Cyanide Inhibition of Porcine Kidney Diamine Oxidase and Bovine Plasma Amine Oxidase: Evidence for Multiple Interaction Sites

Abstract

The interactions of cyanide and phenylhydrazine with porcine kidney diamine oxidase (PKDAO) and bovine plasma amine oxidase (BPAO) (EC 1.4.3.6) have been investigated. Cyanide displays mixed noncompetitive inhibition against amine substrates and also against O2. EPR spectroscopy shows that cyanide binds to an equatorial site on Cu(II) and can be displaced by chloride, which is not an inhibitor, without recovery of activity, indicating that Cu(II)-bound cyanide is not inhibitory. 14CN− studies have shown that one cyanide in PKDAO and two in BPAO are covalently and irreversibly bound per enzyme dimer at an unknown site, even under conditions where cyanide is not bound to Cu(II). These cyanides have no effect on activity or on binding of phenylhydrazine to the enzymes. Cyanide also binds reversibly to the organic cofactor in both enzymes, presumably as a cyanohydrin, leading to the observed mixed noncompetitive inhibition against substrate. In both enzymes, two phenylhydrazines react per enzyme dimer. The kinetics of phenylhydrazine titration are affected by cyanide, which indicates that phenylhydrazine and cyanide react at the same carbonyl group in the enzymes. The results suggest that inhibition of amine oxidases by cyanide is through a carbonyl reagent and a Cu(I) ligand rather than through a Cu(II) ligand.