RESUMEN
Recent findings suggest that the senile plaques in Alzheimer's disease may contain soluble amyloid-ß peptide (Aß) fibril precursors along with insoluble fibrils. These soluble Aß species, including oligomers and protofibrils, have been well-studied in vitro and are formed via non-covalent self-assembly of Aß monomers. While both 40- and 42-residue forms of Aß are observed in the human body, the majority of the Aß aggregation work has been conducted on Aß42 or Aß40 separately, with relatively few investigations of mixtures. In order to study the effect of different combinations of Aß40 and Aß42 on protofibril formation, mixtures of either dry solid peptide, or purified Aß40 and Aß42 monomer solutions were mixed together and protofibril/monomer distributions were quantified. Increases in the Aß42/Aß40 ratio increased protofibril formation but the presence of Aß40 in the mixed Aß solutions had a significant negative impact on protofibril formation compared to equivalent solutions of pure Aß42. Protofibril size was less affected, but ß-sheet structure increased with protofibrils formed from higher Aß42/Aß40 ratio solutions. Direct measurement of Aß42/Aß40 ratios by C-terminal-selective ELISA found very little Aß40 incorporated into protofibrils. The cumulative data emphasizes the critical importance of Aß42, yet establishes Aß40 as a regulator of Aß42 aggregation.