Evaluation of α-glucosidase inhibitory ability: Insights from free and bound polyphenols in five grain brans

The research systematically evaluated the α-glucosidase inhibitory ability of bound and free polyphenols extracted from five different types of grain bran. The findings revealed that both forms of polyphenols exhibited certain functional advantages, with the sorghum bran free polyphenols (SBFPs) showing the most outstanding overall performance (IC50 value: 0.683 ± 0.024 μg/mL). UPLC-Q-TOF-MS analysis revealed changes in the phenolic profiles between free and bound polyphenols from different bran types, with SBFPs being the most compositionally diverse, identifying 32 components. In addition, through spectroscopy (circular dichroism and fluorescence spectroscopy), enzyme inhibition kinetics, and computer simulation of molecular docking to explore the inhibitory mechanisms of SBFPs on α-glucosidase. The results indicated that SBFPs interact reversibly with α-glucosidase primarily via a mixed inhibition form dominated by competitive inhibition. Circular dichroism and fluorescence spectroscopy results demonstrated that SBFPs statically quench the fluorescence intensity of α-glucosidase and induce alterations in the secondary structure of enzyme protein, leading to decreased α-helix content. Molecular docking results of the 32 identified components indicated that naringenin, cyanidin, glycitin, and apigenin had the lowest binding energies with α-glucosidase at -7.38, -7.25, -7.06, and - 6.91 kcal/mol, respectively, primarily interacting with the enzyme through hydrophobic interactions and hydrogen bonds. The study highlights the excellent α-glucosidase inhibitory effects of grain bran polyphenols, particularly SBFPs providing a valuable reference for applying bran as a functional ingredient in the design of slow glucose-release or low GI foods.