Direct speciation of metals in copper-zinc superoxide dismutase isoforms on electrophoresis gels using X-ray absorption near edge structure

Abstract

X-Ray absorption near edge structure (XANES) can be used for the speciation analysis of inorganic elements bound to proteins, after separation using polyacrylamide gel electrophoresis. In this paper, we report new methodological developments for the speciation of oxidation states in copper–zinc superoxide dismutase (CuZnSOD) isoforms separated in non-denaturing conditions according to their isoelectric point. The nature of CuZnSOD isoforms is still elusive, although differences in their metallation state have been suggested. A non-denaturing protocol for isoelectrofocusing electrophoresis was applied to preserve the metallation state of the proteins. XANES has been performed to study the copper and zinc oxidation states. As expected, zinc is present in its Zn(II) oxidation state in all analysed isoforms. Copper is present in the Cu(II) oxidation state in the main acidic isoform, while it is found in both Cu(II) and Cu(I) states in the main basic isoform. These methodological developments enable the direct speciation analysis of metals in CuZnSOD without any protein purification step that could modify their oxidation state.