Combined Inhibitory Effects of Procyanidins and Protocatechuic Acid on Starch Digestive Enzymes

Starch is the main source of energy for the human body. Natural products such as procyanidin (PC) and protocatechuic acid (PCA) can reduce postp'ran'dial blood sugar by controlling the digestion of starch. The inhibition effects of PC and PCA on α-amylase and α-glucosidase were studied using inhibition rate determination, Lineweaver-Burk double reciprocal diagram, ultraviolet spectroscopy, fluorescence quenching, equivalent mapping, and Fourier transform infrared spectroscopy. The results showed that the IC₅₀ values of PC for α-amylase and α-glucosidase inhibition were 0.0524 and 0.0106 mg/mL, respectively, and those of PCA were 1.9426 and 1.0667 mg/mL respectively, which showed that PCA had better inhibition ability. The Lineweaver-Burk double reciprocal diagram showed that the inhibition of PC and PCA on α-amylase was a mixed inhibition, and the inhibition on α-glucosidase was non-competitive inhibition and competitive inhibition, respectively. The Binding of PC and PCA with α-amylase and α-glucosidase mainly depended on hydrophobic and hydrogen bonds, and further altered the enzyme structure by changing the microenvironment of amino acid residues. The inhibitory effects on α-glucosidase were the best when the ratio of PC and PCA was 1:21, and the inhibitory effects on α-amylase were optimal at 1:37. When the ratio of PC and PCA was 1:21, CI was less than 1, which showed a synergistic inhibitory effect on α-glucosidase. When the compounding ratio was 1:37, CI was less than 1, which showed a synergistic inhibitory effect on α-amylase. This study revealed the inhibitory mechanism of the combined use of PC and PCA on starch-digesting enzymes and would provide a theoretical basis for the development of natural hypoglycemic dietary supplements as health food excipients or drugs.