ZINC-INDUCED AGGREGATION OF HUMAN AND RAT BETA-AMYLOID PEPTIDES IN-VITRO

The major pathological feature of Alzheimer's disease is the presence of a high density of amyloid plaques in the brain tissue of patients, The plaques are predominantly composed of human beta-amyloid peptide ( A beta), a 39-43-mer peptide the neurotoxicity of which is related to its aggregation state. Previous work has demonstrated that certain met als that have been implicated as risk factors for Alzheimer's disease (Al, Fe, and Zn) also cause substantial aggregation of A beta. In part icular, we reported that zinc cations at concentrations of >10(-4) M d ramatically accelerate the rate of A beta aggregation at physiological peptide concentrations at 37 degrees C in vitro. In the present study , we investigate the effect of Zn2+ on aggregation of radiolabeled and unlabeled human and rat A beta over a wide range of peptide concentra tions in the presence and absence of salt and blocking protein. Aggreg ation was assayed by centrifugation and filtration using amino acid an alysis, immunoassay, and gamma-counting for quantification over a wide range of concentrations of Zn2+ and A beta above and below physiologi cal values. The results of this study demonstrate the following: (a) R adio-iodinated A beta accurately tracked unlabeled A beta, (b) zinc co ncentrations of at least 10(-4) M were required to induce significant aggregation of A beta, and (c) rat and human A beta species were clear ed from aqueous solutions by similar concentrations of zinc. These res ults stand in significant quantitative disagreement (similar to 100-fo ld in zinc concentration) with one previous study that reported signif icant aggregation of A beta by <1 mu M Zn2+. Differences between the p resent study and the latter study from another laboratory appear to re sult from inappropriate reliance on optical density to measure A beta concentrations and nonspecific loss of A beta to plastic in the absenc e of blocking protein.