A carboxylated Zn-phthalocyanine inhibits fibril formation of Alzheimer's amyloid β peptide

Amyloid (A), a 39-42 amino acid peptide derived from amyloid precursor protein, is deposited as fibrils in Alzheimer's disease brains, and is considered to play a major role in the pathogenesis of the disease. We have investigated the effects of a water-soluble Zn-phthalocyanine, ZnPc(COONa)(8), a macrocyclic compound with near-infrared optical properties, on A fibril formation invitro. A thioflavinT fluorescence assay showed that ZnPc(COONa)(8) significantly inhibited A fibril formation, increasing the lag time and dose-dependently decreasing the plateau level of fibril formation. Moreover, it destabilized pre-formed A fibrils, resulting in an increase in low-molecular-weight species. After fibril formation in the presence of ZnPc(COONa)(8), immunoprecipitation of A(1-42) using A-specific antibody followed by near-infrared scanning demonstrated binding of ZnPc(COONa)(8) to A(1-42). A study using the hydrophobic fluorescent probe 8-anilino-1-naphthalenesulfonic acid showed that ZnPc(COONa)(8) decreased the hydrophobicity during A(1-42) fibril formation. CD spectroscopy showed an increase in the helix structure and a decrease in the sheet structure of A(1-40) in fibril-forming buffer containing ZnPc(COONa)(8). SDS/PAGE and a dot-blot immunoassay showed that ZnPc(COONa)(8) delayed the disappearance of low-molecular-weight species and the appearance of higher-molecular-weight oligomeric species of A(1-42). A cell viability assay showed that ZnPc(COONa)(8) was not toxic to a neuronal cell line (A1), but instead protected A1 cells against A(1-42)-induced toxicity. Overall, our results indicate that ZnPc(COONa)(8) binds to A and decreases the hydrophobicity, and this change is unfavorable for A oligomerization and fibril formation.