Redox reactions of copper complexes formed with different beta-amyloid peptides and their neuropathological [correction of neuropathalogical] relevance

Pharmacotherapeutic targets in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder which is characterized by an increasing impairment in normal memory and cognitive processes that significantly diminishes a person's daily functioning. Despite decades of research and advances in our understanding of disease aetiology and pathogenesis, there are still no effective disease-modifying drugs available for the treatment of AD. However, numerous compounds are currently undergoing pre-clinical and clinical evaluations. These candidate pharma-cotherapeutics are aimed at various aspects of the disease, such as the microtubule-associated τ-protein, the amyloid-β (Aβ) peptide and metal ion dyshomeostasis – all of which are involved in the development and progression of AD. We will review the way these pharmacological strategies target the biochemical and clinical features of the disease and the investigational drugs for each category.

Amyloid beta-Cu2+ complexes in both monomeric and fibrillar forms do not generate H2O2 catalytically but quench hydroxyl radicals

Oxidative stress plays a key role in Alzheimer's disease (AD). In addition, the abnormally high Cu(2+) ion concentrations present in senile plaques has provoked a substantial interest in the relationship between the amyloid beta peptide (Abeta) found within plaques and redox-active copper ions. There have been a number of studies monitoring reactive oxygen species (ROS) generation by copper and ascorbate that suggest that Abeta acts as a prooxidant producing H2O2. However, others have indicated Abeta acts as an antioxidant, but to date most cell-free studies directly monitoring ROS have not supported this hypothesis. We therefore chose to look again at ROS generation by both monomeric and fibrillar forms of Abeta under aerobic conditions in the presence of Cu(2+) with/without the biological reductant ascorbate in a cell-free system. We used a variety of fluorescence and absorption based assays to monitor the production of ROS, as well as Cu(2+) reduction. In contrast to previous studies, we show here that Abeta does not generate any more ROS than controls of Cu(2+) and ascorbate. Abeta does not silence the redox activity of Cu(2+/+) via chelation, but rather hydroxyl radicals produced as a result of Fenton-Haber Weiss reactions of ascorbate and Cu(2+) rapidly react with Abeta; thus the potentially harmful radicals are quenched. In support of this, chemical modification of the Abeta peptide was examined using (1)H NMR, and specific oxidation sites within the peptide were identified at the histidine and methionine residues. Our studies add significant weight to a modified amyloid cascade hypothesis in which sporadic AD is the result of Abeta being upregulated as a response to oxidative stress. However, our results do not preclude the possibility that Abeta in an oligomeric form may concentrate the redox-active copper at neuronal membranes and so cause lipid peroxidation.

Soluble amyloid beta1-28-copper(I)/copper(II)/Iron(II) complexes are potent antioxidants in cell-free systems

Amyloid beta (Abeta) is a central characteristic of Alzheimer's disease (AD). Currently, there is a long-standing dispute regarding the role of Abeta-metal ion (Zn, Cu, and Fe) complexes in AD pathogenesis. Here, we aim to decipher the connection between oxidative damage implicated in AD and Abeta-metal ion complexes. For this purpose we study, using ESR, the modulation of Cu/Fe-induced H 2O 2 decomposition by Abeta 1-28 (Abeta 28), a soluble model of Abeta 40/42. The addition of H 2O 2 to 0.6 nM-360 microM Abeta 28 solutions containing 100 microM Cu(II)/Cu(I)/Fe(II) at pH 6.6 results in a concentration-dependent sigmoidal decay of [*OH] with IC 50 values of 61, 59, and 84 microM, respectively. Furthermore, Abeta 28 reduces 90% of *OH production rate in the Cu(I)-H 2O 2 system in 5 min. Unlike soluble Abeta 28, Abeta 28-Cu aggregates exhibit poor antioxidant activity. The mode of antioxidant activity of soluble Abeta 28 is twofold. The primary (rapid) mechanism involves metal chelation, whereas the secondary (slow) mechanism involves (*)OH scavenging and oxidation of Cu(Fe)-coordinating ligands. On the basis of our findings, we propose that soluble Abeta may play a protective role in the early stages of AD, but not in healthy individuals, where Abeta's concentration is nanomolar. Yet, when Abeta-metal ion complexes undergo aggregation, they significantly lose their protective function and allow oxidative damage to occur.

The structure of the amyloid-beta peptide high-affinity copper II binding site in Alzheimer disease

Neurodegeneration observed in Alzheimer disease (AD) is believed to be related to the toxicity from reactive oxygen species (ROS) produced in the brain by the amyloid-beta (Abeta) protein bound primarily to copper ions. The evidence for an oxidative stress role of Abeta-Cu redox chemistry is still incomplete. Details of the copper binding site in Abeta may be critical to the etiology of AD. Here we present the structure determined by combining x-ray absorption spectroscopy (XAS) and density functional theory analysis of Abeta peptides complexed with Cu(2+) in solution under a range of buffer conditions. Phosphate-buffered saline buffer salt (NaCl) concentration does not affect the high-affinity copper binding mode but alters the second coordination sphere. The XAS spectra for truncated and full-length Abeta-Cu(2+) peptides are similar. The novel distorted six-coordinated (3N3O) geometry around copper in the Abeta-Cu(2+) complexes include three histidines: glutamic, or/and aspartic acid, and axial water. The structure of the high-affinity Cu(2+) binding site is consistent with the hypothesis that the redox activity of the metal ion bound to Abeta can lead to the formation of dityrosine-linked dimers found in AD.

Mechanistic studies of Cu(II) binding to amyloid-beta peptides and the fluorescence and redox behaviors of the resulting complexes

Due in large part to the lack of crystal structures of the amyloid-beta (Abeta) peptide and its complexes with Cu(II), Fe(II), and Zn(II), characterization of the metal-Abeta complex has been difficult. In this work, we investigated the complexation of Cu(II) by Abeta through tandem use of fluorescence and electron paramagnetic resonance (EPR) spectroscopies. EPR experiments indicate that Cu(II) bound to Abeta can be reduced to Cu(I) using sodium borohydride and that both Abeta-Cu(II) and Abeta-Cu(I) are chemically stable. Upon reduction of Cu(II) to Cu(I), the Abeta fluorescence, commonly reported to be quenched upon Abeta-Cu(II) complex formation, can be regenerated. The absence of the characteristic tyrosinate peak in the absorption spectra of Abeta-Cu(II) complexes provides evidence that the sole tyrosine residue in Abeta is not one of the four equatorial ligands bound to Cu(II), but remains close to the metal center, and its fluorescence is sensitive to the copper oxidation state and perturbations in the coordination sphere. Further analysis of the quenching and Cu(II) binding behaviors at different Cu(II) concentrations and in the presence of the competing ligand glycine offers evidence supporting the operation of two binding regimes which demonstrate different levels of fluorescence recovery upon addition of the reducing agent. We provide results that suggest the fluorescence quenching is likely caused by charge transfer processes. Thus, by using tyrosine to probe the coordination site, fluorescence spectroscopy provides valuable mechanistic insights into the oxidation state of copper ions bound to Abeta, the binding heterogeneity, and the influence of solution conditions on complex formation.

Substrate mediated reduction of copper-amyloid-beta complex in Alzheimer's disease

X-Ray absorption near-edge spectroscopy (XANES) has been used to probe the substrate mediated reduction of Cu(2+) in Abeta-Cu(2+) complexes by ascorbate and the neurotoxin 6-hydroxydopamine (6-OHDA), however dopamine and, in particular, cholesterol are incapable of reducing soluble monomeric Abeta-Cu(2+) complexes.

Binding of zinc(II) and copper(II) to the full-length Alzheimer's amyloid-beta peptide

There is evidence that binding of metal ions like Zn2+ and Cu2+ to amyloid beta-peptides (Abeta) may contribute to the pathogenesis of Alzheimer's disease. Cu2+ and Zn2+ form complexes with Abeta peptides in vitro; however, the published metal-binding affinities of Abeta vary in an enormously large range. We studied the interactions of Cu2+ and Zn2+ with monomeric Abeta(40) under different conditions using intrinsic Abeta fluorescence and metal-selective fluorescent dyes. We showed that Cu(2+) forms a stable and soluble 1 : 1 complex with Abeta(40), however, buffer compounds act as competitive copper-binding ligands and affect the apparent K(D). Buffer-independent conditional K(D) for Cu(II)-Abeta(40) complex at pH 7.4 is equal to 0.035 micromol/L. Interaction of Abeta(40) with Zn2+ is more complicated as partial aggregation of the peptide occurs during zinc titration experiment and in the same time period (within 30 min) the initial Zn-Abeta(40) complex (K(D) = 60 micromol/L) undergoes a transition to a more tight complex with K(D) approximately 2 micromol/L. Competition of Abeta(40) with ion-selective fluorescent dyes Phen Green and Zincon showed that the K(D) values determined from intrinsic fluorescence of Abeta correspond to the binding of the first Cu2+ and Zn2+ ions to the peptide with the highest affinity. Interaction of both Zn2+ and Cu2+ ions with Abeta peptides may occur in brain areas affected by Alzheimer's disease and Zn2+-induced transition in the peptide structure might contribute to amyloid plaque formation.

X-ray absorption and diffraction studies of the metal binding sites in amyloid beta-peptide

A major source of neurodegeneration observed in Alzheimer's disease is believed to be caused by the toxicity from reactive oxygen species produced in the brain mediated by the A beta protein and mainly copper species. An atomic model of an amyloid beta-peptide (A beta) Cu2+ complex or at least the structure of the metal binding site is of great interest. Accurate information about the Cu-binding site of A beta protein can facilitate simulation of redox chemistry using high level quantum mechanics. Complementary X-ray diffraction and X-ray absorption techniques can be employed to obtain such accurate information. This review provides a blend of X-ray diffraction results on amyloid structures and selected works on A beta Cu2+ binding based on spectroscopic measurements with emphasis on the X-ray absorption technique.