BLOOD POISONS

A number of plants produce poisons that damage blood structure, and, therefore, can be lethal to humans and livestock.

Lectins.

Lectins (a.k.a. toxalbumins, phytotoxins, or phytohemagglutinins) were originally discovered in plants more than a century ago by their ability to agglutinate (clot together) human red blood cells (erthyrocytes). Indeed, some lectins are so specific in agglutinating certain blood types that they were used in the characterizations of the human blood groups A, B, and O. The lectin is composed of amino acids, and these proteins bind to sugar molecules of glycoproteins attached to the outside of blood cells, but they can also bind to glycoproteins on other cell types and to simple sugars alone, such as sucrose. Victims of lectin blood poisoning can die from anaphylactic shock, as if the blood has been injected by a foreign protein. These plant lectins easily are taken up by the blood stream through the stomach, because they are relatively small proteins.

Lectins have now been found in bacteria, invertebrates, and vertebrates as well as in plants. They are particularly prevalent in leguminous plants, where lectins are localized in the cotyledons of the seeds and in some roots. Many legumes are in a symbiotic relationship with the nitrogen-fixing bacteria (rhizobium types), which supply the plant with abundant nitrogen, a potentially limiting nutrient for many plants. Some of the useful nitrogen is then used by the legume plant to make lectins and other proteins. Lectins may comprise up to 3% of the weight of a mature seed. After germination, however, lectin content decreases sharply in the growing seedling and soon disappears altogether everywhere except in the roots.

The functions of lectins in plant tissues are now being studied through experimentation. First, lectins may be present as an anti-herbivory device aimed against potential seed or seedling consumers. For example, lectins from the seed of the legume Phaseolus vulgaris can be toxic and even deadly for humans (very rarely) and other animals. The lectin binds to the intestinal wall, thereby weakening it and allowing infectious bacteria to enter the body through the gut. A second hypothesis suggests that high quantities of lectins in seed cotyledons promote growth in the young embryo. The presence of some lectins has led to fast growth in some animal as well as plant cells in the laboratory. The third and best studied hypothesis for lectin function involves lectins found in the roots of legumes. It has been discovered that lectins aid in the recognition of the proper strain of symbiotic nitrogen-fixing bacteria (see nitrogen fixation). The lectins apparently bind to a specific sugar on the outside of the appropriate strain of rhizobium.

Some lectin-containing seeds are listed below. Almost all lectins can be destroyed by soaking or cooking the beans properly before eating them.

Lectin (plant name)

Legumes

Euphorbs

Cyanide.

Cyanide (a.k.a. hydrogen cyanide, cyanohydric acid, prussic acid, or bitter almonds) is a potent metabolic poison present in some food crops and other plants. Cyanide is a small molecule composed of a carbon and nitrogen atom joined by a stable triple bond. This poison is best known for its inhibition of many enzymes that are important in animal metabolism. [Enzymes are proteins that act as catalysts in chemical reactions.] Cyanide most notably inhibits cytochrome oxidase, one of a group of enzymes important in cellular respiration. [Respiration is the process by which both animals and plants break down glucose in the presence of oxygen to yield carbon dioxide and water and produce valuable energy to maintain cellular processes and growth.] Without functioning cytochrome oxidase, respiration is inhibited. Cyanide binds tightly to the enzyme and permanently inhibits its functioning.

Cyanide is made as an anti-herbivory compound to discourage plant consumers. Cyanide most often is attached to other molecules in the form of cyanogenic glycosides. An example of one such compound is amygdalin (from stems of cherry, apricot, etc., Prunus spp.). In this form, cyanide is nontoxic to the plant; only in the breakdown of cyanogenic glycosides, during animal consumption or digestion, is hydrogen cyanide gas released. For example, cows feeding on some species of grasses containing cyanogenic glycosides become ill as they chew on the grass. In this fashion, it is hypothesized that cyanide in nonlethal doses effectively deters herbivory.

Progressive symptoms of cyanide poisoning include gasping, staggering, paralysis, convulsions, and coma, and the result can be death. The lethal dose ranges from 0.5 to 3.5 mg per kilogram of body weight, a substantial quantity. Victims are treated by pumping the stomach and administering oxygen. As with other poisons, cyanide can be broken down by proper processing of the plant for safe consumption (see cassava).

Some cyanide containing plants are listed below.

Plant (relative cyanide level)

Dicoumarin.

Dicoumarin is a compound present in leaves of sweet clover (Melilotus). For years it has been known that this is the cause of bleeding disease of cattle, and in the 1920s dicoumarin caused huge problems for cattle ranchers and dairy farmers, causing blood blisters. Moreover, milk can become tainted with this chemical, and it is alleged that Abraham Lincoln's mother died of poisoning from tainted milk.

Dicoumarin prevents clotting of the blood, and was used for human medicine to prevent formation of blood clots. However, most commonly dicoumarin is the active ingredient in a very powerful rat poison called Warfarin, because it causes internal bleeding and can be effective by killing either wounded rats or during birth. However, in rats Warfarin resistance can be genetically produced in a population within several years (one gene).

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