Primary and .beta.-secondary deuterium isotope effects in N-deethylation reactions

Abstract

Lidocaine (1), labeled specifically with deuterium in the alpha-methylene (lidocaine-d4,2) and beta-methyl (lidocaine-d6,3) carbon atoms of the terminal amino group, was used to probe the mechanism of oxidative N-deethylation by rat liver microsomes. The reaction rates were determined by measuring the formation of acetaldehyde colorimetrically. This general assay for oxidative N-deethylation reactions has the advantages of being rapid, producing a relatively stable colored derivative and being linear over the range of 0.25-4 mug of acetaldehyde formed per milliliter of incubate. Deuterium substitution at the methylene carbon atoms, the presumed site of initial oxygen insertion, revealed a kH/kD = 1.49 +/- 0.11 and a KmD/KmH = 1.23. Deuterium substitution on the terminal methyl groups showed a kH/kD = 1.52 +/- 0.10 and a KmD/KmH = 0.92. The results are explained in terms of both primary and secondary isotope effects on a possible rate-determining step in the N-deethylation sequence.

Untitled

“…A small but significant isotope effect (1.33) on the metabolic conversion of imipramine to DMI was thus observed, and this was consistent with the majority of isotope effects observed for N-demethylation reactions (magnitude <2) which have produced modified pharmacological properties (Elison et al, 1963;Abdel-Monem, 1975;Nelson et al, 1975). This isotopic effect extended the biological half-life of imipramine and decreased total systemic clearance, but appeared not to disturb the relative plasma levels of imipramine and DMI, indicating that no pharmacological change would be apparent.…”

Communications

“…A small but significant isotope effect (1.33) on the metabolic conversion of imipramine to DMI was thus observed, and this was consistent with the majority of isotope effects observed for N-demethylation reactions (magnitude <2) which have produced modified pharmacological properties (Elison et al, 1963;Abdel-Monem, 1975;Nelson et al, 1975 (Bax et al, 1980). In part, this is related to a progressive increase in plasma binding of the drug (Routledge et al, 1980) but studies in dogs also suggest a lowering of intrinsic clearance with time (Le Lorier et al, 1977;Vicuna et al, 1978).…”

An ab initio study of electronic factors in metabolic hydroxylation of aliphatic carbon atoms

“…A transition state involving approach of the activated oxygen atom toward the C-H bond and creation of a 2-electron 3-center bond has also been postulated in N-deethylation reactions. 21 Recently, an elegant technique utilizing intramolecular competition has shown unexpectedly high primary kinetic isotope effects in benzylic hydroxylation catalyzed by cytochrome P-450. 22 These results are fully compatible with, and provide strong evidence for, the oxene model.…”