Multiple electrophoretic variants of Cu, Zn superoxide dismutase as expression of the enzyme aging. Effects of H2O2, ascorbate and metal ions

Multiple electrophoretic bands, with R_F identical to the natural molecular variants, are produced by treatment of purified Cu, Zn Superoxide dismutase with either H₂O₂ or ascorbate plus Fe(III) EDTA. The ascorbate reaction is also due to H₂O₂ since it is inhibited by catalase. However while H₂O₂ inactivates the enzyme, the electromorphs produced by ascorbate-Fe(III) EDTA have only slightly less activity than the native enzyme and this property parallels the natural situation. It is concluded that oxidative aging can be responsible for the multiple molecular variants of the natural enzyme, under conditions where the oxidant attack is preferentially directed to amino acid side chains outside the active site. Such conditions may occur when a metal ion coordinated to the protein surface undergoes a redox cycle with biological reductants, like ascorbate.