The polyhydroxylated nortropane alkaloids called calystegines occur in many plants of the Convolvulaceae, Solanaceae, and Moraceae families. Certain of these alkaloids exhibit potent inhibitory activities against glycosidases and the recently demonstrated occurrence of calystegines in the leaves, skins, and sprouts of potatoes (Solanum tuberosum), and in the leaves of the eggplant (S. melongena), has raised concerns regarding the safety of these vegetables in the human diet. We have surveyed the occurrence of calystegines in edible fruits and vegetables of the families Convolvulaceae, Solanaceae, and Moraceae by GC-MS. Calystegines A3, B1, B2, and C1 were detected in all the edible fruits and vegetables tested; sweet and chili peppers, potatoes, eggplants, tomatoes, Physalis fruits, sweet potatoes, and mulberries. Calystegines B1 and C1 were potent competitive inhibitors of the bovine, human, and rat beta-glucosidase activities, with Ki values of 150, 10, and 1.9 microM, respectively for B1 and 15, 1.5, and 1 microM, respectively, for C1. Calystegine B2 was a strong competitive inhibitor of the alpha-galactosidase activity in all the livers. Human beta-xylosidase was inhibited by all four nortropanes, with calystegine C1 having a Ki of 0.13 microM. Calystegines A3 and B2 selectively inhibited the rat liver beta-glucosidase activity. The potent inhibition of mammalian beta-glucosidase and alpha-galactosidase activities in vitro raises the possibility of toxicity in humans consuming large amounts of plants that contain these compounds.