Chemical constituents from Rubus suavissimus and their α-glucosidase inhibitory activities

Rubus suavissimus is mainly distributed in Guilin, Liuzhou, Wuzhou and other regions in Guangxi Zhuang Autonomous Region, so it is called as “Guangxi tiancha” in China. R. suavissimus, together with Siraitia grosvenorii and Stevia rebaudianastevia are praised as three famous sweet plants in Guangxi Zhuang Autonomous Region. Zhuang and Yao people use the leaves of R. suavissimus as a tea to treat diabetes, and it is known as the “divine tea” of Yao medicine. Therefore, R. suavissimus is a combination of sugar, tea and medicine, which has great potential for the development of food and medicine. In order to investigate the secondary metabolites with α-glucosidase inhibitory activity from R. suavissimus, herein, the extraction, separation and purification of secondary metabolites were performed on the leaves of R. suavissimus. The structures of purified compounds were determined based on the data of mass spectrometry (HR-ESI-MS) and nuclear magnetic resonance (1H NMR and 13C NMR). In addition, the α-glucosidase inhibitory activity of the purified compounds were evaluated by pharmacological methods simultaneously. The results were as follows: (1) Ten compounds were purified and their structures were elucidated as rubusoside (1), kaempferol 3-O-robinobioside (2), gallic acid (3), dihydrodehydroconiferyl alcohol (4), 5-methoxydihydrodehydroconiferyl alcohol (5), brevifolincarboxylic acid (6), steviolmonoside (7), steviol (8), 16α, 17-dihydroxykaurane (9), and quercetin 3-O-β-D-galactopyranoside (10). Among them, compounds 2, 4, 5 and 9 were isolated from R. suavissimus for the first time. (2) Compounds 2, 3, 5, 6 and 10 showed strong inhibitory activity on α-glucosidase. The IC50 values of compounds 2, 3, 5, 6 and 10 were (0.14 ± 0.03) mg·mL-1, (0.36 ± 0.02) mg·mL-1, (0.44 ± 0.01) mg·mL-1, (0.53 ± 0.04) mg·mL-1 and (0.14 ± 0.03) mg·mL-1 respectively, which were stronger than the positive control acarbose with the IC50 values as (0.69 ± 0.02) mg·mL-1. Thus, compounds 2, 3, 5, 6 and 10, which were isolated from the leaves of R. suavissimus, could be a potential α-glucosidase inhibitors based on their bioactivity results. Compounds with α-glucosidase inhibitory activity from R. suavissimus will provide the basis for development of related hypoglycemic products.