Study on the noncovalent interactions of saikosaponins and cytochrome c by electrospray ionization mass spectrometry.

RATIONALE: The study of interactions between protein and pharmaceutical molecules including natural extracts has become of increasing interest in biological and biomedical research. An investigation of the interaction between saikosaponins and cytochrome c (Cyt c) by electrospray ionization mass spectrometry (ESI-MS) is described in this study. Saikosaponins are found in Bupleurum falcatum (a flowering plant), and they are glycosides that consist of saccharides and the sapogenins of triterpenoids. METHODS: Seven model molecules of saccharides and triterpenes, namely maltose (Mal II), maltotriose (Mal III), raffinose (Raf), and stachyose (Sta), glycyrrhetinic acid (Gly), ursolic acid (Urs) and oleanic acid (Ole), were chosen to perform a series of ESI-MS control experiments for the exploration of the interaction groups in saikosaponins with Cyt c. The dissociation constants of detected noncovalent complexes were determined by using a direct ESI-MS assay. RESULTS: We have observed in the ESI mass spectra the formation of Cyt c complexes with saikosaponins a and c, and these saccharides, with 1:1 and 1:2 stoichiometry. Our results showed that no complex ions of triterpenes and Cyt c were detected in the ESI-MS and similar Kd values were obtained for the Cyt c complexes of saikosaponins and saccharides. This demonstrates that the glycosyl moiety in the saikosaponins is the effective interaction group with Cyt c. We propose that saikosaponins and saccharides interact with Cyt c by hydrogen bonds. The binding affinity of these six ligands with Cyt c is shown to be in the order Ssa > Ssc > Raf, Mal III > Sta ≥ Mal II. CONCLUSIONS: The ESI-MS methodology presented in this study enables us to investigate the interactions of saikosaponins with Cyt c, and allows the direct determination of binding constants. These results could guide further research for providing insights into the structure-binding relationship of ligands with Cyt c.