α-Glucosidase (EC 3.2.1.20) is an enzyme that plays a key role in raising blood sugar levels and it is a potential target for designing agents to treat diabetes mellitus. The objective of this study was to investigate the inhibition of α-glucosidase by twenty two isoflavone and soyasaponin derivatives from soybeans (Glycine max L.). Among them, daidzein and genistein inhibited α-glucosidase with an IC50 value of 13.7±2.5 and 8.8±2.6 μM, respectively. The inhibition kinetics analyzed by a Dixon plot indicate that daidzein and genistein are noncompetitive inhibitors, and the inhibition constant Ki of daidzein and genistein was calculated to be 14.5 and 10.1 μM, respectively. However, neither glucosides nor malonyl glucosides inhibited α-glucosidase at up to 100 μM. Moreover, soyasaponin Ba and soyasaponin Bb' showed a significant inhibitory effect with the IC50 of ca. 25 and 32 μM through simple reversible slow-binding inhibition. By heat treatment (100oC, 20 min), the contents of genistein, daidzein, soyasaponin Ba, soyasaponin Bb and soyasaponin Bb' significantly increased, but the amount of malonyl-isoflavone glycosides and DDMP soyasaponin decreased drastically.

Keywords: soybean, α-glucosidase, inhibition, isoflavone, soyasaponin