Kinetic Analysis and Starch Digestion Product Composition Reveal the Subtle Relationship between the Anthocyanidin Structure and Inhibitory Activity on Pancreatic α-Amylase

The polyphenols anthocyanins and their aglycones, anthocyanidins, can control postprandial blood glucose through inhibition of α-amylase. In this study, 10 anthocyanins were purified from fruit materials and 6 anthocyanidins were obtained by acid hydrolysis. The identities of these compounds were confirmed by LC-ESI-MS/MS. Inhibition activities of anthocyanidins on α-amylase were higher than those of anthocyanins. The inhibition kinetic assay using 2-chloro-4-nitrophenyl α-maltotrioside and detection of inhibition properties in the composition of α-amylase starch digestion products revealed that an increased number of hydroxyl groups (−OH) in the B-ring promotes the overall inhibition ability of the compound in the enzyme and enhances the competitive inhibition ability. It also increases the inhibition activity on the formation of enzymatic products with degrees of polymerization of 2–3. The mechanism appears to be through increased interaction of the −OHs with key amino acid residues in and around the α-amylase active site, as revealed by molecular docking. Methylation at 3′-OH in the B-ring appears to stabilize the hydrogen bonds between anthocyanidins and the key amino acid residues. However, the simultaneous methylation of 3′ and 5′-OHs caused a decrease in the inhibition activity and transformed the mixed inhibition mode into pure competitive inhibition, which was attributed to steric hindrance.