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 beta peptide (A beta) 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 A beta acts as a prooxidant producing H2O2. However, others have indicated A beta 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 A beta 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 A beta does not generate any more ROS than controls of Cu2+ and ascorbate. A beta does not silence the redox activity of Cu2+/+ via chelation, but rather hydroxyl radicals produced as a result of Fenton-Haber Weiss reactions of ascorbate and Cu2+ rapidly react with A beta; thus the potentially harmful radicals are quenched. In support of this, chemical modification of the A beta peptide was examined using H-1 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 A beta being upregulated as a response to oxidative stress. However, our results do not preclude the possibility that A beta in an oligomeric form may concentrate the redox-active copper at neuronal membranes and so cause lipid peroxidation.