CALYSTEGINS OF PHYSALIS-ALKEKENGI VAR FRANCHETI (SOLANACEAE) - STRUCTURE DETERMINATION AND THEIR GLYCOSIDASE INHIBITORY ACTIVITIES

Five calystegins were extracted from the roots of Physalis alkekengi var. francheti (Solanaceae) with hot water and purified to homogeneity by the combination of a variety of ion-exchange column chromatographies, Their structures have been determined from the H-1- and C-13-NMR spectral data, and two of the compounds were identified as calystegins A(3) and B-2, which have been isolated from the roots of Calystegia sepium (Convolvulaceae). Two of the remaining three were found to be 1 alpha,3 alpha,4 beta-trihydroxy-nor-tropane and 1 alpha,2 alpha,3 alpha,4 beta-tetrahydroxy-nor-tropane and given the trivial name calystegins A(5) and B-3, respectively. The last calystegin was assigned as 1 alpha,2 beta,3 alpha,6 alpha-tetrahydroxy-nor-tropane, which was the same as the relative configuration proposed in the literature for calystegin B-1 isolated from C. sepium. However, the C-13-NMR spectral data for the compound from C. sepium differed substantially from our results. From a personal communication with the authors of the original paper on calystegins, it was clarified that the C-13-NMR chemical shifts of calystegin B-1 in the original paper had been erroneous. Since their corrected C-13-NMR data of calystegin B-1 and its H-1-NMR chemical shifts in the original paper are very close to our present data, we concluded that both compounds from C. sepium and P. alkekengi are identical. Calystegin B-2 has been known to be a potent competitive inhibitor of almond beta-glucosidase (K-i = 1.2 mu W) and coffee bean alpha-galactosidase (K-i = 0.86 mu M). In this study calystegin B-1 (1 alpha,2 beta,3 alpha,6 alpha-tetrahydroxy-nor-tropane) proved to be a potent competitive inhibitor of almond beta-glucosidase (K-i = 1.9 mu M) and bovine liver beta-galactosidase (K-i = 1.6 mu M), but not an inhibitor of alpha-galactosidases. Calystegin A(3) was found to be a weaker inhibitor compared to calystegin B-2 but with the same inhibitory spectrum. Calystegin A(5), a 2-deoxy derivative of calystegin B-2, showed no activity against any glycosidases tested. Since calystegin B-3, a 2-epimer of calystegin B-2, also exhibited only a weak inhibitory activity, it was concluded that the equatorially oriented OH group at C2 is the essential feature for recognition and strong binding by the active site of glycosidases. Based on the structure/activity relationships for the five calystegins isolated from P. alkekengi var. francheti and calystegin C-1 from Morus alba, we propose that the OH group at C6 of calystegin B-1 or C-1, in place of the beta-glycoside oxygen, is protonated by an acidic group in the active site of the beta-glycosidase.