First identification of α-glucosidase inhibitors from okra (Abelmoschus esculentus) seeds

Infusion of roasted okra seeds has long been consumed in Turkey for diabetes mellitus therapy. Previous reports of a hypoglycemic effect observed in rats administrated with okra seed extract indicated a possible connection with inhibition of intestinal alpha-glucosidase. An attempt to identify active components was first herein conducted using alpha-glucosidase-inhibition-guided isolation, yielding two major flavonol glucosides named isoquercetin (2) and quercetin-3-O-beta-glucopyranosyl-(1"' --> 6")-glucoside (3). They selectively inhibited rat intestinal maltase and sucrase, in which isoquercetin (2) was 6-10 times more potent than its related diglucoside 3. This result suggested that an increase in hydrophilicity by the additional glucose residue in 3 led to a significant decline in the inhibitory effect and raised the possible involvement of the free 3-OH in exerting the inhibition. Our postulation was evaluated by examining alpha-glucosidase inhibition of quercetin (1), and the aglycone of 2 and 3, whose 3-OH is free from any glucose moiety. Interestingly, 1 displayed a broad inhibitory effect toward rat intestinal and baker's yeast alpha-glucosidases, with improved potency. A kinetic study of 1 indicated that it inhibited maltase by two distinct mechanisms, in competitive (K(i) 462 microM) and noncompetitive (K(i) 2153 microM) manners, whereas the mechanism underlying the inhibition of sucrase was verified as being of a competitive behavior (K(i) 218 microM).