The Inhibition of Pancreatic α-Amylase by Monomeric, Dimeric and Trimeric Procyanidins Is Dependent upon the Structural Characteristics of Inhibitors and Substrates

Procyanidins are oligomeric flavonoids with several bioactive properties. Their antidiabetic potential is related to their capacity to inhibit enzymes responsible for the absorption of dietary carbohydrates, such as pancreatic α-amylase. Procyanidins possess great structural diversity, including types of monomers and interflavanic bonds (A- or B-), and the degree of polymerization. However, there is a lack of evidence that systematically analyzes the effect of these structural features on their α-amylase inhibitory activity. In this paper, the activity of a mammalian pancreatic α-amylase was assessed using two different substrates, and the inhibitory activity of five commercially available procyanidins and three monomeric flavonoids was compared. The enzyme-binding sites of the eight compounds were predicted by in silico analysis to help explain the different enzyme-inhibitory activities. The inhibitory activity of procyanidins and monomeric flavonoids depended on the substrate used. A-type dimers presented the best activity against a polymeric substrate, while a B-type dimer was the best inhibitor for an oligomeric substrate. The predicted binding site for dimers and monomers was close to the active site. For the B-type trimer, the binding site was on the back side (approximately 180°) of the catalytic triad. In silico predictions suggested that dimeric procyanidins, especially A-type, could better enter the active site cavity, which could explain their superior inhibitory activity. We may conclude that inhibition of pancreatic α-amylase by procyanidins is mainly related to the type of interflavanic bond and the degree of polymerization. Dimers could be the most effective procyanidins to mildly inhibit this enzyme and present antidiabetic potential.