Disaccharidase Inhibitory Activity of Thai Plant Extracts

Background/Objectives: Intestinal α-glucosidases, including maltase, sucrase, and trehalase, are key enzymes responsible for the final steps of carbohydrate digestion. Although Thai medicinal plants possess diverse bioactivities, most previous studies on plant-derived α-glucosidase inhibitors have focused on single-enzyme assays, primarily maltase, and lack systematic comparison of the three major intestinal disaccharidases-maltase, sucrase, and trehalase. This study aimed to determine the kinetic properties of rat intestinal α-glucosidases and evaluate the inhibitory potential of selected Thai plant extracts. Methods: Rat small-intestinal S9 fractions, post-mitochondrial supernatant obtained by centrifugation at 9000× g, containing soluble enzymes and microsomal components responsible for disaccharidase activity, were prepared and disaccharidase activities were quantified using the glucose oxidase-peroxidase method. Kinetic parameters were obtained from Eadie-Hofstee plots using maltose, sucrose, and trehalose as substrates. Fourteen Thai plant extracts (Oryza sativa, Cratoxylum formosum, Garcinia cawa, Aganosma marginata, Polyalthia evecta, Ellipeiopsis cherrevensis, Ancistrocladus tectorius, Micromelum minutum, and Microcos tomentosa) and isolated compounds (Bergapten, Eurycomalactone, Lupinifolin, Osthole) were assessed at 100 and 250 µg/mL for inhibition of maltase, sucrase, and trehalase. Results: Maltase exhibited the highest substrate affinity based on the lowest Km value. Among the tested samples, the 80% ethanol extract of Microcos tomentosa (MT80) inhibited maltase, sucrase, and trehalase activities by approximately 10-60% at 250 µg/mL, and was the only extract showing consistent inhibition across all three enzymes. Other extracts showed selective inhibition toward one or two enzymes. Conclusions: These findings indicate that MT80 possesses a broad-spectrum inhibitory profile against major intestinal α-glucosidases, suggesting a potential advantage for comprehensive regulation of postprandial glucose excursions and supporting its candidacy as a source of novel α-glucosidase inhibitors.