The roots of Beta vulgaris L. (sugar beet, Chenopodiaceae) have been used industrially as a raw material for sugar. The fresh roots and leaves of this plant are used as a vegetable and food garnish in Japanese-style dishes. In Chinese traditional medicine, the roots of sugar beet have been known to exhibit sedative and emmenagogue-like effects. As a part of our screening to find biologically active principles contained in medicinal foodstuffs, which have been used for both medicinal and alimentative purposes, the saponin fractions from the roots and leaves of sugar beet were found to show an inhibitory effect on the elevation of plasma glucose levels in the oral glucose tolerance test in rats. By using a bioassay-guided separation, we have isolated ten new saponins named betavulgarosides I, II, III, IV, V, VI, VII, VIII, IX, and X from the roots and leaves. The absolute structures of betavulgarosides I-X with a novel acetal type acidic substituent or a dioxolant moiety were determined on the basis of physicochemical evidence using 2D NMR technique (H-H, C-H COSY, NOESY, HMBC, H-H, C-H HOHAHA) and chemical findings, which included the chemical correlation of betavulgaroside IV with known saponin momordin I. Oleanolic acid 3-O -monodesmosides such as betavulgarosides II and IV and 28-deglucosylbetavulgaroside V were found to exhibit hypoglycemic activity, while oleanolic acid 3, 28-O-bisdesmosides such as betavulgarosides I, III, and V lacked the activity. Examination of the hypoglycemic activity of oleanolic acid and related glycosides led us to presume the following structure-requirement in oleanolic acid glucuronide-saponin for the activity: the 3-O-glycoside moiety and 28-O-carboxyl group are essential to the activity. On the basis of detailed examination for the hypoglycemic activity of oleanolic acid 3-O-glucuronide, we have deduced that these betavulgarosides decrease the concentration in the blood by inhibiting absorption across the cell membranes of the digestive tract.
