Cyanide as a copper-directed inhibitor of amine oxidases: implications for the mechanism of amine oxidation

Abstract

The interactions of five copper-containing amine oxidases with substrates and substrate analogues in the presence of the copper ligands cyanide, azide, chloride, and 1,10-phenanthroline have been investigated. While cyanide inhibits, to varying degrees, the reaction of phenylhydrazine with porcine kidney amine oxidase (PKAO), porcine plasma amine oxidase (PPAO), bovine plasma amine oxidase (BPAO), and pea seedling amine oxidase (PSAO), it enhances the reaction of Arthrobacter P1 amine oxidase (APAO) with this substrate analogue. This indicates that cyanide exerts an indirect effect on topa quinone (TPQ) reactivity via coordination to Cu(II) rather than through cyanohydrin formation at the TPQ organic cofactor. Moreover, cyanide binding to the mechanistically relevant TPQ• semiquinone form of substrate-reduced APAO and PSAO was not observable by EPR or resonance Raman spectroscopy. Hence, cyanide most likely inhibits enzyme reoxidation by binding to Cu(I) and trapping the Cu(I)-TPQ• form of amine oxidases, and thus preventing the reaction of O2 with Cu(I). In contrast, ligands such as azide, chloride, and 1,10-phenanthroline, which preferentially bind to Cu(II), inhibit by stabilizing the aminoquinol Cu(II)-TPQred redox state, which is in equilibrium with Cu(I)-TPQ•.