Mitochondrial dysfunction is a hallmark of beta-amyloid (Abeta)-induced neuronal toxicity in Alzheimer's disease (AD). Here, we demonstrate that Abeta-binding alcohol dehydrogenase (ABAD) is a direct molecular link from Abeta to mitochondrial toxicity. Abeta interacts with ABAD in the mitochondria of AD patients and transgenic mice. The crystal structure of Abeta-bound ABAD shows substantial deformation of the active site that prevents nicotinamide adenine dinucleotide (NAD) binding. An ABAD peptide specifically inhibits ABAD-Abeta interaction and suppresses Abeta-induced apoptosis and free-radical generation in neurons. Transgenic mice overexpressing ABAD in an Abeta-rich environment manifest exaggerated neuronal oxidative stress and impaired memory. These data suggest that the ABAD-Abeta interaction may be a therapeutic target in AD.

In the course of analyzing the chemical composition of Alzheimer's disease neuritic and vascular amyloid, we have purified stable dimeric and trimeric components of A␤ peptides. These peptides (molecular mass 9.0 and 13.5 kDa) were separated by size exclusion chromatography in the presence of 80% formic acid or 5 M guanidine thiocyanate, pH 7.4. The average ratio of monomers, dimers, and trimers was 55:30:15, respectively. Similar structures were produced over time upon incubation of synthetic A␤-(1-42) at pH 7.4. The stability of these oligomeric forms was also demonstrated by Western blot and mass spectrometry. Atomic force microscopy and electron microscopy rotary shadowing revealed that the monomers polymerized into 8 -10-nm filaments, whereas the dimers generated prolate ellipsoids measuring 3-4 nm in diameter. Although evidence implicates ␤-amyloid peptide (A␤) in the pathogenesis of Alzheimer's disease (AD) 1 (reviewed in Ref. 1), little is known about the nature of the A␤ mediating the pathology. Toxicity initially was attributed to aggregated A␤ in amyloid plaques (1), the morphological hallmarks of AD brains. A␤-(1-42) is the major peptide constituent of amyloid plaques (2), and increased production of the 42-amino acid peptide correlates with an earlier onset of AD (1). However, recent studies show that small quantities of A␤-(1-42) also exists as soluble peptide in the plasma, cerebrospinal fluid, and cerebral cortex of AD and normal individuals and are also secreted by cells in tissue culture (3-13). Utilizing ultracentrifugation, graded membrane filtration, and ELISA, we have recently isolated and quantitated the oligomeric water-soluble A␤ present in the brains of AD and control individuals (13). The levels of insoluble A␤ in AD brains are at least 100 times higher than those found in control brains. The amounts of water-soluble A␤ in AD brains are approximately six times higher than those detected in control brains. Interestingly, we isolated an A␤ fraction, from the A␤ water-soluble oligomeric pool, with a molecular mass of Ͻ10 kDa containing monomeric and/or dimeric forms of A␤ peptide (13). In all probability these peptides represent the initial building blocks that may ultimately aggregate into insoluble A␤ filaments. In the course of analyzing the chemical composition of AD neuritic plaque and vascular amyloid, we have purified stable dimeric and trimeric components of A␤-(1-40/42) (2, 14 -15). In the present study we report the chemical and morphological characteristics of the dimeric A␤ as elucidated by atomic force microscopy and transmission electron microscopy techniques. In addition, the potential for toxicity of the AD brain-derived A␤-(1-40/42) dimer was assessed on glial-neuronal cell cultures. MATERIALS AND METHODSPurification of A␤-(1-42) from AD Brain-Brains were obtained from eight patients who died of AD (postmortem delay 3-6 h). After separation of the leptomeninges, the right hemispheres were frozen at Ϫ70°C. Examination of the left hemispheres revealed numerous neuritic plaq...