2.8. Determination of α‐glucosidase inhibition activity

The method of determination was based on the test condition of literature (Mcdougall et al., ²⁰⁰⁵) with slight adjustment. The reaction was performed in 0.1 M phosphate buffer (pH 6.8) with a final volume of 250 μl. In brief, 50 μl of sample solution and 50 μl of 20 /mL α‐glucosidase were added to the wells of microplate. While they were shaken properly and pre‐incubated for 15 min at 37℃, 50 μl of 10 mM pNPG solution was added and mixed thoroughly to start the reaction for 1 hr at 37℃, and then, the reaction was stopped by the addition of 100 μl 0.2 mM Na₂CO₃. Since pNPG can be hydrolyzed to produce glucose and pNP under the action of α‐glucosidase, the released pNP has a maximum absorption at 405 nm, and the absorbance was measured using a microplate reader. The α‐glucosidase inhibition rate of each sample was calculated according to the formula:

where I is the α‐glucosidase inhibition; A _S is the absorbance of the sample; A _SB is the absorbance of the sample blank; A _C is the absorbance of the control; and A _B is the absorbance of the blank.