α-Glucosidase inhibitory peptides from the enzymolysis of Semen Ziziphi Spinosae protein using an ultrasound-assisted protease: Preparation and inhibitory mechanism.

Peptides are promising sources of safe hypoglycemic drugs. The potential of Semen Ziziphi Spinosae protein (SZSP) as a natural source of α-glucosidase inhibitory peptides was investigated. SZSP was hydrolyzed using an ultrasound-assisted protease, and the four α-glucosidase inhibitory peptides were purified, identified, and screened. Their inhibitory mechanisms were investigated using molecular docking. Ultrasound-assisted enzymolysis enhanced the α-glucosidase inhibition and protein conversion rates, which cleaved the protein into small molecules. Fourier transform infrared spectroscopy results showed that the protease hydrolysis tended to transform α-helicals into β-sheets. A purification, identification, and screening process finally identified four α-glucosidase inhibitory peptides. The IC values of LPLLDK, PRLPEM, LPWK, and FPPR were 120.36 ± 6.73, 139.50 ± 7.21, 248.12 ± 10.27, and 106.67 ± 3.22 μM, respectively. Lineweaver-Burk analyses demonstrated that FPPR was a competitive inhibitor of -glucosidase, while LPLLDK and LPWK exhibited a mixed inhibition mechanism and PRLPEM was a non-competitive inhibitor. Molecular docking studies indicated that polypeptides occupy the active pockets of -glucosidase through hydrogen bonding, hydrophobic interactions, and salt Bridges, preventing -glucosidase from forming complexes with the substrate or non-competitive binding to other sites to form enzyme-substrate inhibitors to inhibit enzyme-substrate intermediates and prevent the release of catalytic reaction products. These results demonstrate that the peptides extracted from SZSP may be beneficial for the treatment of diabetes.