p-nitroanisole, p-methoxyacetanilide, and p-dimethoxybenzene and the respective trideuteromethyl derivatives, when mediated by rat liver microsomes.The direct insertion mode of electron impact mass spectrometry (the advantages and limitations of which are discussed) was used together with conventional methods (observation of formaldehyde release, product analysis by spectrophotometry) to determine the isotope effects. Only the mass spectrometry method was applicable for determining the isotope effect associated with the mono-O-demethylation of p-trideuteromethoxyanisole and an unusually large value (10) was found.An insignificant isotope effect (≯ 1.05) was found for the mono-N-demethylation of l-(o-carbamoylphenyl)-3,3-dimethyltriazene and its di-(trideuteromethyl) analogue. The protium and deuterium forms had closely similar growth-inhibitory activities for the TLX5 lymphoma in mice.