Loss of oxidation-reduction specificity in amyotrophic lateral sclerosis-associated CuZnSOD mutants

Both transgenic mouse and cell culture models of familial amyotrophic later al sclerosis (FALS) support a gain-of-function effect for the mutations in copper-zinc superoxide dismutase (CuZnSOD) associated with FALS, but the na ture of the function gained remains incompletely characterized. We previous ly reported an enhanced peroxidase activity for FALS-associated CuZnSOD mut ants. Because one of the targets of such activity is CuZnSOD itself, we exa mined peroxide-mediated inactivation of wild-type and mutant CuZnSODs, and found that the mutants are more readily inactivated. Inactivation of the mu tants was associated with fragmentation, which did not occur in the wild-ty pe enzyme under these conditions. Furthermore, the reduction of the FALS-as sociated mutants by ascorbate was enhanced markedly when compared to the wi ld-type enzyme. The visible spectra of the mutants showed a consistent blue shift of the peak at 680 nm in the wild-type enzyme, suggesting an alterat ion in copper-site geometry. These results extend previous studies demonstr ating enhanced peroxidase activity in the mutants, and suggest that the tox ic function that leads to motor neuron degeneration may result from a loss of specificity of the redox reactions catalyzed by CuZnSOD.