One-electron oxidation of β-amyloid peptide: sequence modulation of reactivity

Amyloid β peptide (Aβ) is a 39 to 43 amino-acid-long peptide implicated in Alzheimerʹs disease. One of its mechanisms of toxicity is related to its redox properties. Therefore we studied its one electron oxidation using azide free radicals produced in γ and pulse radiolysis, and compared the results with those obtained with the reverse sequence Aβ(40–1). HPLC analysis combined with absorption, fluorescence, Raman spectroscopy, and MALDI-TOF MS were used for product identification. Met35 was shown to be the target in Aβ(1–40); oxidation leads to a major compound that is Aβ with methionine sulfoxide. Similarly, oxidation of fragment Aβ(29–40) also leads to methionine sulfoxide. For Aβ(40–1), Met35 is not reactive and Tyr10 is the target of azide radicals. The major products are peptide dimer linked by dityrosine and trimer. The lowering of the one-electron reduction potential of the MetS+/Met couple, which was proposed, is in agreement with our findings. To our knowledge, this is the first time that such a drastic effect of the primary sequence is observed in a small peptide. In addition, it is also the first experimental demonstration of the sensitivity of the one-electron reduction potential of methionine on neighboring groups.