Ternary Complexes of Iron, Amyloid-β, and Nitrilotriacetic Acid: Binding Affinities, Redox Properties, and Relevance to Iron-Induced Oxidative Stress in Alzheimer's Disease

The interaction of amyloid-beta (A beta) and redox-active metals, two important biomarkers present in the. senile plaques of Alzheimer's disease (AD) brain, has been suggested to enhance the A beta aggregation or facilitate the generation of reactive oxygen species (ROS). This study investigates the nature of the interaction between the metal-binding domain of A beta, viz., A beta(1-16), and the Fe(III) or Fe(II) complex with nitrilotriacetic acid (NTA). Using electrospray ionization mass spectrometry (ESI-MS), the formation of a ternary complex of A beta(1-16), Fe(III), and NTA with a stoichiometry of 1:1:1 was identified. MS also revealed that the NTA moiety can be detached via collision-induced dissociation. The cumulative dissociation constants of both A beta-Fe(III)-NTA and A beta-Fe(II)-NTA complexes were deduced to be 6.3 x 10(-21) and 5.0 x 10(-12) M-2, respectively, via measurement of the fluorescence quenching of the sole tyrosine residue on A beta upon formation of the complex. The redox properties of these two complexes were investigated by cyclic voltammetry. The redox potential of the A beta-Fe(III)-NTA complex was found to be 0.03 V versus Ag/AgCl, which is negatively shifted by 0.54 V when compared to the redox potential of free Fe(III)/Fe(II). Despite such a large potential modulation, the redox potential of the A beta-Fe(III)-NTA complex is still sufficiently high for a range of redox reactions with cellular species to occur. The A beta-Fe(II)-NTA complex electrogenerated from the A beta-Fe(III)-NTA complex was also found to catalyze the reduction of oxygen to produce H2O2, These findings provide significant insight into the role of iron and A beta in the development of AD. The binding of iron by A beta modulates the redox potential to it level at which its redox cycling occurs, In the presence of a biological reductant (antioxidant), redox cycling of iron could disrupt the redox balance within the cellular Milieu. As a consequence, no( only is ROS continuously produced, but oxygen and biological reductants can also be depleted. A cascade of biological processes call therefore be affected. In addition, the strong binding affinity of A beta toward Fe(III) and Fe(II) indicates A beta could compete for iron against other iron-containing proteins. In particular, its strong affinity for Fe(II), which is 8 orders of magnitude stronger than that of transferrin, would greatly interfere with iron homeostasis,