Trimethylamine N-oxygenation and N-demethylation in rat liver microsomes.
The in vitro oxidation of trimethylamine (TMA) to TMA N-oxide (TMAO) and dimethylamine (DMA) was studied in rat liver microsomes. Pretreatment of rats with phenobarbital, 3-methylcholanthrene, ethanol or pregnenolone 16 alpha-carbonitrile had little or no effect on the liver microsomal metabolism of TMA to TMAO or DMA. Changing the atmosphere in the 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 Km for TMA N-demethylation was 5-fold higher than for the N-oxygenation reaction. The results of these studies suggest that the enzyme systems responsible for N-demethylation and N-oxygenation are different and that they are under different regulatory control. Carbon monoxide (CO/O2 = 80/20) had little or no inhibitory effect on either the N-demethylation or N-oxygenation of TMA by liver microsomes from control or pregnenolone 16 alpha-carbonitrile-treated rats. Additional studies indicated that methimazole, an inhibitor of FAD-containing monooxygenase (FMO), was a potent inhibitor of TMA oxidation. Preincubation of liver microsomes from control or pregnenolone 16 alpha-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-generating system in the preincubation medium (a procedure for preventing the thermal inactivation of FMO activity). The data suggest that FMOs are the major enzymes responsible for N-demethylation and N-oxygenation of TMA in rat liver microsomes.
Gut
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National Institute of Public Health, Praha, Czech Republic.
Conney
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AH
D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D000431:Ethanol; D008297:Male; D008744:Methylamines / Q000378:metabolism*; D008745:Methylation; D008748:Methylcholanthrene; D008862:Microsomes, Liver / Q000378:metabolism*; D010100:Oxygen; D010105:Oxygenases / Q000378:metabolism*; D010634:Phenobarbital; D051381:Rats; D017207:Rats, Sprague-Dawley
D008744:Methylamines; D000431:Ethanol; D008748:Methylcholanthrene; D003577:Cytochrome P-450 Enzyme System; D010105:Oxygenases; C020066:dimethylaniline monooxygenase (N-oxide forming); C023336:trimethylamine; D010100:Oxygen; D010634:Phenobarbital