Complement C1q does not bind monomeric beta-amyloid.
The tendency of both labeled and unlabeled beta-amyloid to bind in solution to C1q, the recognition species in the complement cascade, was examined using both hydrodynamic and spectroscopic methods. Potential binding interactions were evaluated using a purified synthetic beta-amyloid 1-40 sequence, alone, and selectively labeled at the amino terminus with spectroscopic probes. The probes permitted both absorbance and fluorescence analyses of beta-amyloid binding interactions. Under conditions used for the analyses beta-amyloid exists exclusively as a monomer in solution, and C1q retains an intact quaternary structure and is capable of binding to IgM. When mixed together the monomeric beta-amyloid does not bind to, or interact with, the complement C1q at concentrations below approximately 100 microM. The data suggest that if beta-amyloid toxicity is associated with complement activation in Alzheimer's disease then monomeric beta-amyloid is likely not responsible for activation through the classical complement pathway.