Stoichiometry and Affinity of the Human Serum Albumin-Alzheimer's Aβ Peptide Interactions

A promising strategy to control the aggregation of the Alzheimer's A beta peptide in the brain is the clearance of A beta from the central nervous system into the peripheral blood plasma. Among plasma proteins, human serum albumin plays a critical role in the A beta clearance to the peripheral sink by binding to A beta oligomers and preventing further growth into fibrils. However, the stoichiometry and the affinities of the albumin-A beta oligomer interactions are still to be fully characterized. For this purpose, here we investigate the A beta oligomer-albumin complexes through a novel and generally applicable experimental strategy combining saturation transfer and off-resonance relaxation NMR experiments with ultrafiltration, domain deletions, and dynamic light scattering. Our results show that the A beta oligomers are recognized by albumin through sites that are evenly partitioned across the three albumin domains and that bind the A beta oligomers with similar dissociation constants in the 1-100 nM range, as assessed based on a Scatchard-like model of the albumin inhibition isotherms. Our data not only explain why albumin is able to inhibit amyloid formation at physiological nM A beta concentrations, but are also consistent with the presence of a single high affinity albumin-binding site per A beta protofibril, which avoids the formation of extended insoluble aggregates.