Glycoalkaloids toxicity

Phytoalexins are low molecular weight compounds produced in plants as a defense mechanism against microorganisms. They do, however, exhibit toxicity to humans and other animals in addition to microbes (30). Coumarins, glycoalkaloids, isocoumarins, isoflavonoids, linear furanocoumarins, stilbenes, and terpenes aU. fall into the category of phytoalexins (31). Because phytoalexins are natural components of plants, and because their concentration may increase as a response to production and management stimuli, it is useful to recogni2e the possible effects of phytoalexins in the human diet. 

Assessment of whether a chemical has the potential to cause adverse effects in humans arises usually from direct observation of an effect in animals or humans, such as the acute poisoning episodes that have occurred when potatoes contain high levels of glycoalkaloids. Epidemiological studies have also been used to infer a possible relationship between intake of a particular type of food, or constituent of that food, and the potential to cause an adverse effect. Such observations led to the characterisation of the aflatoxins as human carcinogens. However, natural toxic substances that occur in plant foods have often been identified through observations in animals, particularly farm animals. It was observations of adverse effects in farm animals that led to the further characterisation of the phytoestrogens and the mycotoxins. In other instances, the concern arises from the chemical similarity to other known toxins. 

Levels of solanaceous glycoalkaloids can be significant, occasionally causing toxicity in humans and livestock. Symptoms of potato poisoning include gastrointestinal disturbances, apathy, drowsiness, mental confusion, visual disturbances, dizziness, hallucinations, and trembling. Symptoms may perist 2-24 hours after consumption, and one recent outbreak showed reduced butyrylcholinesterase levels six days later, which then returned to normal in 4 to 5 weeks. 

Clay eating probably was extremely important in human evolution it enabled hominids who did not use fire to eat plants with toxic antifeedants. However, the glycoalkaloids of the potato are heat stable and insoluble in water. Domestication of tomatoes and potatoes probably went hand in hand with clay eating. Johns (1986) suggested that geophagy is the most basic human detoxification technique with behavior antecedents that are prehominid. ... 

This limited overview on the analysis of four classes of the following secondary potato metabolites is, except for anthocyanins, largely limited to our own studies of glycoalkaloids, calystegine alkaloids, and phenolic compounds. Because interest in these potato constituents arises from potential health benefits and occasional toxicity, we also include in this overview a brief discussion of these aspects that relate to composition and a description of experimental methods. The interested reader should consult the cited references for an entry into the extensive worldwide literature on the diverse analytical and biological aspects for these metabolites. 

Rayburn, J. R., Bantle, J. A., Friedman, M. (1994). Role of carbohydrate side chains of potato glycoalkaloids in developmental toxicity. J. Agric. Food Chem., 42,1511-1515. 

This primary mode of action of tomatine, that involves the formation of complexes with membrane sterols is similar to that described for polyene antibiotics [2, 4], and results in pore formation and loss of membrane integrity. This mode of action is supported by the reduced activity of tomatine on sterol-free bacteria and Oomycete fungi such as Pythium and Phytophthora [15, 28], and the strongly reduced toxicity of hydrolysis products of the glycoalkaloid which fail to bind sterols [57]. 

Several glycoalkaloids present in food are of toxicological interest. Solaninc. found in potatoes, tomatoes, apples, eggplant, and sugar beets, has been responsible lor several cases uf moderate to severe poisoning. Solanine is a cholinesterase inhibitor and toxic doses are probably ca 200 mg. Market potatoes contain about 1-5 mg of solanine per 10(1 g fresh weight, The USDA establishes solanine levels of 20 mg/100 g as the limit for safe consumption. 

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