α-Glucosidase and α-amylase inhibitory activities of guava leaves

The 75% ethanol extract from guava (Psidium guajava Linn.) leaves was extracted further, in turn, with CH2Cl2, EtOAc and n-BuOH to afford four fractions, CH2Cl2-soluble, EtOAc-soluble, n-BuOH-soluble and residual extract fractions. Both the n-BuOH-soluble and EtOAc-soluble fractions showed high inhibitory activity against α-glucosidase and α-amylase. Seven pure flavonoid compounds, quercetin (1), kaempferol (2), guaijaverin (3), avicularin (4), myricetin (5), hyperin (6) and apigenin (7), were isolated (using enzyme assay-guide fractionation method) from the n-BuOH-soluble and EtOAc-soluble fractions. The structures of these pure compounds were determined on the basis of MS and NMR data and the activities of these compounds were evaluated. Compounds 1, 2 and 5 showed high inhibitory activities, with IC50 values of 3.5 mM, 5.2 mM and 3.0 mM against sucrase, with IC50 values of 4.8 mM, 5.6 mM and 4.1 mM against maltase and with IC50 values of 4.8 mM, 5.3 mM and 4.3 mM against α-amylase, respectively. We found that myricetin showed the most powerful activity among these compounds with a 70% inhibition against sucrase at a concentration of 1.5 mg/ml. The hydroxyl group at the 3-position on the A-ring and a number of hydroxyl groups attached to the C-ring played important roles in the inhibition activity. There was an obvious synergistic effect (the mixing action of two compounds) against α-glucosidase, but against α-amylase this was not found. This is the first study of the active compositions of guava leaves and the biological activity of the active compositions against α-glucosidase and α-amylase.