A Cu(I)-semiquinone state in substrate-reduced amine oxidases.

The role of copper in copper-containing amine oxidases has long been a source of debate and uncertainty. Numerous electron paramagnetic resonance (EPR) experiments, including rapid freeze-quench studies, have failed to detect changes in the copper oxidation state in the presence of substrate amines. One suggestion that copper reduction might occur, has never been confirmed. Copper amine oxidases contain another cofactor, recently identified as 6-hydroxydopa quinone (topa quinone), which is reduced by substrates. Copper has been implicated in the reoxidation of the substrate-reduced enzyme, but the failure to detect any copper redox change has led to proposals that Cu(II) acts as a Lewis acid, that it has an indirect role in catalysis, or that it serves a structural role. We present evidence for the generation of a Cu(I)-semiquinone state by substrate reduction of several amine oxidases under anaerobic conditions, and suggest that the Cu(I)-semiquinone may be the catalytic intermediate that reacts directly with oxygen.