Brazilin inhibits the Zn2+-mediated aggregation of amyloid β-protein and alleviates cytotoxicity

Interactions of Zn2+ with amyloid beta-protein (A beta) and the subsequent induction of A beta aggregation have been implicated in the pathogenesis of Alzheimer's disease (AD). The development of small-compound inhibitors against Zn2+-mediated A beta aggregation is therefore greatly desired. In this study, brazilin was used to inhibit Zn2+-mediated A beta aggregation and alleviate its cytotoxicity. The binding properties of brazilin and Zn2+ were first probed using Fourier transform infrared (FTIR) spectroscopy and isothermal titration calorimetry (ITC) assays. Both the FTIR and ITC results have shown that brazilin is able to bind Zn2+ in a physiologically suitable range of concentrations. The dissociation constant (K-d) between brazilin and Zn2+ was about 46.0 +/- 6.8 mu M, which makes brazilin a potential drug model for the chelation of free Zn2+. Moreover, the higher affinity of brazilin for A beta(42) (K-d = 2.5 +/- 1.6 mu M) than that of Zn2+ (K-d = 6.2 +/- 0.9 mu M), enables brazilin to sequester Zn2+ from the A beta(42)-Zn2+ complex. In addition, the inhibitory effects of brazilin on Zn2+-mediated A beta aggregation were examined using the Thioflavin T fluorescence assay, transmission electron microscopy and cytotoxicity assays. It was found that brazilin showed remarkable inhibitory capability against Zn2+-induced aggregation of A beta(42). Furthermore, the Zn2+-mediated cytotoxicity of A beta(42) was also largely mitigated under the influence of brazilin. This study therefore provides further insights into the role of Zn2+ in the A beta(42) aggregation pathway, indicating potential new strategies for the design of small compounds with therapeutic potential for AD.