The inactivation of monoamine oxidase (MAO) by 1-phenylcyclopropylamine (1-PCPA, 1) has been proposed (Silverman, R. B.; Zieske, P. A. Biochemistry 1985, 24, 2128–2138) to proceed via a radical mechanism involving ring opening of an unstable cyclopropylaminyl radical (Scheme I). This intermediate has been shown to partition between attachment to the N-5 position of the covalently-bound flavin cofactor, which results in irreversible inactivation, and attachment to an active-site cysteine residue, which results in an unstable adduct that hydrolyzes to release acrylophenone and active enzyme over time. The stereochemistry and mechanism of the ring-opening step is investigated in this paper. The enantiomerically pure dideuterated 1-PCPA analogues 5 and 6 were prepared and used to inactivate MAO. Extractions of the acrylophenone metabolites released on decomposition of the reversible cysteine adducts that were formed during inactivation by 5 and by 6 were subjected to GCMS analysis. It was found that the inactivation by the R-isomer (5) produced 66% of [ββ-2H2]acrylophenone, 19% of [α-2H]acrylophenone, and 15% of unlabeled acrylophenone; inactivation by the S-isomer (6) resulted in formation of 4% of [β,β−2H2]acrylophenone, 6% of [α−2H]-acrylophenone, and 90% of unlabeled acrylophenone. These results are indicative of a stereoselective ring opening, preferentially forming β,β-dideutero adduct 8 following inactivation by 5 (Scheme II) and the α,α-dideuterio adduct 13 following inactivation by 6 (Scheme III). Stereoselective exchange of a single α deuterium on adduct 13 is proposed to account for the complete washout of deuterium in the product and the observed deuterium isotope effect of 3.1 on the reactivation of 6-inactivated MAO. The difference in the amount of cleavage of the two cyclopropyl bonds with each enantiomer may be due, at least in part, to a secondary deuterium isotope effect on the formation of the dideuterated carbon radical. This secondary deuterium isotope effect also provides a rationalization that favors a mechanism involving electron transfer to the aminium radical cation followed by cyclopropyl ring cleavage and then combination with an active-site cysteine radical. The results do not support a direct SH2 mechanism. The preference for cleavage of one of the cyclopropane bonds is believed to be the result of asymmetry at the active site which is set up for stereospecific removal of the pro-R proton from substrates; the pro-R C-H bond of substrates corresponds to the cyclopropane bond in 1-PCPA that is preferentially broken.
ASJC Scopus subject areas
- Colloid and Surface Chemistry