amyloid-beta-peptides and dityrosine

Copper is involved in Alzheimer's disease (AD) where it appears to affect the aggregation of amyloid-β (Aβ) and to catalyze the production of reactive oxygen species (ROS). Oxidative stress apparently produces Aβ dimers that are covalently linked through two tyrosine residues. Such dityrosine cross-links are considered as potential markers of the disease and seem to be implicated in the pathological disorder. In the present study, pure o,o'-dityrosine (diY) was prepared enzymatically (with horseradish peroxidase; HRP), which was subsequently used to construct calibration lines aimed at quantifying nanomolar amounts of diY in reaction mixtures by fluorescence spectroscopy. Hence, diY concentrations down to 67 nM could be determined, which allowed to find that ca. 3% of dityrosine-bridged dimers of Aβ(1-40) were produced after 3 days at 37 °C in the presence of copper and dihydrogen peroxide. These cross-linked dimers in the presence of copper(II) ions completely inhibit the typical aggregation of Aβ, since β sheets could not be detected applying the usual Thioflavin T (ThT) method. Furthermore, the use of a potent Cu(II) chelator, such as the ATCUN tripeptide, L-histidyl-L-alanyl-L-histidine (HAH), efficiently prevented the copper-mediated generation of ROS and the associated dityrosine-bridged Aβ dimers, suggesting that such metal chelators may find future applications in the field of anti-AD drug design.

Topics: Amyloid beta-Peptides; Armoracia; Calibration; Copper; Horseradish Peroxidase; Limit of Detection; Oligopeptides; Oxidation-Reduction; Peptide Fragments; Protein Multimerization; Spectrometry, Fluorescence; Tyrosine

Alzheimer's disease (AD) is characterised by the formation of amyloid deposits composed primarily of the amyloid beta-peptide (Abeta). This peptide has been shown to bind redox active metals ions such as copper and iron, leading to the production of reactive oxygen species (ROS) and formation of hydrogen peroxide (H(2)O(2)). The generation of H(2)O(2) has been linked with Abeta neurotoxicity and neurodegeneration in AD. Because of the relative stability of a tyrosyl radical, the tyrosine residue (Tyr-10) is believed to be critical to the neurotoxicity of Abeta. This residue has also been shown to be important to Abeta aggregation and amyloid formation. It is possible that the formation of an Abeta tyrosyl radical leads to increased aggregation via the formation of dityrosine as an early aggregation step, which is supported by the identification of dityrosine in amyloid plaque. The role of dityrosine formation in Abeta aggregation and neurotoxicity is as yet undetermined, partly because there are no facile methods for the synthesis of Abeta dimers containing dityrosine. Here we report the use of horseradish peroxidase and H(2)O(2) to dimerise N-acetyl-L-tyrosine ethyl ester and apply the optimised conditions for dityrosine formation to fully unprotected Abeta peptides. We also report a simple fluorescent plate reader method for monitoring Abeta dimerisation via dityrosine formation.

Topics: Amino Acid Sequence; Amyloid beta-Peptides; Chromatography; Dimerization; Electrophoresis; Humans; Hydrogen Peroxide; Molecular Sequence Data; Peptide Fragments; Tyrosine

We have previously reported that amyloid Abeta, the major component of senile plaques in Alzheimer's disease (AD), binds Cu with high affinity via histidine and tyrosine residues [Atwood, C. S., et al. (1998) J. Biol. Chem. 273, 12817-12826; Atwood, C. S., et al. (2000) J. Neurochem. 75, 1219-1233] and produces H(2)O(2) by catalyzing the reduction of Cu(II) or Fe(III) [Huang, X., et al. (1999) Biochemistry 38, 7609-7616; Huang, X., et al. (1999) J. Biol. Chem. 274, 37111-37116]. Incubation with Cu induces the SDS-resistant oligomerization of Abeta [Atwood, C. S., et al. (2000) J. Neurochem. 75, 1219-1233], a feature characteristic of neurotoxic soluble Abeta extracted from the AD brain. Since residues coordinating Cu are most vulnerable to oxidation, we investigated whether modifications of these residues were responsible for Abeta cross-linking. SDS-resistant oligomerization of Abeta caused by incubation with Cu was found to induce a fluorescence signal characteristic of tyrosine cross-linking. Using ESI-MS and a dityrosine specific antibody, we confirmed that Cu(II) (at concentrations lower than that associated with amyloid plaques) induces the generation of dityrosine-cross-linked, SDS-resistant oligomers of human, but not rat, Abeta peptides. The addition of H2O2 strongly promoted Cu-induced dityrosine cross-linking of Abeta1-28, Abeta1-40, and Abeta1-42, suggesting that the oxidative coupling is initiated by interaction of H2O2 with a Cu(II) tyrosinate. The dityrosine modification is significant since it is highly resistant to proteolysis and is known to play a role in increasing structural strength. Given the elevated concentration of Cu in senile plaques, our results suggest that Cu interactions with Abeta could be responsible for causing the covalent cross-linking of Abeta in these structures.

Topics: Alzheimer Disease; Amino Acid Sequence; Amyloid beta-Peptides; Animals; Copper; Cross-Linking Reagents; Humans; Molecular Sequence Data; Oxidation-Reduction; Peptide Fragments; Rats; Sodium Dodecyl Sulfate; Spectrometry, Fluorescence; Spectrometry, Mass, Electrospray Ionization; Tyrosine

Recent reports by Galeazzi and co-workers demonstrated the susceptibility of Abeta(1-42) to undergo dityrosine formation via peroxidase-catalyzed tyrosine cross-linking. We have formed dityrosine cross-links in Abeta(1-40) using these enzymatic conditions as well as a copper-H(2)O(2) method. The efficiency of dityrosine cross-link formation is strongly influenced by the aggregation state of Abeta; more dityrosine is formed when copper-H(2)O(2) or horseradish peroxidase-catalyzed oxidation is applied to fibrillar Abeta vs soluble Abeta. Once formed, dityrosine cross-links are susceptible to further oxidative processes and it appears that cross-links formed in soluble Abeta react through these pathways more readily than those formed in fibrillar Abeta. Because preorganization of fibrils affects the efficiency of dityrosine formation, we examined the effect of dityrosine formation upon local peptide conformation by assessing the solution structure of a small dityrosine dimer derived from Abeta(8-14). Two-dimensional (1)H NMR studies of the short dityrosine dimer offer no evidence of structure. Thus, the fibrillar structure of Abeta enhances formation of dityrosine cross-links, but dityrosine cross-links do not seem to enhance local secondary structure.

Topics: Amyloid beta-Peptides; Antibodies, Monoclonal; Ascorbic Acid; Blotting, Western; Copper; Cross-Linking Reagents; Dimerization; Humans; Hydrogen Peroxide; Magnetic Resonance Spectroscopy; Models, Molecular; Peptide Fragments; Protein Structure, Secondary; Structure-Activity Relationship; Tyrosine

We investigated whether oxidative stress participates in the pathogenic Abeta-induced degenerative mechanism of cultured human cerebrovascular smooth muscle (HCSM) cells, which are intimately involved in cerebral amyloid angiopathy (CAA). Studies using the cell-permeable dye dichlorofluorescein diacetate suggested that free radicals were not robustly detected in HCSM cells exposed to pathogenic Abeta. Furthermore, examination for oxidatively modified proteins, indicated by the presence of dinitrophenylhydrazone and dityrosine moieties, demonstrated no appreciable difference between pathogenic Abeta-treated and untreated HCSM cells. These findings support the notion that pathogenic Abeta-induced toxicity in HCSM cells and neuronal cells occurs by different mechanisms.

Topics: Amyloid beta-Peptides; Cell Death; Cells, Cultured; Cerebral Arteries; Cross-Linking Reagents; Free Radicals; Humans; In Vitro Techniques; Muscle, Smooth, Vascular; Oxidative Stress; Peptide Fragments; Tyrosine