Methionine regulates copper/hydrogen peroxide oxidation products of Abeta.

Metal-catalysed oxidation (MCO) may play a causative role in the pathogenesis of Alzheimer's disease (AD). Amyloid beta peptide (Abeta), the major biomarker of AD, in the presence of copper ions reduces Cu(2+) to Cu(+) and catalyses the formation of H(2)O(2) that subsequently induces radicals through Fenton chemistry. Abeta is also subject to attack by free radicals, where the presence of Cu(2+) in conjunction with H(2)O(2) catalyses oxygenation, primarily at the methionine sulfur atom. This work investigates MCO of Abeta, to gain further insight into the role of oxidative stress in AD. By combining a fluorescence assay with gel electrophoresis to monitor MCO reactions of Abeta (1-28) in the presence and absence of methionine it was determined that methionine can both protect some residues against MCO and promote the oxidation of Tyr(10) specifically. Electrospray ionization mass spectrometric analysis of methionine MCO products indicated the formation of methionine sulfoxide, methionine sulfone and related hydroxylated products. Similar products could be formed from the oxidation of Met(35) of Abeta and may relate to changes in properties of the peptide following MCO.     

Synthesis and structural analysis of copper(II) cysteine complexes

This report describes the synthesis and structural analysis of stable copper(II) cysteine complexes. Pale pink copper(II) cysteine complexes were synthesized in mole ratios of 1:2, 1:4, and 1:6 of copper(II):cysteine in ethanol. Infrared spectroscopy and X-ray absorption spectroscopy confirmed that copper(II) binding occurred via the thiol ligand of cysteine. XANES analysis showed that the oxidation state of copper remained as copper(II) and the local atomic geometry was similar in all of the cysteine complexes. The EXAFS data indicate that the copper(II) cysteine complexes are forming ring type structures with sulfur ligands from the cysteines acting as bridging ligands. X-ray diffraction revealed that the copper(II) cysteine complexes formed monoclinic cells with maximum crystallinity found in the 1:4 copper(II):cysteine complex.     

Interaction of heme nonapeptide derived from cytochrome c with microsomal reductases

The interaction of heme nonapeptide (a proteolytic product of cytochrome c) with purified NADH:cytochrome b5 (EC 1.6.2.2) and NADPH:cytochrome P-450 (EC 1.6.2.4) reductases was investigated. In the presence of heme nonapeptide, NADH or NADPH were enzymatically oxidized to NAD+ and NADP+, respectively. NAD(P)H consumption was coupled to oxygen uptake in both enzyme reactions. In the presence of carbon monoxide the spectrum of a carboxyheme complex was observed during NAD(P)H oxidation, indicating the existence of a transient ferroheme peptide. NAD(P)H oxidation could be partially inhibited by cyanide, superoxide dismutase and catalase. Superoxide and peroxide ions (generated by enzymic xanthine oxidation) only oxidized NAD(P)H in the presence of heme nonapeptide. Oxidation of NAD(P)H was more rapid with O2- than O2-2. We suggest that a ferroheme-O2 and various heme-oxy radical complexes (mainly ferroheme-O-2 complex) play a crucial role in NAD(P)H oxidation.     

Protective Effects of Vitamin E against Oxidative Damage Induced by Aβ1-40Cu（Ⅱ） Complexes

β-amyloid peptide (Aβ) is considered to be responsible for the formation of senile plaques,which is the hallmark of Alzheimer's disease (AD).Oxidative stress,manifested by protein oxidation andlipid peroxidation,among other alterations,is a characteristic of AD brain.A growing body of evidence hasbeen presented in support of Aβ_(1-40) forming an oligomeric complex that binds copper at a CuZn superoxidedismutase-like binding site. Aβ_(1-40)Cu(Ⅱ) complexes generate neurotoxic hydrogen peroxide (H_2O_2) from O_2via Cue reduction,though the precise reaction mechanism is unclear.The toxicity of Aβ_(1-40) or the Aβ_(1-40)Cu(Ⅱ)complexes to cultured primary cortical neurons was partially attenuated when ( )-α-tocopherol (vitamin E)as free radical antioxidant was added at a concentration of 100 μM.The data derived from lactate dehydro-genase (LDH) release and the formation of H_2O_2 confirmed the results from the MTT assay.These findingsindicate that copper binding to Aβ_(1-40) can give rise to greater production of H_2O_2, which leads to a break-down in the integrity of the plasma membrane and subsequent neuronal death.Groups treated with vitaminE exhibited much slighter damage,suggesting that vitamin E plays a key role in protecting neuronal cellsfrom dysfunction or death.     

D-Penicillamine: analysis of the mechanism of copper-catalyzed hydrogen peroxide generation

Recent studies have suggested that the inhibition of lymphocyte mitogenesis by D-penicillamine in the presence of copper could be mediated by the formation and action of hydrogen peroxide. To explore this possibility further, we first sought evidence of H2O2 generation by D-penicillamine in a cell-free system by a) measurement of copper-catalyzed D-penicillamine oxidation and the requirement for oxygen in this process; b) direct measurement of H2O2 formation during D-penicillamine oxidation by the peroxidase-mediated oxidation of fluorescent scopoletin; and c) evaluation of the possible synthesis of O2- during D-penicillamine oxidation. The addition of copper to D-penicillamine in physiologic buffer catalyzed D-penicillamine oxidation in a dose-dependent fashion. D-penicillamine oxidation was accompanied by O2 consumption with a molar ratio of approximately 2:1, but did not occur under anaerobic conditions. Furthermore, D-penicillamine oxidation resulted in the formation of amounts of H2O2 stoichiometrically equivalent to oxygen consumption (i.e., 1:1). Copper-catalyzed D-penicillamine oxidation caused reduction of nitroblue tetrazolium in a reaction blocked by superoxide dismutase, suggesting the formation of O2-. Additional studies confirmed that D-penicillamine inhibited PHA-induced mitogenesis of lymphocytes in the presence of copper, and that catalase protected the cells from this action. Furthermore, when polymorphonuclear leukocytes were incubated with D-penicillamine plus copper, hexose monophosphate shunt activity increased up to threefold with abrogation of this stimulation by catalase. None of the effects of D-penicillamine plus copper on cells were diminished by hydroxyl radical scavengers mannitol or benzoate. These results are consistent with oxygen-dependent copper-catalyzed oxidation of D-penicillamine in aqueous solutions leading to the formation of O2- and H2O2. H2O2 produced by this reaction can inhibit lymphocyte mitogenesis and stimulate neutrophil hexose monophosphate shunt activity in vitro and may be relevant to the therapeutic effects of D-penicillamine in vivo.     

Differences between cysteine and homocysteine in the induction of deoxyribose degradation and DNA damage

The effect of two naturally occurring thiols, such as cysteine and homocysteine, has been examined for their ability to induce deoxyribose degradation and DNA damage. Copper(II) ions have been added to incubation mixtures and oxygen consumption measurements have been performed in order to correlate the observed damaging effects with the rate of metal catalyzed thiol oxidation. Ascorbic acid plus copper has been used as a positive control of deoxyribose and DNA oxidation due to reactive oxygen species. Cysteine or homocysteine in the presence of copper ions induce the degradation of deoxyribose and the yield of 8-hydroxy-2'-deoxyguanosine (8-OHdG), although important differences are observed between the two thiols tested, homocysteine being less reactive than cysteine. DNA cleavage is induced by cysteine in the presence of copper(II) ions but not by homocysteine. Catalase and thiourea, but not superoxide dismutase (SOD), were shown to inhibit the damaging effects of cysteine on deoxyribose or DNA suggesting that H(2)O(2) and *OH radicals are responsible for the observed induced damage. The results indicate that there are differences between the damaging effects of the two thiols tested towards deoxyribose and DNA damage. The pathophysiological importance will be discussed.     

The oxidation of yeast alcohol dehydrogenase-1 by hydrogen peroxide in vitro

Yeast alcohol dehydrogenase (YADH) plays an important role in the conversion of alcohols to aldehydes or ketones. YADH-1 is a zinc-containing protein, and it accounts for the major part of ADH activity in growing baker's yeast. To gain insight into how oxidative modification of the enzyme affects its function, we exposed YADH-1 to hydrogen peroxide in vitro and assessed the oxidized protein by LC-MS/MS analysis of proteolytic cleavage products of the protein and by measurements of enzymatic activity, zinc release, and thiol/thiolate loss. The results illustrated that Cys43 and Cys153, which reside at the active site of the protein, could be selectively oxidized to cysteine sulfinic acid (Cys-SO2H) and cysteine sulfonic acid (Cys-SO3H). In addition, H2O2 induced the formation of three disulfide bonds: Cys43-Cys153 in the catalytic domain, Cys103-Cys111 in the noncatalytic zinc center, and Cys276-Cys277. Therefore, our results support the notion that the oxidation of cysteine residues in the zinc-binding domain of proteins can go beyond the formation of disulfide bond(s); the formation of Cys-SO2H and Cys-SO3H is also possible. Furthermore, most methionines could be oxidized to methionine sulfoxides. Quantitative measurement results revealed that, among all the cysteine residues, Cys43 was the most susceptible to H2O2 oxidation, and the major oxidation products of this cysteine were Cys-SO2H and Cys-SO3H. The oxidation of Cys43 might be responsible for the inactivation of the enzyme upon H2O2 treatment.     

Interactions of quercetin with iron and copper ions: complexation and autoxidation

Quercetin (3,3',4',5,7-pentahydroxyflavone), one of the most abundant dietary flavonoids, has been investigated for its ability to bind Fe(II), Fe(III), Cu(I) and Cu(II) in acidic to neutral solutions. In particular, analysis by UV-visible spectroscopy allows to determine the rate constants for the formation of the 1:1 complexes. In absence of added metal ion, quercetin undergoes a slow autoxidation in neutral solution with production of low hydrogen peroxide (H(2)O(2)) concentrations. Autoxidation is accelerated by addition of the metal ions according to: Cu(I) > Cu(II)>Fe(II) Fe(III). In fact, the iron-quercetin complexes seem less prone to autoxidation than free quercetin in agreement with the observation that EDTA addition, while totally preventing iron-quercetin binding, slightly accelerates quercetin autoxidation. By contrast, the copper-quercetin complexes appear as reactive intermediates in the copper-initiated autoxidation of quercetin. In presence of the iron ions, only low concentrations of H(2)O(2) can be detected. By contrast, in the presence of the copper ions, H(2)O(2) is rapidly accumulated. Whereas Fe(II) is rapidly autoxidized to Fe(III) in the presence or absence of quercetin, Cu(I) bound to quercetin or its oxidation products does not undergo significant autoxidation. In addition, Cu(II) is rapidly reduced by quercetin. By HPLC-MS analysis, the main autoxidation products of quercetin are shown to be the solvent adducts on the p-quinonemethide intermediate formed upon two-electron oxidation of quercetin. Finally, in strongly acidic conditions (pH 1-2), neither autoxidation nor metal complexation is observed but Fe(III) appears to be reactive enough to quickly oxidize quercetin (without dioxygen consumption). Up to ca. 7 Fe(III) ions can be reduced per quercetin molecule, which points to an extensive oxidative degradation.     

Ascorbate does not act as a pro-oxidant towards lipids and proteins in human plasma exposed to redox-active transition metal ions and hydrogen peroxide

The combination of ascorbate, transition metal ions, and hydrogen peroxide (H(2)O(2)) is an efficient hydroxyl radical generating system called "the Udenfriend system." Although the pro-oxidant role of ascorbate in this system has been well characterized in vitro, it is uncertain whether ascorbate also acts as a pro-oxidant under physiological conditions. To address this question, human plasma, used as a representative biological fluid, was either depleted of endogenous ascorbate with ascorbate oxidase, left untreated, or supplemented with 25 microM-1 mM ascorbate. Subsequently, the plasma samples were incubated at 37 degrees C with 50 microM-1 mM iron (from ferrous ammonium sulfate), 60 or 100 microM copper (from cupric sulfate), and/or 200 microM or 1 mM H(2)O(2). Although endogenous and added ascorbate was depleted rapidly in the presence of transition metal ions and H(2)O(2), no cholesterol ester hydroperoxides or malondialdehyde were formed, i.e., ascorbate protected against, rather than promoted, lipid peroxidation. Conversely, depletion of endogenous ascorbate was sufficient to cause lipid peroxidation, the rate and extent of which were enhanced by the addition of metal ions but not H(2)O(2). Ascorbate also did not enhance protein oxidation in plasma exposed to metal ions and H(2)O(2), as assessed by protein carbonyl formation and depletion of reduced thiols. Interestingly, neither the rate nor the extent of endogenous alpha-tocopherol oxidation in plasma was affected by any of the treatments. Our data show that even in the presence of redox-active iron or copper and H(2)O(2), ascorbate acts as an antioxidant that prevents lipid peroxidation and does not promote protein oxidation in human plasma in vitro.     

Major differences in oxysterol formation in human low density lipoproteins (LDLs) oxidized by *OH/O2*- free radicals or by copper.

The aim of our study was to determine the oxysterol formation in low density lipoproteins (LDLs) oxidized by defined oxygen free radicals (*OH/O2*-). This was compared to the oxysterol produced upon the classical copper oxidation procedure. The results showed a markedly lower formation of oxysterols induced by *OH/O2*- free radicals than by copper and thus suggested a poor ability of these radicals to initiate cholesterol oxidation in LDLs. Moreover, the molecular species of cholesteryl ester hydroperoxides produced by LDL copper oxidation seemed more labile than those formed upon *OH/O2*(-)-induced oxidation, probably due to their degradation by reaction with copper ions.     

Direct evidence for inhibition of free radical formation from Cu(I) and hydrogen peroxide by glutathione and other potential ligands using the EPR spin-trapping technique.

Copper-induced oxidative damage is generally attributed to the formation of the highly reactive hydroxyl radical by a mechanism analogous to the Haber-Weiss cycle for Fe(II) and H2O2. In the present work, the reaction between the Cu(I) ion and H2O2 is studied using the EPR spin-trapping technique. The hydroxyl radical adduct was observed when Cu(I), dissolved in acetonitrile under N2, was added to pH 7.4 phosphate buffer containing 100 mM 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Formation of the hydroxyl radical was dependent on the presence of O2 and subsequent formation of H2O2. The kscav/kDMPO ratios obtained were below those expected for a mechanism involving free hydroxyl radical and reflect the interference of nucleophilic addition of H2O to DMPO to form the DMPO/.OH adduct in the presence of nonchelated copper ion. Addition of ethanol or dimethyl sulfoxide to the reaction suggests that a high-valent metal intermediate, possibly Cu(III), was also formed. Spin trapping of hydroxyl radical was almost completely inhibited upon addition of Cu(I) to a solution of either nitrilotriacetate or histidine, even though the copper was fully oxidized to Cu(II) and H2O2 was formed. Bathocuproinedisulfonate, thiourea, and reduced glutathione all stabilized the Cu(I) ion toward oxidation by O2. Upon addition of H2O2, the Cu(I) in all three complexes was oxidized to varying degrees; however, only the thiourea complex was fully oxidized within 2 min of reaction and produced detectable hydroxyl radicals. No radicals were detected from the bathocuproinedisulfonate or glutathione complexes. Overall, these results suggest that the deleterious effects of copper ions in vivo are diminished by biochemical chelators, especially glutathione, which probably has a major role in moderating the toxicological effects of copper.     

Formation of hydrogen peroxide by lens proteins: protein-derived hydrogen peroxide as a potential mechanism of oxidative insult to the lens.

The exposure of dialyzed preparations of lens crystallins to copper (II) ions causes a decrease in protein surface thiol and the production of hydrogen peroxide (H2O2). H2O2 production by gamma and beta crystallin subfractions (which contain the greatest level of thiol) is the predominant source of this H2O2. Protein surface thiols are probable sources of H2O2 formation since N-ethyl maleimide treatment of lens proteins and zinc ions inhibit H2O2 production. These data are consistent with a hypothesis that transition metal-catalyzed oxidation of protein contributes to cataractogenic lens protein oxidations.     

Inactivation of human fibroblast growth factor-1 (FGF-1) activity by interaction with copper ions involves FGF-1 dimer formation induced by copper-catalyzed oxidation.

Although the angiogenic proteins acidic fibroblast growth factor (FGF-1) and basic fibroblast growth factor (FGF-2) both interact with the transition metal copper, itself a putative modulator of angiogenesis, a role for copper in FGF function has not been established. Using nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, we detect the complete conversion of recombinant forms of human FGF-1 monomer protein to FGF-1 homodimers after exposure to copper ions. In contrast, not all forms of bovine FGF-1 isolated from bovine brain or a recombinant preparation of human FGF-2 completely formed homodimers after exposure to copper ions under similar conditions. Since the copper-induced FGF-1 homodimers reverted to the monomer form in the presence of dithiothreitol, specific alkylation of cysteine residues by pyridylethylation prevented FGF-1 homodimer formation, and preformed FGF-1 homodimers could not be dissociated by the metal chelator EDTA, FGF-1 dimer formation appeared to result from the formation of intermolecular disulfide bonds by copper-induced oxidation of sulfhydryl residues. FGF-1 homodimers bound with similar apparent affinity as FGF-1 monomers to immobilized copper ions, both eluting at 60 mM imidazole. Both human FGF-1 monomer and dimer forms had a 6-fold higher apparent affinity for immobilized copper ions, as compared with human FGF-2, which eluted in the monomer form at 10 mM imidazole. Further, in contrast to FGF-1 monomers, which dissociate from immobilized heparin in 1.0 M NaCl, preformed FGF-1 homodimers had reduced apparent affinity for immobilized heparin and eluted at 0.4 M NaCl. In contrast, the apparent affinity of human FGF-2 for immobilized heparin was unaffected after exposure to copper ions. Heparin appeared to modulate the formation of copper-induced intermolecular disulfide bonds for FGF-1 but not FGF-2, since co-incubation of heparin and copper with FGF-1 monomers resulted in dimers and other oligomeric complexes. FGF-1 copper-induced homodimers failed to induce mitogenesis in [3H]thymidine incorporation assays, an effect which could be reversed by treatment with dithiothreitol, whereas FGF-2-induced mitogenic activity was relatively unaffected by pretreatment with copper. The differences between human FGF-1 and FGF-2 in protein-copper interactions may be due to differing free thiol content and arrangement between the two proteins. A recombinant human FGF-1 mutant containing the two cysteines conserved throughout the FGF family of proteins but lacking a cysteine residue (Cys 131) present in wild-type human FGF-1 but not human FGF-2 readily formed copper-induced dimers.(ABSTRACT TRUNCATED AT 400 WORDS)     

A cysteine residue near the propionate side chain of heme is the radical site in ascorbate peroxidase

The radical scavenger 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO(*)) and the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were used in conjunction with mass spectrometry to identify the protein-based radical sites of the H(2)O(2)-tolerant ascorbate peroxidase (APX) of the red alga Galdieria partita and the H(2)O(2)-sensitive stromal APX of tobacco. A cysteine residue in the vicinity of the propionate side chain of heme in both enzymes was labeled with TEMPO(*) and DMPO in an H(2)O(2)-dependent manner, indicating that these cysteine residues form thiyl radicals through interaction of APX with H(2)O(2). TEMPO(*) bound to the cysteine thiyl radicals, and sulfinylated and sulfonylated them. Other oxidized cysteine residues were found in both APXs. Experiments with a cysteine-to-serine point mutation showed that formation of TEMPO adducts and subsequent oxidation of the cysteine residue located near the propionate group of heme leads to loss of enzyme activity, in particular in the Galdieria APX. When treated with glutathione and H(2)O(2), both cysteine residues in both enzymes were glutathionylated. These results suggest that, under oxidative stress in vivo, cysteine oxidation is involved in the inactivation of APXs in addition to the proposed H(2)O(2)-mediated crosslinking of heme to the distal tryptophan residue [Kitajima S, Shimaoka T, Kurioka M & Yokota A (2007) FEBS J274, 3013-3020], and that glutathione protects APX from irreversible oxidation of the cysteine thiol and loss of enzyme activity by binding to the cysteine thiol group.     

Nickel(II) complexes of the multihistidine peptide fragments of human prion protein

Nickel(II) complexes of the peptide fragments of human prion protein containing histidyl residues both inside and outside the octarepeat domain have been studied by the combined application of potentiometric, UV-visible and circular dichroism spectroscopic methods. The imidazole-N donor atoms of histidyl residues are the exclusive metal binding sites below pH 7.5, but the formation of stable macrochelates was characteristic only for the peptide HuPrP(76-114) containing four histidyl residues. Yellow colored square planar complexes were obtained above pH 7.5-8 with the cooperative deprotonation of three amide nitrogens in the [N_im,N⁻,N⁻,N⁻] coordination mode. It was found that the peptides can bind as many nickel(II) ions as the number of independent histidyl residues. All data supported that the complex formation processes of nickel(II) are very similar to those of copper(II), but with a significantly reduced stability for nickel(II), which shifts the complex formation reactions into the slightly alkaline pH range. The formation of coordination isomers was characteristic of the mononuclear complexes with a significant preference for the nickel(II) binding at the histidyl sites outside the octarepeat domain. The results obtained for the two-histidine fragments of the protein, HuPrP(91-115), HuPrP(76-114)H85A and HuPrP(84-114)H96A, made it possible to compare the binding ability of the His96 and His111 sites. These data reveal a significant difference in the nickel(II) and copper(II) binding sites of the peptides: His96 was found to predominate almost completely for nickel(II) ions, while the opposite order, but with comparable concentrations, was reported for copper(II).     

Laccase-catalyzed oxidation of Mn(2+) in the presence of natural Mn(3+) chelators as a novel source of extracellular H(2)O(2) production and its impact on manganese peroxidase

A purified and electrophoretically homogeneous blue laccase from the litter-decaying basidiomycete Stropharia rugosoannulata with a molecular mass of approximately 66 kDa oxidized Mn(2+) to Mn(3+), as assessed in the presence of the Mn chelators oxalate, malonate, and pyrophosphate. At rate-saturating concentrations (100 mM) of these chelators and at pH 5.0, Mn(3+) complexes were produced at 0.15, 0.05, and 0.10 micromol/min/mg of protein, respectively. Concomitantly, application of oxalate and malonate, but not pyrophosphate, led to H(2)O(2) formation and tetranitromethane (TNM) reduction indicative for the presence of superoxide anion radical. Employing oxalate, H(2)O(2) production, and TNM reduction significantly exceeded those found for malonate. Evidence is provided that, in the presence of oxalate or malonate, laccase reactions involve enzyme-catalyzed Mn(2+) oxidation and abiotic decomposition of these organic chelators by the resulting Mn(3+), which leads to formation of superoxide and its subsequent reduction to H(2)O(2). A partially purified manganese peroxidase (MnP) from the same organism did not produce Mn(3+) complexes in assays containing 1 mM Mn(2+) and 100 mM oxalate or malonate, but omitting an additional H(2)O(2) source. However, addition of laccase initiated MnP reactions. The results are in support of a physiological role of laccase-catalyzed Mn(2+) oxidation in providing H(2)O(2) for extracellular oxidation reactions and demonstrate a novel type of laccase-MnP cooperation relevant to biodegradation of lignin and xenobiotics.     

In vitro oxidative activity of cupric complexes of kanamycin A in comparison to in vivo bactericidal efficacy

The interactions of copper(II) complexes of kanamycin A with oxidation-susceptible biomolecules: 2'-deoxyguanosine, plasmid DNA and yeast tRNA(Phe) were studied in both the presence and absence of hydrogen peroxide. The mixture of complex with H(2)O(2) was found to be an efficient oxidant, converting dG to its 8-oxo derivative, generating strand breaks in plasmid DNA and multiple cleavages in tRNA(Phe). Some of these reactions may play a role in toxic effects of aminoglycoside antibiotics. These complexes were screened for their antibacterial activity. The microbiological studies undertaken to compare the bactericidal action of kanamycin A alone and complexed with copper(II) ions in both neutral and oxidative environment revealed that the enhancement of bactericidal action by Cu(II) was not statistically significant.     

Ab initio modelling of the structure and redox behaviour of copper(I) bound to a His–His model peptide: relevance to the β-amyloid peptide of Alzheimer’s disease

A contributing factor to the pathology of Alzheimer's disease is the generation of reactive oxygen species, most probably a consequence of the beta-amyloid (Abeta) peptide coordinating copper ions. Experimental and theoretical results indicate that His13 and His14 are the two most firmly established ligands in the coordination sphere of Cu(II) bound to Abeta. Abeta1-42 is known to reduce Cu(II) to Cu(I). The Abeta-Cu(II) complex has been shown to catalytically generate H(2)O(2) from reducing agents and O(2). Cu(II) in the presence of Abeta has been reported to have a formal reduction potential of +0.72-0.77 V (vs. the standard hydrogen electrode). Quantum chemical calculations using the B3LYP hybrid density functional method with the 6-31G(d) basis set were performed to model the reduction of previously studied Cu(II) complexes representing the His13-His14 portion of Abeta (Raffa et al. in J. Biol. Inorg. Chem. 10:887-902, 2005). The effects of solvation were accommodated using the CPCM method. The most stable complex between Cu(I) and the model compound, 3-(5-imidazolyl)propionylhistamine (1) involves tricoordinated Cu(I) in a distorted-T geometry, with the Npi of both imidazoles as well as the oxygen of the backbone carbonyl bound to copper. This model would be the most likely representation of a Cu(I) binding site for a His-His peptide in aqueous solution. A variety of possible redox processes are discussed.     

Dipeptides containing the alpha-aminoisobutyric residue (Aib) as ligands: preparation,spectroscopic studies and crystal structures of copper(II) complexes with H-Aib-X-OH (X=Gly,L-Leu,L-Phe)

Synthetic procedures are described that allow access to new copper(II) complexes with dipeptides containing the alpha-aminoisobutyric residue (Aib) as ligands. The solid complexes [Cu(H(-1)L(A))](n).nH(2)O (1) (L(A)H=H-Aib-Gly-OH), [Cu(H(-1)L(B))(MeOH)](n).nMeOH (2) (L(B)H=H-Aib-L-Leu-OH) and [Cu(H(-1)L(C))](n) (3) (L(C)H=H-Aib-L-Phe-OH) have been isolated and characterized by single-crystal X-ray crystallography, solid-state IR spectra and UV-Vis spectroscopy in solution (H(-1)L(2-) is the dianionic form of the corresponding dipeptide). Complexes 1 and 3 are three-dimensional coordination polymers with similar structures. The doubly deprotonated dipeptide behaves as a N(amino), N(peptide), O(carboxylate), O'(carboxylate), O(peptide) mu(3) ligand and binds to one Cu(II) atom at its amino and peptide nitrogens and at one carboxylate oxygen, to a second metal at the other carboxylate oxygen, while a third Cu(II) atom is attached to the peptide oxygen. The geometry around copper(II) is distorted square pyramidal with the peptide oxygen at the apex of the pyramid. The structure of 2 consists of zigzag polymeric chains, where the doubly deprotonated dipeptide behaves as a N(amino), N(peptide), O(carboxylate), O'(carboxylate) mu(2) ligand. The geometry at copper(II) is square pyramidal with the methanol oxygen at the apex. The IR data are discussed in terms of the nature of bonding and known structures. The UV-Vis spectra show that the solid-state structures of 1, 2 and 3 do not persist in H(2)O.     

Cupric-amyloid beta peptide complex stimulates oxidation of ascorbate and generation of hydroxyl radical

A growing body of evidence supports an important role for oxidative stress in the pathogenesis of Alzheimer's disease. Recently, a number of papers have shown a synergistic neurotoxicity of amyloid beta peptide and cupric ions. We hypothesized that complexes of cupric ions with neurotoxic amyloid beta peptides (Abeta) can stimulate copper-mediated free radical formation. We found that neurotoxic Abeta (1-42), Abeta (1-40), and Abeta (25-35) stimulated copper-mediated oxidation of ascorbate, whereas nontoxic Abeta (40-1) did not. Formation of ascorbate free radical was significantly increased by Abeta (1-42) in the presence of ceruloplasmin. Once cupric ion is reduced to cuprous ion, it can be oxidized by oxygen to generate superoxide radical or it can react with hydrogen peroxide to form hydroxyl radical. Hydrogen peroxide greatly increased the oxidation of cyclic hydroxylamines and ascorbate by cupric-amyloid beta peptide complexes, implying redox cycling of copper ions. Using the spin-trapping technique, we have shown that toxic amyloid beta peptides led to a 4-fold increase in copper-mediated hydroxyl radical formation. We conclude that toxic Abeta peptides do indeed stimulate copper-mediated oxidation of ascorbate and generation of hydroxyl radicals. Therefore, cupric-amyloid beta peptide-stimulated free radical generation may be involved in the pathogenesis of Alzheimer's disease.