2؉ or Zn 2؉ to A␤ in a negatively charged lipid environment caused a conformational change from ␤-sheet to ␣-helix, accompanied by peptide oligomerization and membrane penetration. These results suggest that metal binding to A␤ generated an allosterically ordered membrane-penetrating oligomer linked by superoxide dismutase-like bridging histidine residues.

Dopamine (DA) and alpha-synuclein (alpha-SN) are two key molecules associated with Parkinson's disease (PD). We have identified a novel action of DA in the initial phase of alpha-SN aggregation and demonstrate that DA induces alpha-SN to form soluble, SDS-resistant oligomers. The DA:alpha-SN oligomeric species are not amyloidogenic as they do not react with thioflavin T and lack the typical amyloid fibril structures as visualized with electron microscopy. Circular dichroism studies indicate that in the presence of lipid membranes DA interacts with alpha-SN, causing an alteration to the structure of the protein. Furthermore, DA inhibited the formation of iron-induced alpha-SN amyloidogenic aggregates, suggesting that DA acts as a dominant modulator of alpha-SN aggregation. These observations support the paradigm emerging for other neurodegenerative diseases that the toxic species is represented by a soluble oligomer and not the insoluble fibril.

The interaction of A␤ peptides with the lipid matrix of neuronal cell membranes plays an important role in the pathogenesis of Alzheimer's disease. By using EPR and CD spectroscopy, we found that in the presence of Cu 2؉ or Zn 2؉ , pH, cholesterol, and the length of the peptide chain influenced the interaction of these peptides with lipid bilayers. In the presence of Zn 2؉ , A␤40 and A␤42 both inserted into the bilayer over the pH range 5.5-7.5, as did A␤42 in the presence of Cu 2؉ . However, A␤40 only penetrated the lipid bilayer in the presence of Cu 2؉ at pH 5.5-6.5; at higher pH there was a change in the Cu 2؉ coordination sphere that inhibited membrane insertion. In the absence of the metals, insertion of both peptides only occurred at pH < 5.5. Raising cholesterol to 0.2 mol fraction of the total lipid inhibited insertion of both peptides under all conditions investigated. Membrane insertion was accompanied by the formation of ␣-helical structures. The nature of these structures was the same irrespective of the conditions used, indicating a single low energy structure for A␤ in membranes. Peptides that did not insert into the membrane formed ␤-sheet structures on the surface of the lipid. Alzheimer's disease (AD)1 is a neurodegenerative disorder affecting the memory and cognitive functions of the brain. A characteristic central nervous system histologic marker in patients with AD is accumulation of the 39 -43-residue amyloid ␤-peptide (A␤) in morphologically heterogeneous neuritic plaques and cerebrovascular deposits (1). A␤ is derived from a proteolytic cleavage of the ␤-amyloid precursor protein (APP). This is a highly conserved and widely expressed integral membrane protein with a single membrane-spanning domain. Possible mechanisms of A␤ toxicity include formation of plasma membrane channels (2) and generation of H 2 O 2 through Cu 2ϩ reduction by the peptide, which make cells more responsive to oxidative stress (3). The mechanisms may be related because reactive oxygen species may cause lipid peroxidation that leads to alterations in the order and fluidity of the bilayer lipids (4) and may also lead to an increase of membrane permeability. The common change observed in cell membrane permeability is an increased intracellular calcium level (5, 6) that could occur either indirectly through A␤ modulating an existing Ca 2ϩ channel or directly through cation-selective channels formed by A␤.The supramolecular structure of membrane-associated A␤, either as an ion channel or a fusogen, is unknown, although Durell et al. (7) have developed theoretical models for the structure of ion channels formed by the membrane-bound A␤40. Recently, Bhatia et al. (8) and Lin et al. (9) used atomic force microscopy, laser confocal microscopy, and fluorescent calcium imaging to examine in real time the acute effects of fresh and globular A␤40, A␤42, and A␤25-35 on cultured endothelial cells. A␤ peptides caused morphological changes within minutes after treatment and led to eventual cellular degeneration. Cellular morphol...

The amyloid ␤ peptide is toxic to neurons, and it is believed that this toxicity plays a central role in the progression of Alzheimer's disease. The mechanism of this toxicity is contentious. Here we report that an A␤ peptide with the sulfur atom of Met-35 oxidized to a sulfoxide (Met(O)A␤) is toxic to neuronal cells, and this toxicity is attenuated by the metal chelator clioquinol and completely rescued by catalase implicating the same toxicity mechanism as reduced A␤. However, unlike the unoxidized peptide, Met(O)A␤ is unable to penetrate lipid membranes to form ion channel-like structures, and ␤-sheet formation is inhibited, phenomena that are central to some theories for A␤ toxicity. Our results show that, like the unoxidized peptide, Met(O)A␤ will coordinate Cu 2؉ and reduce the oxidation state of the metal and still produce H 2 O 2 . We hypothesize that Met(O)A␤ production contributes to the elevation of soluble A␤ seen in the brain in Alzheimer's disease.Genetic evidence from early onset cases of Alzheimer's disease indicates that metabolism of the ␤-amyloid peptide (A␤)