Inhibitory mechanism of two allosteric inhibitors, oleanolic acid and ursolic acid on α-glucosidase.

Glycemic control which can be efficaciously regulated by inhibiting α-glucosidase activity is an effective therapy for diabetes mellitus. This work is to investigate the kinetics and inhibition mechanism of oleanolic acid and ursolic acid on α-glucosidase. Oleanolic acid and ursolic acid exhibited potent inhibitory activities with IC50 values of (6.35±0.02)×10-6 and (1.69±0.03)×10-5 molL-1 respectively in a reversible and non-competitive manner. Both of them binding to α-glucosidase induced the conformational change and intrinsic fluorescence quenching of α-glucosidase. The binding constants of oleanolic acid and ursolic acid with α-glucosidase at 298K were (2.04±0.02)×103 and (1.87±0.02)×103 Lmol-1 , respectively. Docking results showed that oleanolic acid and ursolic acid bound in different allosteric sites of cavity 2 and cavity 4 on α-glucosidase, respectively, which triggered allosteric regulation to perturb conformational dynamics of α-glucosidase, eventually leading to a decrease of catalytic activity of the enzyme. The substrate was not catalyzed by α-glucosidase to generate further products due to formation of a nonreactive ternary complex of oleanolic acid- or ursolic acid-α-glucosidase-substrate. The combination of oleanolic acid and ursolic acid displayed a significant synergistic inhibition on α-glucosidase.