ON THE NUCLEATION AND GROWTH OF AMYLOID BETA-PROTEIN FIBRILS - DETECTION OF NUCLEI AND QUANTITATION OF RATE CONSTANTS

We have studied the fibrillogenesis of synthetic amyloid beta-protein- (1-40) fragment (A beta) in 0.1 M HCl, At low pH, A beta formed fibril s at a rate amenable to detailed monitoring by quasi-elastic light-sca ttering spectroscopy. Examination of the fibrils with circular dichroi sm spectroscopy and electron microscopy showed them to be highly simil ar to those found in amyloid plaques. We determined the hydrodynamic r adii of A beta aggregates during the entire process of fibril nucleati on and growth. Above an A beta concentration of approximate to 0.1 mM, the initial rate of elongation and the final size of fibrils were ind ependent of A beta concentration. Below an A beta concentration of 0.1 mM, the initial elongation rate was proportional to the peptide conce ntration, and the resulting fibrils were significantly longer than tho se formed at higher concentration. We also found that the surfactant n -dodecylhexaoxyethylene glycol monoether (C(12)E(6)) slowed nucleation and elongation of fibrils in a concentration-dependent manner. Our ob servations are consistent with a model of A beta fibrillogenesis that includes the following key steps: (i) peptide micelles form above a ce rtain critical A beta concentration, (ii) fibrils nucleate within thes e micelles or on heterogeneous nuclei (seeds), and (iii) fibrils grow by irreversible binding of monomers to fibril ends, Interpretation of our data enabled us to determine the sizes of fibril nuclei and A beta micelles and the rates of fibril nucleation (from micelles) and fibri l elongation, Our approach provides a powerful means for the quantitat ive assay of A beta fibrillogenesis.