Some aspects of the role of cytochrome P‐450 isozymes in the n‐oxidative transformation of secondary and tertiary amine compounds

Abstract

Indirect evidence of the participation of cytochrome P-450 (P-450) in the microsomal N-oxygenation of secondary and tertiary nitrogen functions is presented by studies employing diagnostic modifiers of the hemoprotein system as well as antibodies directed toward the diverse P-450 isoforms and NADPHcytochrome P-450 reductase. Experiments with recombinant hemoproteins or P-450 isozymes directly purified from the tissues of various animal species support the results obtained by the inhibitor assays. Although the intermediacy of aminium radicals is thought to be restrictive to P-450-catalyzed N-oxygenation of secondary and tertiary amine groups bearing accessible hydrogens on the a-carbon, numerous exceptions to this rule are documented. It is proposed that aminium radicals partition between oxygen rebound and a-hydrogen abstraction to yield a finite level of N-oxygenated product in all P-450-mediated amine oxidations, the partition ratio depending on the amine structure and particular P-450 isozyme operative. In some instances, N-oxygenation appears to proceed by peroxidatic mechanisms. The relative contribution of P-450 to the N-oxygenation of secondary and tertiary amines in crude preparations or live animals, where competition with the flavin-containing monooxygenase (FMO) OCcurs, seems to be a function of the relative amounts and catalytic capacities of the two enzyme systems. Both parameters are species and tissue dependent. Accordingly, the extent to which P-450 contributes to total N-oxidative turnover of the amine substrates varies from minor to major.

Oxidizing species in the mechanism of cytochrome P450

“…The P450-catalyzed oxidation of amines to the corresponding N-oxides or N-hydroxyl products is less common than Ndealkylation. 145,146 These reactions are usually formulated as single-electron abstractions followed by recombination of the nitrogen radical cation with the ferryl oxygen (Scheme 18). 145 These reactions are less accessible to direct query with mechanistic probes, so the favored mechanism rests primarily on the evidence for radical cation intermediates in N-dealkylation reactions.…”

“…145,146 These reactions are usually formulated as single-electron abstractions followed by recombination of the nitrogen radical cation with the ferryl oxygen (Scheme 18). 145 These reactions are less accessible to direct query with mechanistic probes, so the favored mechanism rests primarily on the evidence for radical cation intermediates in N-dealkylation reactions. Conflicting evidence is provided, however, by the finding that electron donating and withdrawing substituents do not influence the oxidation of anilines and dimethylanilines to hydroxylamines and N-oxides, respectively, in a systematic way.…”

Quantitation and kinetic properties of hepatic microsomal and recombinant flavin-containing monooxygenases 3 and 5 from humans

“…These enzymes catalyze the oxidative metabolism of a variety of nitrogen-, sulfur-, and phosphorous-containing compounds, some of which are of toxicological importance [2,3]. The involvement of FMO-mediated reactions in human drug metabolism has been associated with a number of substrates including dimethylaniline [4][5][6], tertiary amines [7,8] including imipramine [6] and chlorpromazine [9], thiobenzamide [5,6], and tamoxifen [10].…”

Redox Partner Interaction Sites in Cytochrome P450 Monooxygenases:In SilicoAnalysis and Experimental Validation

“…Previously, a comprehensive list of positions mentioned in literature was compiled and the current knowledge about the electron transfer mechanism was discussed. [23,24] A conservation analysis of all CYPs showed class-specific conservation of heme-interacting residues, but no significant conservation of the CYP-redox partner interaction interface was observed. [15] While both literature and sequence data point to the CYP-reductase interface being essential for electron transfer, no sufficiently detailed mechanistic model is yet available to describe the productive transfer of the first and the second electron to the CYP-substrate complex.…”