α-Glucosidase inhibitory action of natural acylated anthocyanins. 2. α-Glucosidase inhibition by isolated acylated anthocyanins

Four diacylated pelargonidin (Pg: SOA-4 and SOA-6), cyanidin (Cy: YGM-3), and peonidin (Pn: YGM-6) 3-sophoroside-5-glucosides isolated from the red flowers of the morning glory, Pharbitis nil cv. Scarlett O'Hara (SOA), and the storage roots of purple sweet potato, Ipomoea batatas cv. Ayamurasaki (YGM), were subjected to an α-glucosidase (AGH) inhibitory assay, in which the assay was performed with the immobilized AGH (iAGH) system to mimic the membrane-bound AGH at the small intestine. As a result, the acylated anthocyanins showed strong maltase inhibitory activities with IC₅₀ values of <200 μM, whereas no sucrase inhibition was observed. Of these, SOA-4 [Pg 3-(2-O-(6-O-(E-3-O-(β-D-glucopyranosy)caffey)-β-Dglucopyranosyl)- 6-O-E-caffeyl-β-D-glucopyranoside)-5-O-β-D-glucopyranoside] possessed the most potent maltase inhibitory activity (IC₅₀ = 60 μM). As a result of a marked reduction of iAGH inhibitory activity by deacylating the anthocyanins, that is, Pg(or Cy or Pn) sophoroside-5-glucoside, acylation of anthocyanin with caffeic (Caf) or ferulic (Fer) acid was found to be important in the expression of iAGH (maltase) inhibition. In addition, the result that Pg-based anthocyanins showed the most potent maltase inhibition, with an IC₅₀ value of 4.6 mM, and the effect being in the descending order of potency of Pg > Pn = Cy strongly suggested that no replacement at the 3′(5′)-position of the aglycon B-ring may be essential for inhibiting iAGH action.