Soybean phytochemicals responsible for bacterial neuraminidase inhibition and their characterization by UPLC-ESI-TOF/MS

Ethanol extract of soybean (Glycine max (L.) Merr.) showed good inhibitory activity against bacterial neuraminidase (BNA), which plays a pivotal role in the pathogenesis of a number of microbial diseases. The saponin portion fractionated through preparative HPLC (IC50 = 2.25 mu g mL(-1)) was found to be responsible for the observed BNA inhibition. Estimation of the inhibitory effects by individual compounds showed that the soyasaponins of group B (Ba, Bb, Bb ', Bc, and Bd) exhibited extremely high inhibitions (IC50 = 0.25-0.48 mu M), whereas group A (Aa, Ab, and Ac) was almost inactive. Kinetic studies determined that group B soyasaponins were noncompetitive inhibitors. Furthermore, molecular docking experiments confirmed that soyasaponin Ba (group B) could undergo binding interactions with various residues in the binding pocket. In contrast, soyasaponin Aa (group A) failed to enter the binding pocket due to its extra scaffold structure of oligosaccharides bonded to the 22-hydroxyl position. The metabolites in the soybean extract were fully characterized using UPLC-ESI-TOF/MS.