Novel Insights into the Inhibitory Effect and Mechanism of Proanthocyanidins from Pyracantha fortuneana Fruit on α-Glucosidase

Proanthocyanidins were extracted from Pyracantha fortuneana fruit (PFF), and their structures were investigated through C-13 nuclear magnetic resonance (C-13 NMR), high performance liquid chromatography (HPLC) and high resolution mass spectrometry (HRMS). The results showed that these compounds were predominantly constituted of procyanidin with A-type and B-type linkage and coexistence of procyanidins glucoside. Spectroscopy methods were used to analyze the inhibitory activity of proanthocyanidins on alpha-glucosidase. The results demonstrated that these compounds exhibited excellent inhibitory effect on alpha-glucosidase with the IC50 value of 0.15 +/- 0.01 mu g/mL, and they reversibly inhibited alpha-glucosidase in a non-competitive type. The fluorescence quenching analysis revealed that proanthocyanidins statically quenched the fluorescence spectra by forming an inhibitor-alpha-glucosidase complex. Molecular docking results further indicated that the driving powers of the interaction between proanthocyanidins and alpha-glucosidase were hydrogen bonds and hydrophobic force. The main inhibitory mechanism of proanthocyanidins on alpha-glucosidase may be due to the insertion of proanthocyanidins into the pocket of the enzyme altering the catalytic configuration of the active site in a manner, thus reducing substrate binding affinity. The findings of this work provided a new perspective that proanthocyanidins from PFF with a possibility to be used as novel natural anti-diabetic agents in functional food industries.