Natural Plant Toxins In Foods

The following is a survey of some of the most well studied and characterized plant toxins.

Allyl Isothiocyanates

Allyl isothiocyanates are a group of major naturally occurring compounds that confer the pungent flavor to foods such as mustard and horseradish, where it is present at about 50 to 100 ppm. These compounds are in Brassica vegetables such as broccoli and cabbage, and in cassava and other tropical staple foods, but at much lower concentrations. Normal dietary exposure to isothiocyanate-containing foods releases milligram amounts of isothiocyanates. Nominal processing steps (chopping, rinsing, milling) renders the food safe when wash water is discarded. In high doses, isothiocyanates are carcinogenic in rats but nonmutagenic in bacteria. Isothiocyanates do not occur in foods per se but occur as glucosinolate conjugates that are hydrolyzed when the plant releases enzymes as it is disturbed, such as during chopping, processing, or ingestion (Fig. 1). The major concern with isothiocyanates is their goitrogenic properties in that they inhibit binding of iodine in the thyroid gland. Because iodine is required for the formation of the critical thyroid hormones thyroxine (T4) and triiodothyronine (T3), isothiocyanate-induced hyperthyroidism (goiter) mimics iodine deficiency. Hyperthyroidism is a physiological response as the thyroid attempts to compensate for reductions in both T4 and T3 production.

Endemic goiter is seen in geographical areas like India and Africa where consumption of poorly processed foods is coincident with iodine deficiency. Like many food toxins, allyl isothiocyanates are "double-edged swords" in that they have been shown to be chemopreventive in certain animal-testing protocols.

Canavanine

Despite their reputation as being the ultimate health food, alfalfa sprouts contain up to 15,000 ppm canavanine, an arginine analogue that can substitute for this amino acid in cellular proteins, thereby altering their function. Canavanine is also produced in other legumes such as the jack bean. Canavanine inhibits nitric oxide synthetase and induces heat-shock proteins in human cells in vitro (1). By virtue of its antimetabolic action, canavanine is under current consideration as an antitumor drug in combination with other antimetabolites such as 5-fluorouracil, but it has not yet been tested for carcinogenicity. Canavanine may cause autoimmune disorders such as lupus erythematosus in people (2). Primates fed alfalfa sprouts develop a severe toxic syndrome resembling human lupus.

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