Figure 1. Mechanism of formation of isothiocyanate goitrogen from glucosinolates.
Seeds from apples, apricots, cherries, peaches, pears, plums, and quinces as well as almonds, sorghum, lima beans, cassava, corn, yams, chickpeas, cashew nuts, and kirsch contain compounds that are toxic due to their release of free hydrogen cyanide, which occurs when the plant tissue is disturbed as during chopping, processing, or ingestion. These conditions initiate the hydrolysis of the glycoside by the action of ^-glucuronidases and other enzymes naturally present in the plant tissue and in the intestinal lumen. Although acid also initiates this process, it doesn't appear to occur in the digestive tract to any great extent, despite the acid environment in the stomach. Hydrolysis by ^-glucuronidases produce the sugar and a cy-anohydrin, the latter spontaneously or enzymatically degrading to form free hydrogen cyanide (Fig. 2). There are several such cyanogenic glycosides, of which linamarin, amygdalin, and dhurrin are examples (Fig. 2). In the 1970s, amygdalin, as laetrile, was a fad remedy touted as a cure and/or preventive for cancer and other ailments. Underground "clinics" briefly flourished where patients were given large quantities of amygdalin or amygdalin-rich seeds and nuts.
Cyanide is one of the most acutely toxic chemicals. It binds to and inactivates heme enzymes, specifically mitochondrial cytochrome aa3 oxidase, resulting in an acute, life-threatening anoxia. The two-step therapy is initiated with sodium nitrite, which induces methemoglobinemia, permitting the release of cyanide from heme proteins, followed by sodium thiosulfate, which acts as a substrate for rhodanese, an endogenous hepatic enzyme that catalyzes the conversion of free cyanide to the less toxic thiocyanate.
Cases of acute human poisoning from the cyanide released from certain varieties of lima beans, cassava, and bitter almonds are a regular occurrence (3). Due to the importance of cassava as a subsistence crop in Africa and South America, cyanogenic glycosides in that food probably represent the greatest health risk. High-cyanide varieties of cassava, distinguished by their bitter taste, may contain over 600 ppm cyanide on a dry weight basis, whereas "sweet" varieties contain significantly less. Processing steps such as sun drying, soaking, boiling, and fer-
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