Yeast and mammalian α-glucosidase inhibitory constituents from Himalayan rhubarb Rheum emodi Wall.ex Meisson

The methanolic extract of rhizome of Himalayan rhubarb Rheum emodi displayed mild yeast as well as mammalian intestinal α-glucosidase inhibitory activity. However, further fractionation of active extract led to the isolation of several potent molecules in excellent yields, displaying varying degrees of inhibition on two test models of α-glucosidase. Rhapontigenin, desoxyrhapontigenin, chrysophanol-8-O–β-d-glucopyranoside, torachrysone-8-O-β-d-glucopyranoside displayed potent yeast α-glucosidase inhibition. However chrysophanol-8-O–β-d-glucopyranoside, desoxyrhaponticin and torachrysone-8-O–β-d-glucopyranoside displayed potent to moderate mammalian α-glucosidase inhibitory activity. Other compounds displayed mild activity on both the tests. Except desoxyrhapontigenin and rhapontigenin that increased Vmax, other compounds including crude extract decreased the Vmax significantly (p<0.02) in yeast α-glucosidase test. Further kinetic analysis on mammalian α-glucosidase inhibition showed that chrysophanol-8-O–β-d-glucopyranoside, desoxyrhaponticin and torachrysone-8-O-β-d-glucopyranoside may be classified as mixed-noncompetitive inhibitors. However, desoxyrhapontigenin and rhapontigenin may be classified as modulators of enzyme activity. Presence and position of glycoside moiety in compounds appear important for better inhibition of mammalian α-glucosidase. This is the first report assigning particularly, mammalian intestinal α-glucosidase inhibitory activity to these compounds. Chrysophanol-8-O–β-d-glucopyranoside, desoxyrhaponticin, desoxyrhapontigenin and rhapontigenin have been isolated in substantial yields from R. emodi for the first time. Therefore, these compounds may have value in the treatment and prevention of hyperglycemia associated diabetes mellitus.