Effects of Synergistic Inhibition on α-glucosidase by Phytoalexins in Soybeans.

To determine the mechanism of action of the effects of phytoalexins in soybeans, we analyzed alpha-glucosidase inhibition kinetics using Michaelis-Menten plots and Lineweaver-Burk plots. The results showed that the type of inhibition with glyceollin was competitive, that of genistein was noncompetitive, that of daidzein was uncompetitive, and luteolin showed a mixed mode of action. The K-i values were determined using a Dixon plot as glyceollin, 18.99 mu M; genistein, 15.42 mu M; luteolin, 16.81 mu M; and daidzein, 9.99 mu M. Furthermore, potential synergistic effects between glyceollin and the three polyphenols were investigated. A combination of glyceollin and luteolin at a ratio of 3:7 exhibited synergistic effects on alpha-glucosidase inhibition, having a combination index (CI) of 0.64244, according to the CI-isobologram equation. Collectively, these results showed that a combination of glyceollin and luteolin has the potential to inhibit alpha-glucosidase activity via a synergistic mode of inhibition.