α-Glucosidase inhibitors from the bulb of Eleutherine americana

► Extract of the bulb of Eleutherine americana showed strong maltase-inhibitory activity. ► Eleutherinoside A with naphtopyrone structure is a unique glucosidase inhibitor. ► This is the first study of E. americana in relation to its anti-diabetic potencies. One effective way to treat diabetes is by suppressing carbohydrate digestion due to the utilisation of α-glucosidase inhibitors (AGIs). The determination of prospective herbs, done in vitro by using enzyme assay, resulted in the finding of Eleutherine americana, which showed a potent inhibitory activity. A 50% aqueous methanol-soluble extract of bulbs of E. americana was chromatographed successively and the active fractions were further purified with preparative high performance liquid chromatography (HPLC) to isolate active compounds against α-glucosidase. Structure determination by mass and NMR analysis revealed that these isolated compounds were eleutherol (1), eleutherinoside A (2), and eleuthoside B (3) based on comparisons with the reference data. Considering the amount and the inhibitory activity of each naphthalene in the whole extracts, the bulb of E. americana inhibitory activity against α-glucosidase might be a result of compound 2 (IC50=0.5mM, yield=5mg/50g plant sample, with a characteristic structure which has never been found in other AGIs. AGIs play an important role for the treatment of diabetes, therefore these results may suggest novel alternatives for diabetes treatment management.