Oxidation of nitroxyl anion to nitric oxide by copper ions

Abstract

This study made use of a nitric oxide‐sensitive electrode to examine possible means of generating nitric oxide from nitroxyl anion (NO⁻) released upon the decomposition of Angeli's salt. Our results show that copper ions (from CuSO₄) catalyze the rapid and efficient oxidation of nitroxyl to nitric oxide. Indeed, the concentrations of copper required to do so (0.1–100 μM) are roughly 100‐times lower than those required to generate equivalent amounts of nitric oxide from S‐nitroso‐N‐acetyl‐D,L‐penicillamine (SNAP). Experiments with ascorbate (1 mM), which reduces Cu²⁺ ions to Cu⁺, and with the Cu²⁺ chelators, EDTA and cuprizone, and the Cu⁺ chelator, neocuproine, each at 1 mM, suggest that the oxidation is catalyzed by copper ions in both valency states. Some compounds containing other transition metals, i.e. methaemoglobin, ferricytochrome c and Mn(III)TMPyP, were much less efficient than CuSO₄ in catalyzing the formation of nitric oxide from nitroxyl, while FeSO₄, FeCl₃, MnCl₂, and ZnSO₄ were inactive. Of the copper containing enzymes examined, Cu‐Zn superoxide dismutase and ceruloplasmin were weak generators of nitric oxide from nitroxyl, even at concentrations (2500 and 30 u ml⁻¹, respectively) vastly greater than are present endogenously. Two others, ascorbate oxidase (10 u ml⁻¹) and tyrosinase (250 u ml⁻¹) were inactive. Our findings suggest that a copper‐containing enzyme may be responsible for the rapid oxidation of nitroxyl to nitric oxide by cells, but the identity of such an enzyme remains elusive. British Journal of Pharmacology (2000) 131, 356–362; doi:10.1038/sj.bjp.0703550