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Betulinic acid (BA), an important pentacyclic triterpene widely distributed in many foods, possesses high antidiabetic activity. In this study, BA was found to exhibit stronger inhibition of α-glucosidase than acarbose with an IC value of (1.06 ± 0.02) × 10 mol L in a mixed-type manner. BA bound with α-glucosidase to form a BA-α-glucosidase complex, resulting in a more compact structure of the enzyme. The obtained concentrations and spectra profiles of the components resolved by the multivariate-curve resolution-alternating least-squares confirmed the formation of the BA-α-glucosidase complex. Molecular docking showed that BA tightly bound to the active cavity of α-glucosidase, which might hinder the entrance of the substrate leading to a decline in enzyme activity. The chemical modification of α-glucosidase verified the results of the computer simulation that the order of importance of the four amino acid residues in the binding process was His > Tyr > Lys > Arg.

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