Methionine Does Not Reduce Cu(II)-b-Amyloid! —Rectification of the Roles of Methionine-35 and Reducing Agents in Metal-Centered Oxidation Chemistry of Cu(II)-β-Amyloid

Giordano F. Z. da Silva,^a# Vasiliky Lykourinou,^a Alexander Angerhofer,^b* and Li-June Ming^a*

^a Department of Chemistry and Institute for Biomolecular Science, University of South Florida, Tampa, Florida 33620-5250.

^b Department of Chemistry, University of Florida, Gainesville, Florida 32611

Abstract

The potential risk of metal-centered oxidative catalysis has been overlooked in the research of the copper complexes of the Alzheimer’s disease-related β-amyloid (Aβ) peptides. Cu²⁺ complexes of Aβ_1–40 and its 1–16 and 1–20 fragments have recently been shown to exhibit significant metal-centered oxidative activities toward several catecholamine neurotransmitters with and without H₂O₂ around neutral pH [G.F.Z da Silva, L.-J. Ming, Angew. Chem. Int. Ed. 46 (2007) 3337]. The results further support the metallo-Aβ-associated oxidative stress theory often considered to be connected to the neuropathology of the disease. The metal-centered oxidative catalysis of CuAβ_1-16/20 challenges the long-standing proposed redox role of Met35 in Aβ because Aβ_1–16/20 do not contain a Met. External Met has been determined by kinetic, optical, and electron paramagnetic resonance methods to bind directly to the Cu²⁺ center of CuAβ_1–40 and CuAβ_1–20 with K_d = 2.8 mM and 11.3 mM, respectively, which reflects less accessibility of the metal center in the full-length CuAβ_1–40. However, Met does not serve as a reducing agent for the Cu(II) which thus must amplify the observed oxidative catalysis of CuAβ_1–20 through a non-redox mechanism. Conversely, the CuAβ-catalyzed oxidation reaction of dopamine is inhibited by bio-available reducing agents such as ascorbate (competitive K_ic = 66 μM) and glutathione (non-competitive, K_inc = 53 μM). These data indicate that the oxidation chemistry of metallo-Ab is not initiated by Met35. The results yield further molecular and mechanistic insights into the roles of metallo-Aβ in this disease.

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