CYANIDE AS A COPPER-DIRECTED INHIBITOR OF AMINE OXIDASES - IMPLICATIONS FOR THE MECHANISM OF AMINE OXIDATION

The interactions of five copper-containing amine oxidases with substra tes and substrate analogues in the presence of the copper ligands cyan ide, azide, chloride, and 1,10-phenanthroline have been investigated. While cyanide inhibits, to varying degrees, the reaction of phenylhydr azine with porcine kidney amine oxidase (PKAO), porcine plasma amine o xidase (PPAO), bovine plasma amine oxidase (BPAO), and pea seedling am ine oxidase (PSAO), it enhances the reaction of Arthrobacter P1 amine oxidase (APAO) with this substrate analogue. This indicates that cyani de exerts an indirect effect on topa quinone (TPQ) reactivity via coor dination to Cu(II) rather than through cyanohydrin formation at the TP Q organic cofactor, Moreover, cyanide binding to the mechanistically r elevant 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 trapp ing the Cu(I)-TPQ(.) form of amine oxidases, and thus preventing the r eaction of O-2 with Cu(I), In contrast, ligands such as azide. chlorid e, and 1,10-phenanthroline, which preferentially bind to Cu(II). inhib it by stabilizing the aminoquinol Cu(II)-TPQ(red) redox state, which i s in equilibrium with Cu(I)-TPQ(.).