α-Glucosidase inhibitors are effective drugs in type 2 diabetes therapy. This research aimed to investigated α-glucosidase inhibitors from two Thai medicinal plants, leaves of Moringa oleifera and Syzygium samarangense, using chromatographic technique and bioassay guidance. Isolation of methanolic extract of M. oleifera leaves yielded two flavonoids named keampferol and kaempferyl-3-O-β-glucopyranoside together with two aromatic rhamnosides named p-hydroxybenzaldehyde-O-α-L-rhamnopyranoside and 1-O-(4-hydroxymethylphenyl)-α-L-rhamnopyranoside. Of compounds isolated, keampferol exhibited the most promising inhibition (IC₅₀ 46.46 µM) against baker’s yeast α-glucosidase, which was 10 times more potent than antidiabetic drug acarbose, whereas inhibitory effect of aromatic rhamnosides was not observed. Isolation of ethylacetate extract of S. samarangense leaves yielded 13 compounds, classified into four groups: flavonols (kaempferol, quercetin, myricetin and myricitrin), flavanones (5-hydroxy-7-methoxy-6-methyl flavanone, pinostrobin, demethoxymatteucinol, 7-hydroxy-5-methoxy-6, 8-dimethyl flavanone, strobopinin and pinocembrin), chalcones (2', 4'-dihydroxy-6'-methoxy-3', 5'-dimethylchalcone and aurentiacin) and gallic acid. The results indicated flavonols showed broad and potent inhibition (IC₅₀ 3.71- 94.33 µM). More interestingly, 2', 4'-Dihydroxy-6'-methoxy-3', 5'-dimethylchalcone and strobopinin containing methyl group (s) in ring A, which are common to genus Syzygium, also displayed promising inhibition. 2', 4'-Dihydroxy-6'-methoxy-3', 5'-dimethylchalcone inhibited maltase and sucrase with IC₅₀ values of 94 and 83 µM, respectively, whereas strobopinin inhibited these enzymes which IC₅₀ values of 175 and 84 µM, respectively. 2', 4'-dihydroxy-6'-methoxy-3', 5'-dimethylchalcone inhibited maltase and sucrase by mixed-type manner: Ki = 3.92, 2.32 mM and Ki' = 18 and 6.67 mM, respectively. Strobopinin inhibited maltase and sucrase by mixed-type manner: Ki = 1.84, 1.95 mM and Ki' = 6.13 and 9.09 mM, respectively. Notably, the inhibition mechanism of 2', 4'-dihydroxy-6'-methoxy-3', 5'-dimethylchalcone and strobopinin are first reported herein.
