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

Amyloid β (Aβ), 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 Aβ fibril formation in vitro. A thioflavin T fluorescence assay showed that ZnPc(COONa)₈significantly inhibited Aβ fibril formation, increasing the lag time and dose‐dependently decreasing the plateau level of fibril formation. Moreover, it destabilized pre‐formed Aβ fibrils, resulting in an increase in low‐molecular‐weight species. After fibril formation in the presence of ZnPc(COONa)₈, immunoprecipitation of Aβ₁‐₄₂using Aβ‐specific antibody followed by near‐infrared scanning demonstrated binding of ZnPc(COONa)₈to Aβ₁‐₄₂. A study using the hydrophobic fluorescent probe 8‐anilino‐1‐naphthalenesulfonic acid showed that ZnPc(COONa)₈decreased the hydrophobicity during Aβ₁‐₄₂fibril formation. CD spectroscopy showed an increase in the α helix structure and a decrease in the β sheet structure of Aβ₁‐₄₀in fibril‐forming buffer containing ZnPc(COONa)₈. SDS/PAGE and a dot‐blot immunoassay showed that ZnPc(COONa)₈delayed the disappearance of low‐molecular‐weight species and the appearance of higher‐molecular‐weight oligomeric species of Aβ₁‐₄₂. A cell viability assay showed that ZnPc(COONa)₈was not toxic to a neuronal cell line (A1), but instead protected A1 cells against Aβ₁‐₄₂‐induced toxicity. Overall, our results indicate that ZnPc(COONa)₈binds to Aβ and decreases the hydrophobicity, and this change is unfavorable for Aβ oligomerization and fibril formation.