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

Glycemic control which can be efficaciously regulated by inhibiting a-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 alpha-glucosidase. Oleanolic acid and ursolic acid exhibited potent inhibitory activities with IC50 values of (6.35 +/- 0.02) x 10(-6) and (1.69 +/- 0.03) x 10(-5) mol L-1 respectively in a reversible and non-competitive manner. Both of them binding to alpha-glucosidase induced the conformational change and intrinsic fluorescence quenching of alpha-glucosidase. The binding constants of oleanolic acid and ursolic acid with alpha-glucosidase at 298 K were (2.04 +/- 0.02) x 10(3) and (1.87 +/- 0.02) x 10(3) Lmol(-1), respectively. Docking results showed that oleanolic acid and ursolic acid bound in different allosteric sites of cavity 2 and cavity 4 on alpha-glucosidase, respectively, which triggered allosteric regulation to perturb conformational dynamics of alpha-glucosiciase, eventually leading to a decrease of catalytic activity of the enzyme. The substrate was not catalyzed by alpha-glucosidase to generate further products due to formation of a nonreactive ternary complex of oleanolic acid- or ursolic acid-alpha-glucosidase-substrate. The combination of oleanolic acid and ursolic acid displayed a significant synergistic inhibition on alpha-glucosidase. (C) 2017 Elsevier B.V. All rights reserved.