TRIMETHYLAMINE N-OXYGENATION AND N-DEMETHYLATION IN RAT-LIVER MICROSOMES

The in vitro oxidation of trimethylamine (TMA) to TMA N-oxide (TMAO) a nd dimethylamine (DMA) was studied in rat liver microsomes. Pretreatme nt of rats with phenobarbital, 3-methyl-cholanthrene, ethanol or pregn enolone 16alpha-carbonitrile had little or no effect on the liver micr osomal metabolism of TMA to TMAO or DMA. Changing the atmosphere in th e incubation vessel from 20% oxygen/80% nitrogen (air) to 100% oxygen had a selective stimulatory effect on the N-oxygenation of TMA but did not affect TMA N-demethylation. In addition, the K(m) for TMA N-demet hylation was 5-fold higher than for the N-oxygenation reaction. The re sults of these studies suggest that the enzyme systems responsible for N-demethylation and N-oxygenation are different and that they are und er different regulatory control. Carbon monoxide (CO/O2 = 80/20) had l ittle or no inhibitory effect on either the N-demethylation or N-oxyge nation of TMA by liver chromosomes from control or pregnenolone 16alph a-carbonitrile-treated rats. Additional studies indicated that methima zole, an inhibitor of FAD-containing monooxygenase (FMO), was a potent inhibitor of TMA oxidation. Preincubation of liver microsomes from co ntrol or pregnenolone 16alpha-carbonitrile-treated rats at 37-degrees for 10 min without NADP(H) (a procedure that irreversibly inactivated FMO activity) resulted in >95% inhibition of TMA N-demethylation and N -oxygenation, and this inhibition was prevented by including a NADPH-g enerating system in the preincubation medium (a procedure for preventi ng the thermal inactivation of FMO activity). The data suggest that FM Os are the major enzymes responsible for N-demethylation and N-oxygena tion of TMA in rat liver microsomes.