Hydrogen peroxide-mediated degradation of protein: Different oxidation modes of copper- and iron-dependent hydroxyl radicals on the degradation of albumin

Cupric ions (Cu2+) added to hydrogen peroxide (H2O2) were found to generate hydroxyl radicals (HO.) capable of benzoate hydroxylation. Although ferrous (Fe2+) and ferric (Fe3+) ions, when added to H2O2, resulted in very little production of HO., the addition of EDTA to the reaction mixture markedly increased their catalytic activity. In the absence of albumin, catalase (a H2O2 scavenger) and mannitol (an HO. radical scavenger) effectively inhibited the formation of HO. in H2O2/Cu2+ and H2O2/Fe2+/EDTA oxidation systems. On analysis using SDS-polyacrylamide gel electrophoresis, catalase was shown to prevent the degradation of albumin by both oxidation systems, whereas mannitol was an effective scavenger of the H2O2/Fe2+/EDTA oxidation system but not of the H2O2/Cu2+ oxidation system. Furthermore, the effect of alteration of benzoate hydroxylation and H2O2 consumption on the H2O2/Cu2+ and H2O2/Fe2+/EDTA oxidation systems resulted in opposite behavior that was dependent upon the presence or absence of albumin. These observations suggest that copper ions bind to albumin and induce site-specific degradation by HO. generated at the copper-binding site, whereas the Fe2+/EDTA-catalyzed oxidation system induces non-specific degradation of albumin by HO. generated by the Fenton reaction between (HO2)-O-2 and free Fe2+/EDTA in solution.