The plant Momordica charantia L. (Cucurbitaceae) is widely cultivated in Asian countries, and the fruit of this plant has been used as a bitter stomachic, a laxative, an antidiabetic, and an anthelmintic agent for children in traditional Chinese, Indian, Vietnamese, and Indonesian medicines. The alcoholic extract of M. charantia fruits has been reported to inhibit the increase of serum glucose levels in glucose-loaded rats.1) In addition, many cucurbitane-type triterpenes and their glycosides were isolated from the roots, 2) fruits, [3][4][5][6][7][8][9][10][11][12][13] seeds, 14,15) leaves, and vines [16][17][18][19] of this plant.a-Glucosidase inhibitors are oral anti-diabetic drugs used for diabetes mellitus type 2 that work by preventing the digestion of carbohydrates. Carbohydrates are normally converted into simple sugars, which can be absorbed through the intestine. Hence, inhibition of a-glucosidase enzyme can significantly decrease the postprandial increase in blood glucose levels after a mixed carbohydrate diet and could be a key strategy in the control of diabetes mellitus. In fact, a potent a-glucosidase inhibitor, acarbose, has been shown to effectively reduce the intestinal absorption of sugars in humans.
20)The main drawback of acarbose is its side effects, such as abdominal distention, flatulence, meteorism, and possibly diarrhea.21) Therefore, natural inhibitors from dietary plants that have minimal side effects and have been shown to have stronger inhibitory activity against a-glucosidase may be useful as effective therapies for postprandial hyperglycemia.22) Interestingly, in our search for bioactive metabolites, we have examined the constituents of the methanol extract of M. charantia fruits, and led to the isolation of fourteen cucurbitane-type triterpene glycosides, including three new compounds named charantosides A-C (1, 5, 6). For these reasons, the a-glucosidase inhibitory activity of all these compounds was tested. Herein, we report the isolation, structure elucidation, and biological activities of compounds 1-14 (Fig. 1).
Results and DiscussionThe methanol extract of M. charantia fruits yielded three new compounds (1, 5, 6). Compound 1 was obtained as a white powder. The absorption bands at 3407 cm Ϫ1 and 1728 cm Ϫ1 in the IR spectrum indicated hydroxyl and carbonyl functionalities, respectively. A [MϩH] ϩ pseudomolecular ion peak observed at m/z: 649.4337 (Calcd for C 37 H 61 O 9 : 649.4316) in positive HR-electrospray ionization (ESI)-MS, and the 13 C-NMR spectroscopic data indicated a molecular formula of C 37 H 60 O 9 . The 13 C-NMR spectrum of 1 showed 37 carbon signals, of which were attributed to a methoxy group, a sugar moiety, and a triterpene moiety. The distortionless enhancement by polarization transfer (DEPT) experiment permitted differentiation of the 37 carbon signals including eight methyl, eight methylene, fifteen methine, and six quaternary carbons. The 1 H-and 13 C-NMR data assign- Fourteen cucurbitane-type triterpene glycosides (1-14) were isolated from a methano...
