New evidence that the Alzheimer β-amyloid peptide does not spontaneously form free radicals: An ESR study using a series of spin-traps

The direct formation of free radicals from Aβ has been suggested to be a key neurotoxic mechanism in Alzheimer's disease (AD). We have explored the possibility of the spontaneous formation of peptide-derived free radicals during the incubation of Aβ 1-40 by ESR spectroscopy using N-tert-butyl-α-phenylnitrone (PBN), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN), and 3,5-dibromo-4-nitrosobenzenesulfonic acid sodium salt (DBNBS) as spin traps. Employing PBN, we observed spectra during the incubation of β-amyloid peptide, at 37°C, which included adducts of 2-methyl-2-nitrosopropane (MNP), despite rigorous purification of the PBN before incubation. The formation of some of these adducts was found to be enhanced by ambient laboratory light. Our experiments have led us to propose a hypothesis that PBN undergoes hydrolysis and decomposition in the presence of oxidants, which explains the origin of all of the PBN and MNP adducts observed (even when the PBN is highly purified). Hydrogen peroxide, formed during incubation, could play a major role as an oxidant in these experiments. Of the other three spin traps, only DMPO gave (very weak) spectra, but these could be assigned to its hydroxyl radical adduct, formed as an artifact by the nucleophilic addition of water to DMPO, catalyzed by trace levels of iron ions. Thus, while spectra are observed during our experiments, none of them can be assigned to adducts of radicals derived from the peptide and, therefore, our data do not support the suggestion that radicals are spontaneously formed from β-amyloid peptide.