Evidence that the beta-amyloid plaques of Alzheimer's disease represent the redox-silencing and entombment of A beta by zinc

A beta binds Zn2+, Cu2+ and Fe3+ in vitro, and these metals are markedly el evated in the neocortex and especially enriched in amyloid plaque deposits of individuals with Alzheimer's disease (AD). Zn2+ precipitates A beta in v itro, and Cu2+ interaction with A beta promotes its neurotoxicity, correlat ing with metal reduction and the cell-free generation of H2O2 (A beta 1-42 > A beta 1-40 > ratA beta 1-40). Because Zn2+ is redox-inert, we studied th e possibility that it may play an inhibitory role in H2O2-mediated A beta t oxicity. In competition to the cytotoxic potentiation caused by coincubatio n with Cu2+, Zn2+ rescued primary cortical and human embryonic kidney 293 c ells that were exposed to A beta 1-42, correlating with the effect of Zn2in suppressing Cu2+-dependent H2O2 formation from A beta 1-42. Since plaque s contain exceptionally high concentrations of Zn2+, we examined the relati onship between oxidation (8-OH guanosine) levels in AD-affected tissue and histological amyloid burden and found a significant negative correlation. T hese data suggest a protective role for Zn2+ in AD, where plaques form as t he result of a more robust Zn2+ antioxidant response to the underlying oxid ative attack.