Monoamine oxidase (MAO) plays an essential role in the regulation of various neurotransmitter and xenobiotic amines. Inhibitors of MAO have been employed in the treatment of depression and as adjuncts in Parkinson's disease therapy. X-Band and Q-band electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopic techniques are employed to characterize a signal assigned as a stable red anionic semiquinone radical in the resting state of MAO B. It is shown that the radical signal is not affected during substrate (either benzylamine or phenylethylamine) turnover, by anaerobic incubation with substrate, or by covalent modification of the active site flavin cofactor in the catalytically active dimer. Upon denaturation, however, the semiquinone absorbances and EPR signals are lost. Photoreduction of the native enzyme in the presence of ethylenediaminetetraacetate generates an EPR signal that is not the same as that obtained in the resting state and shows different proton ENDOR signals. These results suggest that the two flavin prosthetic groups that exist in catalytically active monoamine oxidase B are physically distinct.

To test whether addition of electron-withdrawing substituents to good substrates of monoamine oxidase (MAO) will transform them into inactivators, a series of fluorine-substituted benzylamines and a series of substituted 2-phenylethylamines were synthesized and tested as substrates and inactivators of monoamine oxidase B. All of the compounds were substrates. None of the benzylamines was an inactivator, but several of the phenethylamines were. These results suggest that either stronger electron withdrawing character or additional stabilizing factors are needed to stabilize the hypothesized enzyme adduct.