Investigating the inhibitory effects of Seidlitzia rosmarinus extract on the amyloid fibril formation of ҡ-casein in the presence of dextran

Background: Formation of amyloid fibrils has been associated with different protein aggregation diseases. Many studies indicate that many proteins can be converted in vitro into amyloid structures. Isolated ҡ-casein (ҡ-CN) spontaneously forms amyloid fibrils under physiological conditions, so it is a convenient model for researching generic aspect of fibril formation. Materials and Methods: In this study the effect of aqueous extract of S. rosmarinus on the amyloid formation of ҡ-CN in the presence and absence of crowding agent, dextran, have been examined using Thioflavin T binding (ThT) assay, fluorescence spectroscopy, and circular dichroism (CD) spectroscopy. Results: ThT binding assay showed that dextran increased the rate of amyloid fibril formation and S. rosmarinus extract retarded the amyloid fibril formation in κ-CN. In the presence of dextran however, the effect of   S. rosmarinus extract on the amyloid formation of ҡ-CN was less than in its absence. Fluorescence spectroscopy results also demonstrated that dextran led to unfolding and increased the exposure hydrophobic area in ҡ-CN. S. rosmarinus extract efficiency decreased the exposure of hydrophobic regions in κ-CN, whereas in the presence of dextran this effect of extract was reduced. CD spectroscopy results exhibited that incubation of κ-CN with S. rosmarinus extract prevented a structural transition to a β-sheet. CD spectroscopy results also indicated that by adding dextran to reduced κ-CN β-sheet structures observed, which indicates structural change. S. rosmarinus extract however, prevented transition to β-sheet structural. Conclusion: In conclusion our finding suggests that S. rosmarinus extract prevents amyloid fibril formation in κ-CN, although this effect decreased in the presence of dextran.