Hemin, copper and amyloid-β: A medley involved in Alzheimer's disease. An interaction that fine regulates the reactivity

Metal ions have been shown to play a critical role in amyloid-β (Aβ) neurotoxicity and plaque formation which are key hallmarks of Alzheimer's disease. Amyloid-β peptides can bind both copper and hemin and this interaction modulates the redox chemistry of these metals. The characterization of the binding of hemin through UV-Vis spectroscopic titration with Aβ(4-16) shows a significantly higher affinity than that with Aβ(1-16). Also, the characterization of the hemin-catalyzed oxidation through different assays (peroxidase-like activity, ascorbate oxidation, HPLC-MS analysis of peptide oxidation) displays a greater reactivity in the presence of Aβ(4-16) when compared to that of Aβ(1-16). Since the Aβ(4-16) peptide sequence contains the typical amino-terminal copper and nickel binding motif (ATCUN), this leads to investigate the potential formation of ternary hemin/copper/Aβ(4-16) adducts. The evaluation of K₁ and K₂ (constants that regulate the formation of five-coordinated high-spin complex and of six-coordinated low-spin complex, respectively) for mixed systems indicates that the presence of copper stabilizes the 1:1 hemin-Aβ(4-16) species, partially hindering the formation of the low-spin complex. On the other hand, the formation of the ternary hemin/copper/Aβ(4-16) complex gives rise to a less efficient catalyst, resulting in a reduction of the overall oxidative reactivity. These results suggest that the reactivity of metal ions is finely modulated by the formation of complexes with amyloid peptides and this property is also regulated differently by the various in vivo relevant isoforms.