Superoxide dismutase-like activities of copper(II) complexes tested in serum

The superoxide dismutase-like activities of a series of coordination complexes of copper were evaluated and compared to the activities of bovine erythrocyte superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) in serum using the nitroblue tetrazolium chloride (NBT)-reduction assay and electron paramagnetic resonance (EPR) spectroscopy. A 40% inhibition was observed for the initial rate of the NBT reduction by superoxide dismutase in serum, but more than 40% inhibition was achieved with CuSO4, Cu(II)-dimethylglyoxime, Cu(II)-3,8-dimethyl-4,7-diazadeca-3,7-dienediamide, Cu2[N,N'-(2-(O-hydroxy-benzhydrylidene)amino)ethyl]2-1,2-ethane dia mine), Cu(II)-(diisopropylsalicylate)2, Cu(II)-(p-bromo-benzoate)2, Cu(II)-(nicotinate)2 and Cu(II)-(1,2-diamino-2-methylpropane)2. The electron paramagnetic resonance technique of spin trapping was used to detect the formation of superoxide (O2-.) and other free radicals in the xanthine-xanthine oxidase system under a variety of conditions. Addition of the spin trapping agent 5,5-dimethylpyrroline 1-oxide (DMPO) to the xanthine-xanthine oxidase system in fetal bovine serum produced the O2-.-spin adduct of DMPO (herein referred to as superoxide spin adduct, DMPO-OOH) as the well known short-lived nitroxyl whose characteristic EPR spectrum was recorded before its rapid decay to undetectable levels. The hydroxyl radical (HO.) adduct of the spin trap DMPO (herein referred to as DMPO-OH) was detected to a very small extent. When CuSO4, or the test complexes of copper, were added to the xanthine-xanthine oxidase system in serum containing the spin trap, the yield of DMPO-OOH was negligible. In addition to their superoxide dismutase-like activity, CuSO4 and the copper complexes also behaved as Fenton-type catalysts as seen by the accumulation of varying amounts of the hydroxyl spin adduct DMPO-OH. Both the Fenton-type catalysis and the superoxide dismutase-like action of these compounds were lost when a chelator such as EDTA was included in the xanthine-xanthine oxidase incubation mixture. Addition of superoxide dismutase instead of the copper compounds to this enzyme system abolished the formation of superoxide adduct DMPO-OOH, and no hydroxyl adduct DMPO-OH was detected. This effect of superoxide dismutase remained unaltered by EDTA.