Chemical model studies on the monoamine oxidase-B catalyzed oxidation of 4-substituted 1-cyclopropyl-1,2,3,6-tetrahydropyridines

Why Does Monoamine Oxidase (MAO) Catalyze the Oxidation of Some Tetrahydropyridines?

Results pertaining to the mechanism of the oxidation of the tertiary amine 1-methyl-4-(1-methyl-1-H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP, a close analog of the Parkinsonism inducing compound MPTP) by 3-methyllumiflavin (3MLF), a chemical model for the FAD cofactor of monoamine oxidase, are reported. MMTP and related compounds are among the few tertiary amines that are monoamine oxidase B (MAO-B) substrates. The MMTP/3MLF reaction is catalytic in the presence of O2 and the results under anaerobic conditions strongly suggest the involvement of radical intermediates, consistent with a single electron transfer mechanism. These observations support a new hypothesis to explain the MAO-catalyzed oxidations of amines. In general, electron transfer is thermodynamically unfavorable, and as a result, most 1° and 2° amines react via one of the currently accepted polar pathways. Steric constraints prevent 3° amines from reacting via a polar pathway. Those select 3° amines that are MAO substrates possess certain structural features (e. g., a C-H bond that is α- both to nitrogen and a C=C) that dramatically lower the pKa of the corresponding radical cation. Consequently, the thermodynamically unfavorable electron transfer equilibrium is driven towards products by an extremely favorable deprotonation step in the context of Le Chatelier's principle.

Stereoelectronic and Resonance Effects on the Rate of Ring Opening of N-Cyclopropyl-Based Single Electron Transfer Probes

N-Cyclopropyl-N-methylaniline (5) is a poor probe for single electron transfer (SET) because the corresponding radical cation undergoes cyclopropane ring opening with a rate constant of only 4.1 × 10⁴ s^-1, too slow to compete with other processes such as radical cation deprotonation. The sluggish rate of ring opening can be attributed to either (i) a resonance effect in which the spin and charge of the radical cation in the ring-closed form is delocalized into the phenyl ring, and/or (ii) the lowest energy conformation of the SET product (5^•+) does not meet the stereoelectronic requirements for cyclopropane ring opening. To resolve this issue, a new series of N-cyclopropylanilines were designed to lock the cyclopropyl group into the required bisected conformation for ring opening. The results reveal that the rate constant for ring opening of radical cations derived from 1'-methyl-3',4'-dihydro-1'H-spiro[cyclopropane-1,2'-quinoline] (6) and 6'-chloro-1'-methyl-3',4'-dihydro-1'H-spiro[cyclopropane-1,2'-quinoline] (7) are 3.5 × 10² s^-1 and 4.1 × 10² s^-1, effectively ruling out the stereoelectronic argument. In contrast, the radical cation derived from 4-chloro-N-methyl-N-(2-phenylcyclopropyl)aniline (8) undergoes cyclopropane ring opening with a rate constant of 1.7 × 10⁸ s^-1, demonstrating that loss of the resonance energy associated with the ring-closed form of these N-cyclopropylanilines can be amply compensated by incorporation of a radical-stabilizing phenyl substituent on the cyclopropyl group. Product studies were performed, including a unique application of EC-ESI/MS (Electrochemistry/ElectroSpray Ionization Mass Spectrometry) in the presence of ¹⁸O₂ and H₂¹⁸O to elucidate the mechanism of ring opening of 7^•+ and trapping of the resulting distonic radical cation.

Model Electrochemical-Mass Spectrometric Studies of the Cytochrome P450-Catalyzed Oxidations of Cyclic Tertiary Allylamines

Single-electron transfer and hydrogen atom transfer pathways have been proposed to account for the cytochrome P450-catalyzed alpha-carbon oxidations of amines. With the aid of electrochemistry-electrospray ionization mass spectrometry, the electrochemical potentials required for the one-electron oxidations of N-methyl- and selected N-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridinyl derivatives and the chemical fates of the resulting aminyl radical cations have been investigated. Comparison of the results of these studies with those observed in the corresponding enzyme catalyzed oxidations suggests that aminyl radical cations are not obligatory intermediates in the cytochrome P450-catalyzed alpha-carbon oxidations of this class of substrates.

Studies on the cytochrome P450 catalyzed oxidation of 13C labeled 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine by 13C NMR

A recent study from Hanzlik's laboratory (J. Am. Chem. Soc. 2002, 124, 8268) has provided compelling evidence of a hydrogen atom transfer pathway for the cytochrome P450-catalyzed oxidative N-decyclopropylation of N-cyclopropyl-N-methylaniline. In the present paper, we report an analogous pathway for the oxidative decyclopropylation of a 13C-labeled 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridinyl substrate. Three 13C-enriched metabolites were characterized: (1) a diastereomeric pair of N-cyclopropyl-N-oxides; (2) the N-cyclopropylpyridinium species; and (3) cyclopropanone hydrate. These results extend the hydrogen atom transfer pathway to include aliphatic tertiary amine substrates. Consideration of all of the available evidence, however, leads us to conclude that the cytochrome P450-catalyzed alpha-carbon oxidations of cyclopropylamines may proceed via both the single electron and hydrogen atom transfer pathways.

Cyclopropylamine inactivation of cytochromes P450: Role of metabolic intermediate complexes

The inactivation of cytochrome P450 enzymes by cyclopropylamines has been attributed to a mechanism involving initial one-electron oxidation at nitrogen followed by scission of the cyclopropane ring leading to covalent modification of the enzyme. Herein, we report that in liver microsomes N-cyclopropylbenzylamine (1) and related compounds inactivate P450 to a large extent via formation of metabolic intermediate complexes (MICs) in which a nitroso metabolite coordinates tightly to the heme iron, thereby preventing turnover. MIC formation from 1 does not occur in reconstituted P450 systems with CYP2B1/2, 2C11 or 2E1, or in microsomes exposed to gentle heating to inactivate the flavin-containing monooxygenase (FMO). In contrast, N-hydroxy-N-cyclopropylbenzylamine (3) and N-benzylhydroxylamine (4) generate MICs much faster than 1 in both reconstituted and microsomal systems. MIC formation from nitrone 5 (PhCH = N(O)cPr) is somewhat faster than from 1, but very much faster than the hydrolysis of 5 to a primary hydroxylamine. Thus the major overall route from 1 to a P450 MIC complex would appear to involve FMO oxidation to 3, further oxidation by P450 and/or FMO to nitrone 5' (C2H4C = N(O)CH2Ph), hydrolysis to 4, and P450 oxidation to alpha-nitrosotoluene as the precursor to oxime 2 and the major MIC from 1.

Studies on the oxidation of 1,4-disubstituted-1,2,3,6-tetrahydropyridines

Interest in the parkinsonian-inducing proneurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine has prompted extensive studies into the oxidative pathways mediating its bioactivation to the corresponding pyridinium species, a potent inhibitor of the mitochondrial electron transport chain. The initial step in the overall reaction is the two-electron ring alpha-carbon oxidation to give the 1-methyl-4-phenyl-2,3-dihydropyridinium species, a reaction that is catalyzed by monoamine oxidase B. The same a-carbon oxidation is catalyzed by members of the cytochrome P-450 family of oxidases. This paper examines the impact that various structural features of 1,4-disubstituted-1,2,3,6-tetrahydropyridinyl derivatives have on the oxidative fate of this class of compound.

Rat liver microsomal enzyme catalyzed oxidation of 4-phenyl-trans-1-(2-phenylcyclopropyl)-1,2,3,6-tetrahydropyridine

As part of our ongoing studies to characterize the catalytic pathway(s) for the monoamine oxidase and cytochrome P450 catalyzed oxidations of 1,4-disubstituted 1,2,3,6-tetrahydropyridinyl derivatives, we have examined the metabolic fate of 4-phenyl-trans-1-(2-phenylcyclopropyl)-1,2,3,6-tetrahydropyridine in NADPH supplemented rat liver microsomes. Three metabolic pathways have been identified: (1) allylic ring alpha-carbon oxidation to yield the dihydropyridinium species, (2) nitrogen oxidation to yield the N-oxide and (3) N-dealkylation to yield 4-phenyl-1,2,3,6-tetrahydropyridine and cinnamaldehyde. A possible mechanism to account for the formation of cinnamaldehye involves an initial single electron transfer from the nitrogen lone pair to the iron oxo system Fe(+3)(O) to form the corresponding cyclopropylaminyl radical cation that will be processed further to the final products. The reaction pathway leading to the dihydropyridinium metabolite may also proceed via the same radical cation intermediate but direct experimental evidence to this effect remains to be obtained.

beta-scission of the N-O bond in alkyl hydroxamate radicals: a fast radical trap

[reaction--see text ] The rate of the beta-scission of the N-O bond in the alkyl hydroxamate radical is faster than 2 x 10(8) s(-)(1). This reaction may be useful as a radical trap.

N-Alkyl-N-cyclopropylanilines as mechanistic probes in the nitrosation of N,N-dialkyl aromatic amines

A group of N-cyclopropyl-N-alkylanilines has been synthesized, and their reaction with nitrous acid in aqueous acetic acid at 0 degrees C was examined. All compounds reacted rapidly to produce the corresponding N-alkyl-N-nitrosoaniline by specific cleavage of the cyclopropyl group from the nitrogen. The transformations were unaffected by the nature of the alkyl substituent (Me, Et, (i)()Pr, Bn). The reaction of 4-chloro-N-2-phenylcyclopropyl-N-methylaniline with nitrous acid gave 4-chloro-N-methyl-N-nitrosoaniline (76%), cinnamaldehyde (55%), 3-phenyl-5-hydroxyisoxazoline (26%), and 5-(N-4-chlorophenylmethylamino)-3-phenylisoxazoline (8%). Both the selective cleavage of the cyclopropyl group from the aromatic amine nitrogen and nature of the products derived from the cyclopropane ring support a mechanism involving the formation of an amine radical cation. This step is followed by rapid cyclopropyl ring opening to produce an iminium ion with a C-centered radical which either combines with NO or is oxidized.

Studies on the monoamine oxidase-B-catalyzed biotransformation of 4-azaaryl-1-methyl-1,2,3,6-tetrahydropyridine derivatives

The substrate properties of a series of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridinyl (MPTP) analogues in which the C-4 phenyl group has been replaced with various 4-azaaryl moieties have been examined in an effort to evaluate the contribution of electronic, polar, and steric parameters to the MAO-B-catalyzed oxidation of this type of cyclic tertiary allylamine to the corresponding dihydropyridinium metabolite. No significant correlation could be found with the calculated energy of the C-H bond undergoing cleavage. A general trend, however, was observed between the magnitude of the log P value with the magnitude of kcat/Km. The results indicate that the placement of a polar nitrogen atom in the space occupied by the phenyl group of MPTP leads to a dramatic decrease in substrate properties. Enhanced substrate properties, however, were observed when benzoazaarenes replaced the corresponding five-membered azaarenes. These results are consistent with our previously published molecular model of the active site of MAO-B.

Rat liver microsomal enzyme catalyzed oxidation of 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine

NADPH supplemented rat liver microsomal enzyme preparations catalyze the conversion of 1-cyclopropyl4-phenyl-1,2,3,6-tetrahydropyridine to the p-hydroxyphenyl (low yield), descyclopropyl (high yield) and 2,3-dihydropyridinium and, subsequently, pyridinium (intermediary yield) metabolites. When the methine proton of the cyclopropyl group was replaced with a deuteron, a normal deuterium isotope effect (1.4) was observed on the formation of the decyclopropylated metabolite and an inverse isotope effect (0.6) on the dihydropyridinium metabolite. A larger deuterium isotope effect (3.6) was observed on the ring alpha-carbon oxidation pathway with the 2,2,6,6-d4 analogue as substrate. These results and the observation that the ratios of the rates of these two alpha-carbon oxidation pathways are independent of initial substrate concentrations suggest that both pathways are catalyzed by the same active site of one form of P450. These transformations are discussed in terms of metabolic pathways that have been proposed for the cytochrome P450 catalyzed alpha-carbon oxidation of amines.