Interactions of silver nanoparticles with antioxidant enzymes

Oxidative stress is accepted as a key mechanism of silver nanoparticles (AgNPs) toxicity in living organisms. Therefore, mechanistic studies related to the impact of AgNPs on the structure and function of antioxidant enzymes at the molecular level are essential for a comprehensive evaluation of their toxicity. By using a combination of spectroscopic, imaging and fractionation techniques we explored the interactions between citrate-coated AgNPs (20 nm) and two key antioxidant enzymes: catalase (CAT) and superoxide dismutase (SOD). Both enzymes interacted with AgNPs by forming surface complexes. Only CAT was able to promote AgNPs dissolution, without impact of the released Ag ions on its hemic cofactor. Instead, our results suggest that the formation of the AgNPs-CAT complex induced conformation changes in the CAT, which resulted in an impairment of its enzymatic activity together with Ag(I) adsorption. By contrast, AgNPs-SOD complex formation has only marginal influence on the protein conformation, and has no impact on its metallic cofactors and thus its enzymatic activity. Overall, the results showed that the changes in the proteins conformation and the dissolution of AgNPs depended on the protein structure and could result in different degrees of enzymatic activity modulation.