Mammalian Metabolism of Toxic Glycosides from Plants

Abstract

This review examines the occurrence, chemistry and mode of action of toxic plant glycosides in mammalian systems. A diverse array of toxic glycosides occurs in higher plants, prominent among which are glucosinolates, cyanogenic glycosides, glycoalkaloids, saponins, aliphatic nitrotoxins and cardenolides. Occurring less abundantly are the toxic glycosides with diterpenoid, sesquiterpenoid, azoxy, phenolic and sterol aglycones. Hydrolysis of the glycosidic bond is the essential first step in the bioactivation of cyanogens, nitrotoxins and glucosinolates. However, glycosides with polycyclic aglycones may show less biological activity after hydrolysis. Therefore, in the digestive tract, β-gluco-sidases of microbial origin play a pivotal role in the biotransformation and absorption of the glycosides and their aglycones.

Glycoalkaloids are teratogens, inhibitors of acetylcholinesterase, and, like saponins, they have the ability to disrupt membranes. Nitrotoxins, as inhibitors of succinate dehydrogenase, produce a neurotoxic syndrome. Cyanogenic glycosides, through the action of cyanide, prevent oxygen utilization by the inhibition of cytochrome oxidase and cardenolides have a direct impact on the myocardium. Cycasin, a carcinogen, can also produce a gastrointestinal disease. Breakdown products of glucosinolates are goitrogens and, like ranunculin, they can also yield irritant oils. Isoflavones are phytoestrogens which mainly affect the fertility of sheep. Terpenes in bracken are carcinogens while other terpenes in the potato family are calcinogens. Other glycosides cause hypoglycemia and hemolysis.