Chemical structures of saponins from seeds of edible horse chestnuts (Aesculus turbinata) after treatment with wooden ashes and their hypoglycemic activity

The seeds of Japanese horse chestnuts (Aesculus turbinata) contain large amounts of mixed triterpenoidal saponins called "escins". Recent studies have shown that escins have several biological activities including anti-inflammatory action and inhibitory effect on the absorption of ethanol or glucose. For the edible utilization of the seeds, the seeds are usually treated with wooden ashes to remove harshness. Here, we found the novel compounds derived from escin in the seeds of horse chestnuts after the food processing with wooden ashes. The instrumental analyses revealed the chemical structures of four types of the escin derivatives. Those were identified as deacetylated forms of escins I a, II a, I b and II b. However, tigloyl and angeloyl moieties of escins were more resistant to the hydrolysis with wood ashes. To determine the biological activity of deacetylated forms of escins, the fraction including four types of deacetylated escins was used for an oral glucose tolerance test in mice. The analysis revealed that a single oral administration of 300 mg/kg of those saponins was effective to show significant hypoglycemic activity. Taken together, the results suggested the potential usefulness of the saponins generated during the food processing of horse chestnuts with wooden ashes for functional food.