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

The direct formation of free radicals from A beta has been suggested to be a key neurotoxic mechanism in Alzheimer's disease (AD). We have explored th e possibility of the spontaneous formation of peptide-derived free radicals during the incubation of A beta 1-40 by ESR spectroscopy using N-tert-buty l-alpha -phenylnitrone (PBN), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), alph a-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN), and 3,5-dibromo-4-nitroso benzenesulfonic acid sodium salt (DBNBS) as spin traps. Employing PEN. we o bserved spectra during the incubation of beta -amyloid peptide, at 37 degre esC, which included adducts of 2-methyl-2-nitrosopropane (MNP), despite rig orous purification of the PBN before incubation. The formation of some of t hese adducts: was found to be enhanced by ambient laboratory light, Our exp eriments 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 purif ied). Hydrogen peroxide, formed during incubation, could play a major role as an oxidant in these experiments. Of the other three spin traps, only DMP O gave (very weak) spectra, but these could be assigned to its hydroxyl rad ical adduct, formed as an artifact by the nucleophilic addition of water to DMPO, catalyzed by trace levels of iron ions. Thus, while spectra are obse rved during our experiments, none of them can be assigned to adducts of rad icals derived from the peptide and, therefore, our data do not support the suggestion that radicals are spontaneously formed from beta -amyloid peptid e. (C) 2001 Elsevier Science Inc.