OBSERVATION OF A FLAVIN SEMIQUINONE IN THE RESTING STATE OF MONOAMINE-OXIDASE-B BY ELECTRON-PARAMAGNETIC-RESONANCE AND ELECTRON-NUCLEAR DOUBLE-RESONANCE SPECTROSCOPY

Monoamine oxidase (MAO) plays an essential role in the regulation of v arious neurotransmitter and xenobiotic amines, Inhibitors of MAO have been employed in the treatment of depression and as adjuncts in Parkin son's disease therapy, X-Band and Q-band electron paramagnetic resonan ce (EPR) and electron nuclear double resonance (ENDOR) spectroscopic t echniques are employed to characterize a signal assigned as a stable r ed anionic semiquinone radical in the resting state of MAO B. It is sh own that the radical signal is not affected during substrate (either b enzylamine or phenylethylamine) turnover, by anaerobic incubation with substrate, or by covalent modification of the active site flavin cofa ctor in the catalytically active dimer. Upon denaturation, however, th e semiquinone absorbances and EPR signals are lost. Photoreduction of the native enzyme in the presence of ethylenediaminetetraacetate gener ates an EPR signal that is not the same as that obtained in the restin g state and shows different proton ENDOR signals, These results sugges t that the two flavin prosthetic groups that exist in catalytically ac tive monoamine oxidase B are physically distinct.