4-Alkyl radical extrusion in the cytochrome P-450-catalyzed oxidation of 4-alkyl-1,4-dihydropyridines

Abstract

Rat liver microsomal cytochrome P-450 oxidizes the 4-methyl, 4-ethyl (DDEP), and 4-isopropyl derivatives of 3,5-bis(carbethoxy)-2,6-dimethyl-1,4-dihydropyridine to mixtures of the corresponding 4-alkyl and 4-dealkyl pyridines. A fraction of the total microsomal enzyme is destroyed in the process. The 4-dealkyl to 4-alkyl pyridine metabolite ratio, the extent of cytochrome P-450 destruction, and the rate of spin-trapped radical accumulation are correlated in a linear inverse manner with the homolytic or heterolytic bond energies of the 4-alkyl groups of the 4-alkyl-1,4-dihydropyridines. No isotope effects are observed on the pyridine metabolite ratio, the destruction of cytochrome P-450, or the formation of ethyl radicals when [4-2H]DDEP is used instead of DDEP. N-Methyl- and N-ethyl-DDEP undergo N-dealkylation rather than aromatization but N-phenyl-DDEP is oxidized to a mixture of the 4-ethyl and 4-deethyl N-phenylpyridinium metabolites. In contrast to the absence of an isotope effect in the oxidation of DDEP, the 4-deethyl to 4-ethyl N-phenylpyridinium metabolite ratio increases 6-fold when N-phenyl[4-2H]DDEP is used. The results support the hypothesis that cytochrome P-450 catalyzes the oxidation of dihydropyridines to radical cations and show that the radical cations decay to nonradical products by multiple, substituent-dependent, mechanisms.

On the mechanism of amine oxidations by P450

“…A plot of log (V,,,,,) for the P450 oxidation of substituted N,N-dimethylanilines versus Hammett 0 gives p = -0.6 (Burka et al 1985). 1,4-Dihydropyridine Hantzsch esters (August0 et al 1982, Lee et al 1988 and cyclopropylamines (Hanzlik 1979, Hanzlik and Tullman 1982, Macdonald et al 1982 are suicide inactivators of P450.…”

“…For example, the photochemical reaction of trans-stilbene with N-methyl-N-(trideuteriomethy1)-Ntert-butylamine, which was proposed to proceed via deprotonation of the N-methyl-N-(trideuteriomethy1)-N-tert-butylamine cation radical by the stilbene anion radical, gave an isotope effect of 2.2 (Lewis and Ho 1980). Other examples include the alkaline potassium ferricyanide oxidation of N-methyl-N,N-di-(n-buty1)amine versus N-(trideuteriomethy1)-N,N-di-(n-buty1)amine (kH/kD = 3.6) (Lindsay Smith and Mead 1973) and the electrooxidation of these amines in alkaline medium (&/AD = 2.9) (Lindsay Smith and Masheder 1976).…”

“…The fact that 1,4-dihydropyridine Hantzsch esters and cyclopropylamines are suicide inactivators of P450 has been taken as evidence that these compounds react by an electron transfer mechanism. The proposed mechanism (August0 et al , Lee et al 1988) for reaction of the Hantzsch esters is shown in the top half of figure 1. P450 is thought to oxidize the amines to their cation radicals, which are then proposed to undergo aromatization by loss of the alkyl group at C+ as a free radical.…”

Heme-Reversible Impairment of CYP2B1/2 Induction in Heme-Depleted Rat Hepatocytes in Primary Culture: Translational Control by a Hepatic α-Subunit of the Eukaryotic Initiation Factor Kinase?

“…However, such acute hepatic heme depletion was observed after treatment of rats with the P450 suicide inactivator 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP) Correia, 1983, 1985;Ren and Correia, 2000). DDEP irreversibly destroys the heme of hepatic P450s 2C11, 2C6, and 3A, but not CYP2B1 (Lee et al, 1988;Sugiyama et al, 1989). Such CYP2C11 and CYP2C6 destruction through heme pyrrole N-ethylation generates N-ethylprotoporphyrins, potent inhibitors of the terminal heme synthetic enzyme ferrochelatase.…”

Metabolic Aromatization ofN-Alkyl-1,2,3,4-Tetrahydroquinoline Substructures to Quinolinium by Human Liver Microsomes and Horseradish Peroxidase

“…Therefore, it is assumed that the P450-catalyzed aromatization of the alkyltetrahydroquinolines undergoes an initial SET step to generate a radical nitrogen cation (II; Scheme 3). The postulation of the pathway from structure IV to structure VI (Scheme 3) was originated from the radical mechanism for the P450-catalyzed oxidation of 4-alkyl-1,4-dihydropyridine derivatives bearing various substitutions (Augusto et al, 1982;Böcker and Guengerich, 1986;Guengerich and Böcker, 1988;Lee et al, 1988). Augusto et al (1982) reported the radical mechanism for the oxidation of the dihydropyridines on the basis of spin-trapping of the alkyl radical released from the 4-position, and the mechanism was further supported by more evidence, e.g., no significant isotope effect for the 4-position hydrogen on the dehydrogenation (Guengerich and Böcker, 1988).…”