2.8. Assay for α-Glucosidase Inhibition

The yeast α-glucosidase (G0660, Sigma-Aldrich) inhibition assay was performed using the substrate p-nitrophenyl-α-D-glucopyranoside according to the previously described method [21–23]. Briefly, samples and acarbose were prepared by dissolving at 2 mg/mL (with extracts) or 0.8 mM (with pure compound) in DMSO, and 0.5 U/mL α-glucosidase (40 mL) was mixed in 120 mL of 0.1 M phosphate buffer (pH 6.8). After 5 min preincubation, 5 mM p-nitrophenyl-α-D-glucopyranoside solution (40 mL) was added, and the solution was incubated at 37°C for 30 min. Pipette the following reagents into a 96-well plate. Each concentration of samples was carried out in triplicate. The absorbance of released 4-nitrophenol was measured at 405 nm by using a microplate reader (xMark, Bio-Rad, USA). Acarbose was used as the positive control. The inhibitory activity was calculated by the following equation: α-glucosidase inhibitory activity (%) = (1–A/A0) × 100, where A is the absorbance of the sample and A0 is the absorbance of the blank, respectively. The IC₅₀ value was calculated by GraphPad Prism.