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Probing the kinetics of beta-amyloid self-association.

Spontaneous conversion of beta-amyloid peptide (Abeta) from soluble monomer to insoluble fibrillar precipitate may underlie the neurodegeneration associated with Alzheimer's disease. A complete description of Abeta self-association kinetics requires identification of the oligomeric species present and the pathway of association, as well as quantitation of rate constants and reaction order. Previously published work delineating progress in developing quantitative kinetic models is reviewed briefly. The use of light scattering to probe Abeta fibril size and shape as a function of time and concentration is described. Data describing Abeta self-association at physiological pH starting from the urea-denatured state are presented. High-molecular-weight species formed rapidly, within minutes of dilution. At 70 microM, the size of the major fibrillar species was virtually constant with time, whereas at 280 microM two populations were detected, each of which increased in size with time. Incipient precipitation, prior to the appearance of a visible solid phase, was detected reliably by dynamic light scattering. Surprisingly, precipitates appeared earliest at the lowest concentration, and the physical state of the precipitate changed markedly with concentration. A gel diffusion assay showed that monomer Abeta dissociates from preformed fibrils, indicating that reversible heterogeneous association must be incorporated into any realistic model of Abeta association.