A substrate-cofactor free radical intermediate in the reaction mechanism of copper amine oxidase

Reduction of copper amine oxidase with substrate led to the appearance of a free radical which can be detected in anaerobiosis by ESR and optical spectroscopy. The origin of this radical was examined through studies of the semiquinones of 6-hydroxydopamine, an analogue of the recently identified cofactor 6-hydroxydopa. The ESR spectrum of the 6-hydroxydopamine radical was too narrow to account for the enzyme radical signal; however, after spontaneous reaction with primary amines the hyperfine splittings and spectral width obtained by modulation broadening became very similar to those observed for the oxidase radical species. This effect was ascribed to covalent binding of a nitrogen atom directly to the aromatic ring structure, suggesting that the amine oxidase radical is an amino-6-hydroxydopa semiquinone. Identical ESR spectra were obtained using the amines putrescine, cadaverine, p-[(dimethylamino)methyl]benzylamine, and ethylenediamine; these oxidase substrates gave identical enzyme radical spectra as well. The interaction between cofactor and substrate was proved unambiguously by the technique of isotopic labeling: addition of [15N2]ethylenediamine instead of the normal 14N-labeled compound changed the ESR spectra of both the enzyme radical and its 6-hydroxydopamine counterpart. The results were confirmed by optical spectroscopy measurements; 6-hydroxydopamine and oxidized 6-hydroxydopamine gave spectra identical to those of reduced and oxidized amine oxidase, respectively. The 6-hydroxydopamine radical showed a sharp peak at 440 nm; upon addition of amines the maximum shifted to 460 nm, as found for the enzyme. It is proposed that copper amine oxidase represents the first example of a mixed substrate-cofactor radical within the family of tyrosine radical enzymes.