The denatured Cu, Zn superoxide dismutase (SOD1) has the pro-oxidant activity that is suggested to be related with the pathogenesis of amyotrophic lateral sclerosis (ALS). We showed from the changes in the coordinated metal ions that the Cu ion in the Cubinding site is the catalytic site of the pro-oxidant activity, and a redox-active metal ion in the Zn-binding site has the auxiliary function to enhance the pro-oxidant activity. The auxiliary function is suggested to arise from the intramolecular electron transfer between the coordinated metal ions in the denatured SOD1. The oxidation/reduction cycle of Cu in the Cu-binding site is assisted with changing the oxidation state of a metal ion in the Zn-binding site. The magnitude of the toxicity of the denatured SOD1 is discussed based on the ability of the auxiliary function.

Cu, Zn superoxide dismutase (SOD1) is a representative antioxidant enzyme that catalyzes dismutation of reactive oxygen species in cells. However, (E,E)-SOD1 mutants in which both copper and zinc ions were deleted exhibit pro-oxidant activity, contrary to their antioxidant nature, at physiological temperatures, following denaturation and subsequent recombination of Cu2+. This oxidative property is likely related to the pathogenesis of amyotrophic lateral sclerosis (ALS); however, the mechanism by which Cu2+ re-binds to the denatured (E,E)-SOD1 has not been elucidated, since the concentration of free copper ions in cells is almost zero. In this study, we prepared the (Cu,E) form in which only a zinc ion was deleted using ALS-linked mutant H43R (His43→Arg) and found that (Cu,E)-H43R showed an increase in the pro-oxidant activity even at physiological temperature. The increase in the pro-oxidant activity of (Cu,E)-H43R was also observed in solution mimicking intracellular environment and at high temperature. These results suggest that the zinc-deficient (Cu,E) form can contribute to oxidative stress in cells, and that the formation of (E,E)-SOD1 together with the subsequent Cu2+ rebinding is not necessary for the acquisition of the pro-oxidant activity.