Lentiginosine, a dihydroxyindolizidine alkaloid that inhibits amyloglucosidase

Lentiginosine, a dihydroxyindolizidine alkaloid, was extracted from the leaves of Astragalus lentiginosus with hot methanol and was purified to homogeneity by ion-exchange, thin-layer, and radial chromatography. A second dihydroxyindolizidine, the 2-epimer of lentiginosine, was also purified to apparent homogeneity from these extracts. Gas chromatography of the two isomers (as the TMS derivatives) showed that they were better than 95% pure; lentiginosine eluted at 8.65 min and the 2-epimer at 9.00 min. Both compounds had a molecular ion in their mass spectra of 157, and the NMR spectra demonstrated that both were dihydroxyindolizidines differing in the configuration of the hydroxyl group at carbon 2. Lentiginosine was found to be a reasonably good inhibitor of the fungal alpha-glucosidase, amyloglucosidase (Ki = 1 x 10(-5) M), but it did not inhibit other alpha-glucosidases (i.e., sucrase, maltase, yeast alpha-glucosidase, glucosidase I) nor any other glycosidases. The 2-epimer had no activity against any of the glycosidases tested.