Revised mechanism for inactivation of mitochondrial monoamine oxidase by N-cyclopropylbenzylamine

Abstract

A mechanism previously proposed for inactivation of monoamine oxidase (MAO) by N-cyclopropylbenzylamine (N-CBA) [Silverman, R. B., & Hoffman, S. J. (1980) J. Am. Chem. Soc. 102, 884-886] is revised. Inactivation of MAO by N-[1-3H]CBA results in incorporation of about 3 equiv of tritium into the enzyme and release of [3H]acrolein. Treatment of inactivated enzyme with benzylamine, a reactivator for N-CBA-inactivated MAO, releases only 1 equiv of tritium as [3H]acrolein concomitant with reactivation of the enzyme. Even after MAO is inactivated by N-[1-3H]CBA, the reaction continues. At pH 7.2, a linear release of [3H]acrolein is observed for 70 h, which produces 55 equiv of [3H]acrolein while 2.3 equiv of tritium is incorporated into the enzyme. At pH 9, only 3.5 equiv of [3H]acrolein is detected in solution after 96 h, but 40 equiv of tritium is incorporated into the enzyme, presumably as a result of greater ionization of protein nucleophiles at the higher pH. N-[1-3H]Cyclopropyl-alpha-methylbenzylamine (N-C alpha MBA) produces the same adduct as N-CBA but gives only 1-1.35 equiv of tritium bound after inactivation of the enzyme. Denaturation of labeled enzyme results in reoxidation of the flavin without release of tritium, indicating attachment is not to the flavin but rather to an amino acid residue. Enzyme inactivated with N-[1-3H]C alpha MBA is reactivated by benzylamine with the release of 1 equiv of [3H]acrolein, which must have come from an adduct attached to an active site amino acid residue.(ABSTRACT TRUNCATED AT 250 WORDS)

A Comprehensive Listing of Bioactivation Pathways of Organic Functional Groups

“…The presence of a cyclopropylamine moiety within a potential drug candidate should be viewed with some caution since this group is a well-known mechanism-based inactivator of cytochrome P450 [124,125]. Cyclopropylamines have also been implicated in the suicide inactivation of other human enzymes, such as the inactivation of MAO by tranylcypromine [126] and benzylcyclopropylamine [127], and alcohol dehydrogenase (ADH) by the mushroom toxin coprine [128].…”

“…For enzymes which are unequivocally known to oxidize amines by a single electron transfer (SET) mechanism and be irreversibly inhibited by cyclopropylamines, such as MAO-B, a cyclopropyl ring fragmented distonic cation radical intermediate 100 or resulting α, β-unsaturatedcarbonyl metabolite 101 are the reactive enzyme modifying species [126,127]. However, in the case of cytochrome P450, for which the mechanism of cyclopropylamine oxidation has no definitive consensus, the suicide intermediate may be generated either by SET or by direct hydroxylation (HAT) at the α-carbon of the cyclopropyl group.…”

Mechanism-Based Inactivation of Cytochrome P450 Enzymes: Chemical Mechanisms, Structure-Activity Relationships and Relationship to Clinical Drug-Drug Interactions and Idiosyncratic Adverse Drug Reactions

“…Cyclopropylamines have also been implicated in the irreversible inactivation of other drug metabolizing enzymes, as exemplified by the inactivation of monoamine oxidase (MAO) by the anti-depressant tranylcypromine [145,146], and alcohol dehydrogenase by the mushroom toxin coprine [147]. (21)) is thought to involve SET oxidation on the amine nitrogen to the cation radical intermediate, followed by spontaneous cyclopropane ring opening to a ring fragmented distonic cation radical intermediate or an , -unsaturatedcarbonyl metabolite, which then covalently bind to the enzyme [148][149][150]. (19).…”

Mass spectrometric studies on 4‐aryl‐1‐cyclopropyl‐1,2‐dihydropyridinyl derivatives: an examination of a novel fragmentation pathway

“…One mechanism proposed to account for this 2‐electron α‐carbon oxidation proceeds via an initial single electron transfer (SET) step to form the corresponding aminyl radical cation 1 •+ . Ring α‐carbon deprotonation of 1 •+ yields the neutral radical 2 • that undergoes further oxidation to the 2,3‐dihydropyridinium product 3H + 5, 6. An alternative proposal assumes an initial hydrogen atom transfer (HAT) step to yield 2 • directly 7–9.…”