Monomerized Cu, Zn-superoxide dismutase induces oxidative stress through aberrant Cu binding

Mutations in the Cu,Zn-superoxide dismutase (SOD1) gene cause familial amyotrophic lateral sclerosis (FALS). Lowering intracellular Cu improves the FALS-like phenotype of mutant SOD1 mice. Using immobilized Cu-affinity chromatography, we have previously shown that mutant SOD1 is expressed as two affinity fractions, one with high affinity for Cu (SOD1(HAC)) and one with low affinity (SOD1(LAC)), whereas wild-type SOD1 is expressed only as SOD1(LAC). Here we further characterize SOD1(HAC) to ascertain the toxicity of mutant SOD1 species. We found that SOD1(HAC) was modified at cysteine residues (Cys) and could be generated from wild-type SOD1 by oxidation of Cys. SOD1(HAC) mainly consisted of monomer, whereas SOD1(LAC) was a dimer. Mutant SOD1s possessed ectopic thiol oxidase activity that was exaggerated by loading it with adventitial Cu, but this activity was minimal in wild-type SOD1. Wild-type SOD1 could be induced to develop the activity by oxidation of Cys. Conversely, mutant SOD1 decreased the activity by being forced away from its monomeric state with a cross-linker. A significant decrease in free thiol concentration was observed in Neuro2a cells transfected with mutant SOD1s when they were treated with Cu. SOD1(HAC) may be pathogenic in FALS by being a monomeric species that gains a redox activity by aberrantly coordinating Cu(2+).