Metal binding modes of Alzheimer's amyloid beta-peptide in insoluble aggregates and soluble complexes

Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimer's disease. Zn(II) induces the Abeta aggregation at acidic-to-neutral pH, while Cu(II) is an effective inducer only at mildly acidic pH. We have examined Zn(II) and Cu(II) binding modes of Abeta and their pH dependence by Raman spectroscopy. The Raman spectra clearly demonstrate that three histidine residues in the N-terminal hydrophilic region provide primary metal binding sites and the solubility of the metal-Abeta complex is correlated with the metal binding mode. Zn(II) binds to the N(tau) atom of the histidine imidazole ring and the peptide aggregates through intermolecular His(N(tau))-Zn(II)-His(N(tau)) bridges. The N(tau)-metal ligation also occurs in Cu(II)-induced Abeta aggregation at mildly acidic pH. At neutral pH, however, Cu(II) binds to N(pi), the other nitrogen of the histidine imidazole ring, and to deprotonated amide nitrogens of the peptide main chain. The chelation of Cu(II) by histidine and main-chain amide groups results in soluble Cu(II)-Abeta complexes. Under normal physiological conditions, Cu(II) is expected to protect Abeta against Zn(II)-induced aggregation by competing with Zn(II) for histidine residues of Abeta.     

Metal ion-dependent effects of clioquinol on the fibril growth of an amyloid {beta} peptide

Although metal ions such as Cu(2+), Zn(2+), and Fe(3+) are implicated to play a key role in Alzheimer disease, their role is rather complex, and comprehensive understanding is not yet obtained. We show that Cu(2+) and Zn(2+) but not Fe(3+) renders the amyloid beta peptide, Abeta(1-40), nonfibrillogenic in nature. However, preformed fibrils of Abeta(1-40) were stable when treated with these metal ions. Consequently, fibril growth of Abeta(1-40) could be switched on/off by switching the molecule between its apo- and holo-forms. Clioquinol, a potential drug for Alzheimer disease, induced resumption of the Cu(2+)-suppressed but not the Zn(2+)-suppressed fibril growth of Abeta(1-40). The observed synergistic effect of clioquinol and Zn(2+) suggests that Zn(2+)-clioquinol complex effectively retards fibril growth. Thus, clioquinol has dual effects; although it disaggregates the metal ion-induced aggregates of Abeta(1-40) through metal chelation, it further retards the fibril growth along with Zn(2+). These results indicate the mechanism of metal ions in suppressing Abeta amyloid formation, as well as providing information toward the use of metal ion chelators, particularly clioquinol, as potential drugs for Alzheimer disease.     

Copper and zinc binding modulates the aggregation and neurotoxic properties of the prion peptide PrP106-126.

The abnormal form of the prion protein (PrP) is believed to be responsible for the transmissible spongiform encephalopathies. A peptide encompassing residues 106-126 of human PrP (PrP106-126) is neurotoxic in vitro due its adoption of an amyloidogenic fibril structure. The Alzheimer's disease amyloid beta peptide (Abeta) also undergoes fibrillogenesis to become neurotoxic. Abeta aggregation and toxicity is highly sensitive to copper, zinc, or iron ions. We show that PrP106-126 aggregation, as assessed by turbidometry, is abolished in Chelex-100-treated buffer. ICP-MS analysis showed that the Chelex-100 treatment had reduced Cu(2+) and Zn(2+) levels approximately 3-fold. Restoring Cu(2+) and Zn(2+) to their original levels restored aggregation. Circular dichroism showed that the Chelex-100 treatment reduced the aggregated beta-sheet content of the peptide. Electron paramagnetic resonance spectroscopy identified a 2N1S1O coordination to the Cu(2+) atom, suggesting histidine 111 and methionine 109 or 112 are involved. Nuclear magnetic resonance confirmed Cu(2+) and Zn(2+) binding to His-111 and weaker binding to Met-112. An N-terminally acetylated PrP106-126 peptide did not bind Cu(2+), implicating the free amino group in metal binding. Mutagenesis of either His-111, Met-109, or Met-112 abolished PrP106-126 neurotoxicity and its ability to form fibrils. Therefore, Cu(2+) and/or Zn(2+) binding is critical for PrP106-126 aggregation and neurotoxicity.     

Histidine-13 is a crucial residue in the zinc ion-induced aggregation of the A beta peptide of Alzheimer's disease.

Metal ions such as Zn(2+) and Cu(2+) have been implicated in both the aggregation and neurotoxicity of the beta-amyloid (Abeta) peptide that is present in the brains of Alzheimer's sufferers. Zinc ions in particular have been shown to induce rapid aggregation of Abeta. Rat Abeta binds zinc ions much less avidly than human Abeta, and rats do not form cerebral Abeta amyloid. Rat Abeta differs from human Abeta by the substitution of Gly for Arg, Phe for Tyr, and Arg for His at positions 5, 10, and 13, respectively. Through the use of synthetic peptides corresponding to the first 28 residues of human Abeta, rat Abeta, and single-residue variations, we use circular dichroism spectroscopy, sedimentation assays, and immobilized metal ion affinity chromatography to show that the substitution of Arg for His-13 is responsible for the different Zn(2+)-induced aggregation behavior of rat and human Abeta. The coordination of Zn(2+) to histidine-13 is critical to the zinc ion induced aggregation of Abeta.     

Inhibitory effect of copper(II) on zinc(II)-induced aggregation of amyloid beta-peptide

Aggregation of amyloid beta-peptide (Abeta), a key pathological event in Alzheimer's disease, has been shown in vitro to be profoundly promoted by Zn(II). This fact suggests that some factors in the normal brain protect Abeta from the Zn(II)-induced aggregation. We demonstrate for the first time that Cu(II) effectively inhibits the Abeta aggregation by competing with Zn(II) for histidine residues. The Raman spectrum of a metal-Abeta complex in the presence of both Zn(II) and Cu(II) shows that the cross-linking of Abeta through binding of Zn(II) to the N(tau) atom of histidine is prevented by chelation of Cu(II) by the N(pi) atom of histidine and nearby amide nitrogens. The inhibitory effect is strongest at a Cu/Abeta molar ratio of around 4. Above this ratio, Cu(II) itself promotes the Abeta aggregation by binding to the phenolate oxygen of Tyr10. These results emphasize the importance of regulation of Cu(II) levels to inhibit Abeta aggregation, and are consistent with an altered metal homeostasis in Alzheimer's disease.     

Characterization of copper interactions with alzheimer amyloid beta peptides: identification of an attomolar-affinity copper binding site on amyloid beta1-42

Cu and Zn have been shown to accumulate in the brains of Alzheimer's disease patients. We have previously reported that Cu(2+) and Zn(2+) bind amyloid beta (Abeta), explaining their enrichment in plaque pathology. Here we detail the stoichiometries and binding affinities of multiple cooperative Cu(2+)-binding sites on synthetic Abeta1-40 and Abeta1-42. We have developed a ligand displacement technique (competitive metal capture analysis) that uses metal-chelator complexes to evaluate metal ion binding to Abeta, a notoriously self-aggregating peptide. This analysis indicated that there is a very-high-affinity Cu(2+)-binding site on Abeta1-42 (log K(app) = 17.2) that mediates peptide precipitation and that the tendency of this peptide to self-aggregate in aqueous solutions is due to the presence of trace Cu(2+) contamination (customarily approximately 0.1 microM). In contrast, Abeta1-40 has much lower affinity for Cu(2+) at this site (estimated log K(app) = 10.3), explaining why this peptide is less self-aggregating. The greater Cu(2+)-binding affinity of Abeta1-42 compared with Abeta1-40 is associated with significantly diminished negative cooperativity. The role of trace metal contamination in inducing Abeta precipitation was confirmed by the demonstration that Abeta peptide (10 microM) remained soluble for 5 days only in the presence of high-affinity Cu(2+)-selective chelators.     

Copper binding to the amyloid-beta (Abeta) peptide associated with Alzheimer's disease: folding, coordination geometry, pH dependence, stoichiometry, and affinity of Abeta-(1-28): insights from a range of complementary spectroscopic techniques

There is now direct evidence that copper is bound to amyloid-beta peptide (Abeta) in senile plaque of Alzheimer's disease. Copper is also linked with the neurotoxicity of Abeta and free radical damage, and Cu(2+) chelators represent a possible therapy for Alzheimer's disease. We have therefore used a range of complementary spectroscopies to characterize the coordination of Cu(2+) to Abeta in solution. The mode of copper binding is highly pH-dependent. EPR spectroscopy indicates that both coppers have axial, Type II coordination geometry, square-planar or square-pyramidal, with nitrogen and oxygen ligands. Circular dichroism studies indicate that copper chelation causes a structural transition of Abeta. Competition studies with glycine and l-histidine indicate that copper binds to Abeta-(1-28) at pH 7.4 with an affinity of K(a) approximately 10(7) m(-1). (1)H NMR indicates that histidine residues are involved in Cu(2+) coordination but that Tyr(10) is not. Studies using analogues of Abeta-(1-28) in which each of the histidine residues have been replaced by alanine or in which the N terminus is acetylated suggest that the N terminus and His(13) are crucial for Cu(2+) binding and that His(6) and His(14) are also implicated. Evidence for the link between Alzheimer's disease and Cu(2+) is growing, and our studies have made a significant contribution to understanding the mode of Cu(2+) binding to Abeta in solution.     

Methylation of the imidazole side chains of the Alzheimer disease amyloid-beta peptide results in abolition of superoxide dismutase-like structures and inhibition of neurotoxicity

The toxicity of the amyloid-beta peptide (Abeta) is thought to be responsible for the neurodegeneration associated with Alzheimer disease. Generation of hydrogen peroxide has been implicated as a key step in the toxic pathway. Abeta coordinates the redox active metal ion Cu2+ to catalytically generate H2O2. Structural studies on the interaction of Abeta with Cu have suggested that the coordination sphere about the Cu2+ resembles the active site of superoxide dismutase 1. To investigate the potential role for such structures in the toxicity of Abeta, two novel Abeta40 peptides, Abeta40(HistauMe) and Abeta40(HispiMe), have been prepared, in which the histidine residues 6, 13, and 14 have been substituted with modified histidines where either the pi- or tau-nitrogen of the imidazole side chain is methylated to prevent the formation of bridging histidine moieties. These modifications did not inhibit the ability of these peptides to form fibrils. However, the modified peptides were four times more effective at generating H2O2 than the native sequence. Despite the ability to generate more H2O2, these peptides were not neurotoxic. Whereas the modifications to the peptide altered the metal binding properties, they also inhibited the interaction between the peptides and cell surface membranes. This is consistent with the notion that Abeta-membrane interactions are important for neurotoxicity and that inhibiting these interactions has therapeutic potential.     

Amyloid-beta peptide disruption of lipid membranes and the effect of metal ions

Beta-amyloid peptide (Abeta), which is cleaved from the larger trans-membrane amyloid precursor protein, is found deposited in the brain of patients suffering from Alzheimer's disease and is linked with neurotoxicity. We report the results of studies of Abeta1-42 and the effect of metal ions (Cu2+ and Zn2+) on model membranes using 31P and 2H solid-state NMR, fluorescence and Langmuir Blodgett monolayer methods. Both the peptide and metal ions interact with the phospholipid headgroups and the effects on the lipid bilayer and the peptide structure were different for membrane incorporated or associated peptides. Copper ions alone destabilise the lipid bilayer and induced formation of smaller vesicles but when Abeta1-42 was associated with the bilayer membrane copper did not have this effect. Circular dichroism spectroscopy indicated that Abeta1-42 adopted more beta-sheet structure when incorporated in a lipid bilayer in comparison to the associated peptide, which was largely unstructured. Incorporated peptides appear to disrupt the membrane more severely than associated peptides, which may have implications for the role of Abeta in disease states.     

Examining the zinc binding site of the amyloid-beta peptide.

The amyloid beta-peptide (Abeta) is a principal component of insoluble amyloid plaques which are characteristic neuropathological features of Alzheimer's disease. Abeta also exists as a normal soluble protein that undergoes a pathogenic transition to an aggregated, fibrous form. This transition can be affected by extraneous proteinaceous and nonproteinaceous elements, such as zinc ions, which may promote aggregation and/or stabilization of the fibrils. Protein chelation of zinc is typically mediated by histidines, cysteines and carboxylates. Previous studies have demonstrated that the Abeta-Zn2+ binding site is localized within residues 6-28 and that histidines may serve as the principal sites of interaction. To localize key residues within this region, a series of Abeta peptides (residues 1-28) were synthesized that contained systematic His/Ala substitutions. Circular dichroism and electron microscopy were used to monitor the effects of Zn2+ on the peptide beta-sheet conformation and fibril aggregation. Our results indicate that substitution of either His13 or His14 but not His6 eliminates the zinc-mediated effects. These observations indicate a specific zinc binding site within Abeta that involves these central histidine residues.     

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.     

Evidence that the beta-amyloid plaques of Alzheimer's disease represent the redox-silencing and entombment of abeta by zinc

Abeta binds Zn(2+), Cu(2+), and Fe(3+) in vitro, and these metals are markedly elevated in the neocortex and especially enriched in amyloid plaque deposits of individuals with Alzheimer's disease (AD). Zn(2+) precipitates Abeta in vitro, and Cu(2+) interaction with Abeta promotes its neurotoxicity, correlating with metal reduction and the cell-free generation of H(2)O(2) (Abeta1-42 > Abeta1-40 > ratAbeta1-40). Because Zn(2+) is redox-inert, we studied the possibility that it may play an inhibitory role in H(2)O(2)-mediated Abeta toxicity. In competition to the cytotoxic potentiation caused by coincubation with Cu(2+), Zn(2+) rescued primary cortical and human embryonic kidney 293 cells that were exposed to Abeta1-42, correlating with the effect of Zn(2+) in suppressing Cu(2+)-dependent H(2)O(2) formation from Abeta1-42. Since plaques contain exceptionally high concentrations of Zn(2+), we examined the relationship between oxidation (8-OH guanosine) levels in AD-affected tissue and histological amyloid burden and found a significant negative correlation. These data suggest a protective role for Zn(2+) in AD, where plaques form as the result of a more robust Zn(2+) antioxidant response to the underlying oxidative attack.     

Identifying the minimal copper- and zinc-binding site sequence in amyloid-beta peptides

With a combination of complementary experimental techniques, namely sedimentation assay, Fourier transform infrared spectroscopy, and x-ray absorption spectroscopy, we are able to determine the atomic structure around the metal-binding site in samples where amyloid-beta (Abeta) peptides are complexed with either Cu(II) or Zn(II). Exploiting information obtained on a selected set of fragments of the Abeta peptide, we identify along the sequence the histidine residues coordinated to the metal in the various peptides we have studied (Abeta(1-40), Abeta(1-16), Abeta(1-28), Abeta(5-23), and Abeta(17-40)). Our data can be consistently interpreted assuming that all of the peptides encompassing the minimal 1-16 amino acidic sequence display a copper coordination mode that involves three histidines (His(6), His(13), and His(14)). In zinc-Abeta complexes, despite the fact that the metal coordination appears to be more sensitive to solution condition and shows a less rigid geometry around the binding site, a four-histidine coordination mode is seen to be preferred. Lacking a fourth histidine along the Abeta peptide sequence, this geometrical arrangement hints at a Zn(II)-promoted interpeptide aggregation mode.     

Selective destabilization of soluble amyloid beta oligomers by divalent metal ions

Aggregation of the amyloid beta (Abeta) peptide yields both fibrillar precipitates and soluble oligomers, and is associated with Alzheimer's disease (AD). In vitro, Cu(2+) and Zn(2+) strongly bind Abeta and promote its precipitation. However, less is known about their interactions with the soluble oligomers, which are thought to be the major toxic species responsible for AD. Using fluorescence correlation spectroscopy to resolve the various soluble species of Abeta, we show that low concentrations of Cu(2+) (1 microM) and Zn(2+) (4 microM) selectively eliminate the oligomeric population (within approximately 2h), while Mg(2+) displays a similar effect at a higher concentration (60 microM). This uncovers a new aspect of Abeta-metal ion interactions, as precipitation is not substantially altered at these low metal ion concentrations. Our results suggest that physiological concentrations of Cu(2+) and Zn(2+) can critically alter the stability of the toxic Abeta oligomers and can potentially control the course of neurodegeneration.     

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.     

Copper catalyzed oxidation of Alzheimer Abeta.

Abeta derived from amyloid plaques of Alzheimer's disease-affected brain contain several oxidative posttranslational modifications. In this study we have characterized the amino acid content of human amyloid-derived Abeta and compared it with that of human synthetic Abeta subjected to metal-catalyzed oxidation. Human amyloid derived Abeta has an increased content of arginine (46%) and glutamate/glutamine residues (28%), but a decreased content of histidine residues (-32%) as compared to the expected amino acid content. Incubation of synthetic human Abeta with Cu(II), but not Fe(III), in the presence of H2O2 similarly induced a decrease in histidine residues (-79%), but also a decrease in tyrosine residues (-28%). Our results suggest that histidine and tyrosine are most vulnerable to metal mediated oxidative attack, consistent with our earlier findings that Cu coordinated via histidine residues is redox competent. Our results suggest that the loss of histidine residues in human amyloid-derived Abeta may be a result of Cu oxidation, and that unidentified post-translational mechanisms operate to modify other amino acids of Abeta in vivo.     

Copper-mediated amyloid-beta toxicity is associated with an intermolecular histidine bridge

Amyloid-beta peptide (Abeta) is pivotal to the pathogenesis of Alzheimer disease. Here we report the formation of a toxic Abeta-Cu2+ complex formed via a histidine-bridged dimer, as observed at Cu2+/peptide ratios of >0.6:1 by EPR spectroscopy. The toxicity of the Abeta-Cu2+ complex to cultured primary cortical neurons was attenuated when either the pi -or tau-nitrogen of the imidazole side chains of His were methylated, thereby inhibiting formation of the His bridge. Toxicity did not correlate with the ability to form amyloid or perturb the acyl-chain region of a lipid membrane as measured by diphenyl-1,3,5-hexatriene anisotropy, but did correlate with lipid peroxidation and dityrosine formation. 31P magic angle spinning solid-state NMR showed that Abeta and Abeta-Cu2+ complexes interacted at the surface of a lipid membrane. These findings indicate that the generation of the Abeta toxic species is modulated by the Cu2+ concentration and the ability to form an intermolecular His bridge.     

Structural changes of region 1-16 of the Alzheimer disease amyloid beta-peptide upon zinc binding and in vitro aging

Amyloid deposits within the cerebral tissue constitute a characteristic lesion associated with Alzheimer disease. They mainly consist of the amyloid peptide Abeta and display an abnormal content in Zn(2+) ions, together with many truncated, isomerized, and racemized forms of Abeta. The region 1-16 of Abeta can be considered the minimal zinc-binding domain and contains two aspartates subject to protein aging. The influence of zinc binding and protein aging related modifications on the conformation of this region of Abeta is of importance given the potentiality of this domain to constitute a therapeutic target, especially for immunization approaches. In this study, we determined from NMR data the solution structure of the Abeta-(1-16)-Zn(2+) complex in aqueous solution at pH 6.5. The residues His(6), His(13), and His(14) and the Glu(11) carboxylate were identified as ligands that tetrahedrally coordinate the Zn(II) cation. In vitro aging experiments on Abeta-(1-16) led to the formation of truncated and isomerized species. The major isomer generated, Abeta-(1-16)-l-iso-Asp(7), displayed a local conformational change in the His(6)-Ser(8) region but kept a zinc binding propensity via a coordination mode involving l-iso-Asp(7). These results are discussed here with regard to Abeta fibrillogenesis and the potentiality of the region 1-16 of Abeta to be used as a therapeutic target.     

Time-dependent DNA condensation induced by amyloid beta-peptide

The major protein component of the amyloid deposition in Alzheimer's disease is a 39-43 residue peptide, amyloid beta (Abeta). Abeta is toxic to neurons, although the mechanism of neurodegeneration is uncertain. Evidence exists for non-B DNA conformation in the hippocampus of Alzheimer's disease brains, and Abeta was reportedly able to transform DNA conformation in vitro. In this study, we found that DNA conformation was altered in the presence of Abeta, and Abeta induced DNA condensation in a time-dependent manner. Furthermore, Abeta sheets, serving as condensation nuclei, were crucial for DNA condensation, and Cu(2+) and Zn(2+) ions inhibited Abeta sheet-induced DNA condensation. Our results suggest DNA condensation as a mechanism of Abeta toxicity.     

Both the D-(+) and L-(-) enantiomers of nicotine inhibit Abeta aggregation and cytotoxicity

The underlying cause of Alzheimer's disease is thought to be the aggregation of monomeric beta-amyloid (Abeta), through a series of toxic oligomers, which forms the mature amyloid fibrils that accumulate at the center of senile plaques. It has been reported that L-(-)-nicotine prevents Abeta aggregation and toxicity, and inhibits senile plaque formation. Previous NMR studies have suggested that this could be due to the specific binding of L-(-)-nicotine to histidine residues (His6, His13, and His14) in the peptide. Here, we have looked at the effects of both of the L-(-) and D-(+) optical enantiomers of nicotine on the aggregation and cytotoxicity of Abeta(1-40). Surprisingly, both enantiomers inhibited aggregation of the peptide and reduced the toxic effects of the peptide on cells. In NMR studies with Abeta(1-40), both enantiomers of nicotine were seen to interact with the three histidine residues. Overall, our data indicate that nicotine can delay Abeta fibril formation and maintain a population of less toxic Abeta species. This effect cannot be due to a highly specific binding interaction between nicotine and Abeta, as previously thought, but could be due instead to weaker, relatively nonspecific binding, or to the antioxidant or metal chelating properties of nicotine. D-(+)-nicotine, being biologically much less active than L-(-)-nicotine, might be a useful therapeutic agent.