Chemical constituents of knema angustifolia stem and biological activities

Abstract

The medicinal plant Knema angustifolia has been used in Thai folk medicine for the treatment of various diseases, especially related to body tonic or blood tonic agent. Chemical data on this plant are scarce. A novel para-benzoquinone, angustiquinone (3), along with ten known compounds, rapanone (1), embelin (2), quercetin (4), kaempferol (5), bergenin (6), 11-O-acetyl bergenin (7), catechin (8), isovanillic acid (9), protocatechuic acid (10) and gallic acid (11) were isolated from the n-hexane and EtOAc extracts of the stems of K. angustifolia. Their structures were unambiguously elucidated using extensive 1D and 2D NMR analyses, high-resolution mass spectrometry along with comparison with literature data. Compounds 1-2 and 4-11 were tested for anti-tyrosinase and α-glucosidase inhibitory activity. Quercetin (4) and kaempferol (5) revealed as potent inhibitors of tyrosinase activity, in which the IC50 42.3±0.26, 156.5±0.34 µM, respectively. Notably, rapanone (1) and embelin (2) exhibited excellent α-glucosidase inhibitory activity with IC50 1.3±0.17 and 9.25±0.26 µM, respectively which much higher than commercial drug acarbose (93.6±0.49 µM).