Amyloid β−Cu²⁺ Complexes in both Monomeric and Fibrillar Forms Do Not Generate H₂O₂ Catalytically but Quench Hydroxyl Radicals

Oxidative stress plays a key role in Alzheimer’s disease (AD). In addition, the abnormally high Cu²⁺ ion concentrations present in senile plaques has provoked a substantial interest in the relationship between the amyloid β peptide (Aβ) found within plaques and redox-active copper ions. There have been a number of studies monitoring reactive oxygen species (ROS) generation by copper and ascorbate that suggest that Aβ acts as a prooxidant producing H₂O₂. However, others have indicated Aβ acts as an antioxidant, but to date most cell-free studies directly monitoring ROS have not supported this hypothesis. We therefore chose to look again at ROS generation by both monomeric and fibrillar forms of Aβ under aerobic conditions in the presence of Cu²⁺ with/without the biological reductant ascorbate in a cell-free system. We used a variety of fluorescence and absorption based assays to monitor the production of ROS, as well as Cu²⁺ reduction. In contrast to previous studies, we show here that Aβ does not generate any more ROS than controls of Cu²⁺ and ascorbate. Aβ does not silence the redox activity of Cu^2+/+ via chelation, but rather hydroxyl radicals produced as a result of Fenton−Haber Weiss reactions of ascorbate and Cu²⁺ rapidly react with Aβ; thus the potentially harmful radicals are quenched. In support of this, chemical modification of the Aβ peptide was examined using ¹H NMR, and specific oxidation sites within the peptide were identified at the histidine and methionine residues. Our studies add significant weight to a modified amyloid cascade hypothesis in which sporadic AD is the result of Aβ being upregulated as a response to oxidative stress. However, our results do not preclude the possibility that Aβ in an oligomeric form may concentrate the redox-active copper at neuronal membranes and so cause lipid peroxidation.