Antidiabetic principles of natural medicines. III. Structure-related inhibitory activity and action mode of oleanolic acid glycosides on hypoglycemic activity.

We examined the structure-related activity of oleanolic acid glycosides with respect to their inhibitory effect on the increase in serum glucose in oral glucose-loaded rats and their mechanism of action using oleanolic acid 3-O-glucuronide and momordin Ic. Both the 3-O-monodesmoside structure and 28-carboxyl group were confirmed to be essential for such activity, and the 3-O-glucuronide was more potent than 3-O-glucoside. On the other hand, the 28-ester glucoside moiety and 6'-methyl ester of the glucuronide moiety reduced such activity. Oleanolic acid 3-O-glucuronide and momordin Ic, both of which inhibited the increase in serum glucose in oral glucose-loaded rats, did not lower serum glucose in normal or intraperitoneal glucose-loaded rats, or alloxan-induced diabetic mice. These glycosides were found to suppress gastric emptying in rats, and also inhibit glucose uptake in the rat small intestine in vitro. These results indicate that oleanolic acid 3-O-glucuronide and momordin Ic, given orally, have neither insulin-like activity nor insulin releasing-activity. They exhibit their hypoglycemic activity by suppressing the transfer of glucose from the stomach to the small intestine and by inhibiting glucose transport at the brush border of the small intestine.