Antioxidant role of amyloid β protein in cell-free and biological systems: implication for the pathogenesis of Alzheimer disease

In contrast to many studies showing the pro-oxidative nature of amyloid peptide, this work shows that aggregated A beta 42 peptide in varying concentrations (2-20 mu M) in cell-free systems inhibits the formation of hydroxyl radicals and H2O2 from a mixture of iron (20 mu M FeSO4) and ascorbate (2 mM) as measured by benzoate hydroxylation assay and coumarin carboxylic acid assay. Aggregated A beta 42 in similar concentrations further prevents protein and lipid oxidation in isolated rat brain mitochondria incubated alone or with FeSO4 and ascorbate. Moreover, mitochondria exposed to FeSO4 and ascorbate show enhanced formation of reactive oxygen species and this phenomenon is also abolished by aggregated A beta 42. It is suggested that the antioxidant property of A beta 42 in various systems is mediated by metal chelation and it is nearly as potent as a typical metal chelator, such as diethylenetriaminepentaacetic acid, in preventing oxidative damage. However, aggregated A beta 42 causes mitochondrial functional impairment in the form of membrane depolarization and a loss of phosphorylation capacity without involving reactive oxygen species in the process. Thus, the present results suggest that the amyloid peptide exhibits a protective antioxidant role in biological systems, but also has toxic actions independent of oxidative stress. (C) 2012 Elsevier Inc. All rights reserved.