Interaction of the ginsenosides with κ-casein and their effects on amyloid fibril formation by the protein: Multi-spectroscopic approaches

The interaction of the ginsenosides (GS) including ginsenoside Rg1, Rb1 and Re with K-casein and the effects of GS inhibiting amyloid fibril formation by K-casein have been investigated in vitro by fluorescence and ultraviolet spectra. Results showed that Rg1 and Rb1 had dose-dependent inhibitory effects on reduced and carboxymethylated K-casein (RCMK-CN) fibril formation, while Re resulted in an increase in the rate of fibril formation. The enhancement in RLS intensity was attributed to the formation of new complex between GS and RCMK-CN, and the corresponding thermodynamic parameters (MI, AS and AG) were assayed. The steady-state ultraviolet-visible absorption spectra had also been tested to observe if the ground-state complex formed, and it showed the same result as RLS spectra. The binding constants and the number of binding sites between GS and RCMK-CN at different temperatures had been evaluated from relevant fluorescence data. According to the Forster non-radiation energy transfer theory, the binding distance between RCMK-CN and GS was calculated. The fluorescence lifetime of RCMK-CN was longer in the presence of GS than in absence of GS, which was evident that the hydrophobic interaction plays a major role in the binding of GS to RCMK-CN. From the results of synchronous fluorescence, it could be deduced that the polarity around RCMK-CN Trp97 residue decreased and the hydrophobicity increased after addition of Rg1 or Rb1. Based on all the above results, it is explained that Rg1 and Rb1 inhibited amyloid fibril formation by K-casein because the molecular spatial conformation and physical property of K-casein changed causing by the complex formation between GS and K-casein. (C) 2016 Elsevier B.V. All rights reserved.