α-glucosidase inhibitory and antihyperglycemic effects of polyphenols in the fruit of Viburnum dilatatum Thunb.

Small crimson fruit of Viburnum dilatatum Thunb. (gamazumi), a wild deciduous low tree belonging to a family of Caprifoliaceae, has strong antioxidant activity, and cyanidin 3-sambubioside (C3S) and 5-caffeoyl quinic acid (5-CQA) are identified as active compounds. The freeze-dried powder of V. dilatatum fruit juice (CEV) was orally administered to streptozotocin-induced diabetic rats for 4 weeks repeatedly. Consequently, the elevation of plasma glucose level after oral administration of 2 g/kg glucose was suppressed by the repeated administration of CEV. The action was dependent on the dose of CEV, and plasma glucose level in rats administered 500 mg/kg of CEV was decreased significantly from that in rats without CEV. Increase of insulin secretion was not found in rats with or without administration of CEV. It was expected that CEV had some effects on glucose uptake. In five compounds identified from V. dilatatum fruit, C3S and 5-CQA showed inhibitory activity on sucrase and maltase. Inhibitory activity of cyanidin 3-glucoside and cyanidin aglycon (Cy) was not found markedly, and so it was thought that the activity was a characteristic property in Cy diglycosides. Moreover, 5-CQA and C3S were main polyphenol in the fruit of V. dilatatum. These results suggest that V. dilatatum fruit has the alpha-glucosidase inhibitory activities and the antihyperglycemic action.