The microsomal demethylation of N,N-dimethylbenzamides. Substituent and kinetic deuterium isotope effects.
The metabolism of N,N-dimethylbenzamides by phenobarbital-induced rat liver microsomes results in the formation of N-methylbenzamides and formaldehyde. The reaction proceeds via the formation of an intermediate N-hydroxymethyl-N-methylbenzamide, which, for the microsomal oxidation of N,N-dimethylbenzamide, was isolated and characterized. Confirmation of the N-hydroxymethyl-N-methylbenzamide was obtained by its independent synthesis from N-methylbenzamide and formaldehyde. The intermolecular kinetic deuterium isotope effects for the reaction are 0.9 (+/- 0.1) for Vmax and 1.4 (+/- 0.1) for Vmax/Km. The intramolecular kinetic deuterium isotope effect, determined from the relative amounts of N-methylbenzamide and N-trideuteriomethylbenzamide formed in the microsomal demethylation of N-trideuteriomethyl-N-methylbenzamide, is 6.0 +/- 0.3. There is no correlation of Vmax or Vmax/Km with the substituent in the aromatic ring, nor with the calculated ionization potentials of the benzamides. The results are interpreted in terms of a mechanism in which the benzamide undergoes direct hydrogen atom abstraction to form a carbon centred radical. This carbon centred radical subsequently forms an N-hydroxymethyl-N-methylbenzamide that decomposes to formaldehyde and an N-methylbenzamide. Semi-empirical AM1 self consistent field molecular orbital calculations identify that loss of a hydrogen atom from the E-methyl group is thermodynamically more favourable than from the Z-methyl group by ca. 5 kJ/mol.
Constantino
L
L
CECF, Faculdade de Farmácia de Lisboa, Avenida das Forças Armadas, Portugal.
Rosa
E
E
Iley
J
J
D000577:Amides / Q000378:metabolism; D000818:Animals; D001549:Benzamides / Q000138:chemical synthesis / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D003903:Deuterium; D007700:Kinetics; D008297:Male; D013058:Mass Spectrometry; D008745:Methylation; D008862:Microsomes, Liver / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
D000577:Amides; D001549:Benzamides; C038531:N-hydroxymethyl-N-methylbenzamide; D003903:Deuterium; C038530:N-methylbenzamide