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

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. The aim of this study was to test in vitro the ability of S. rosmarinus extract to inhibit the formation of amyloid fibrils in ҡ-CN.

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.

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 κCN, 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.

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.