SOD1 gains pro-oxidant activity upon aberrant oligomerization: change in enzymatic activity by intramolecular disulfide bond cleavage.

Copper-zinc superoxide dismutase (SOD1) has been proposed as one of the causative proteins of amyotrophic lateral sclerosis (ALS). The accumulation of non-native conformers, oligomers, and aggregates of SOD1 in motor neurons is considered responsible for this disease. However, it remains unclear which specific feature of these species induces the onset of ALS. In this study, we showed that disulfide-linked oligomers of denatured SOD1 exhibit pro-oxidant activity. Substituting all the cysteine residues in the free thiol state with serine resulted in the loss of both the propensity to oligomerize and the increase in pro-oxidant activity after denaturation. In contrast, these cysteine mutants oligomerized and acquired the pro-oxidant activity after denaturation in the presence of a reductant that cleaves the intramolecular disulfide bond. These results indicate that one of the toxicities of SOD1 oligomers is the pro-oxidant activity induced by scrambling of the disulfide bonds. Small oligomers such as dimers and trimers exhibit stronger pro-oxidant activity than large oligomers and aggregates, consistent with the trend of the cytotoxicity of oligomers and aggregates reported in previous studies. We propose that the cleavage of the intramolecular disulfide bond accompanied by the oligomerization reduces the substrate specificity of SOD1, leading to the non-native enzymatic activity.

Enhancement of Oxidative Reaction by the Intramolecular Electron Transfer between the Coordinated Redox-Active Metal Ions in SOD1.

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 Cu-binding 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.