A 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)_8 significantly 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 Aβ_<1-42> using Aβ-specific antibody followed by near-infrared scanning demonstrated binding of ZnPc(COONa)_8 to Aβ_<1-42>. A study using the hydrophobic fluorescent probe 8-anilino-1-naphthalenesulfonic acid showed that ZnPc(COONa)_8 decreased the hydrophobicity during Aβ_<1-42> fibril formation. CD spectroscopy showed an increase in the α helix structure and a decrease in the β sheet structure of Aβ_<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 Aβ_<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 Aβ_<1-42>-induced toxicity. Overall, our results indicate that ZnPc(COONa)_8 binds to Aβ and decreases the hydrophobicity, and this change is unfavorable for Aβ oligomerization and fibril formation.