Amyloid β peptides do not form peptide-derived free radicals spontaneously, but can enhance metal-catalyzed oxidation of hydroxylamines to nitroxides

Amyloid beta (A beta) peptides play an important, role in the pathogenesis of Alzheimer's disease. Free radical generation by A beta peptides was suggested to be a key mechanism of their neurotoxicity. Reports that neurotoxic free radicals derived from A beta-(1-40) and A beta-(25-35) peptides react with the spin trap N-tert-butyl-alpha-phenylnitrone (PBN) to form a PBN/*A beta peptide radical adduct with a specific triplet ESR. signal assert that the peptide itself was the source of free radicals. We now report that three A beta peptides, A beta-(1-40), A beta-(25-35), and A beta-(40-1), do not yield radical adducts with PEN from the Oklahoma Medical Research Foundation (OMRF). Incontrast to OMRF PEN, incubation of Sigma PEN in phosphate buffer without A beta peptides produced a three-line ESR spectrum, It was shown that this nitroxide is di-tert-butylnitroxide and is formed in the Sigma PEN solution as a result of transition metal-catalyzed auto-oxidation of the respective hydroxylamine present as an impurity in the Sigma PEN. Under some conditions, incubation of PEN from Sigma with A beta-(1-40) or A beta-(25-35) can stimulate the formation of di-tert-butylnitroxide. It was shown that A beta peptides enhanced oxidation of cyclic hydroxylamine 1-hydroxy-4-oxo-2,2,6,6-tetramethylpiperidine (TEMPONE-H), which was strongly inhibited by the treatment of phosphate buffer with Chelex-100. It was shown that ferric and cupric ions are effective oxidants of TEMPONE-H. The data obtained allow us to conclude that under some conditions toxic A beta peptides A beta-(1-40) and A beta-(25-35) enhance metal-catalyzed oxidation of hydroxylamine derivatives, but do not spontaneously form peptide-derived free radicals.