LECTIN; A TOXIC PLANT PROTEIN

Lectin is the general term for plant proteins that have highly specific binding sites for carbohydrates. They are widely distributed among various sources such as soybeans, peanuts, jack beans, mung beans, lima beans, kidney beans, fava beans, vetch, yellow wax beans, hyacinth beans, lentils, peas, potatoes, bananas, mangoes, and wheat germ. Most plant lectins are glycoproteins, except concanavalin A from jack beans, which is carbohydrate free. The most toxic lectins in food include ricin in castor bean (oral toxic dose in man: 150 to 200 mg; intravenous toxic dose: 20 mg) and the lectins of kidney bean and hyacinth bean. The mode of action of lectins may be related to their ability to bind to specific cell receptors, in a way comparable to that of antibodies. Because they are able to agglutinate red blood cells, they are also known as hemaglutinins. The binding of bean lectin on rat intestinal mucosal cells has been demonstrated in vitro, and it has been suggested that this action is responsible for the oral toxicity of the lectins. Such bindings may disturb the intestines’ absorptive capacity for nutrients and other essential compounds. The lectins, being proteins, can easily be inactivated by moist heat. Germination decreases the hemaglutinating activity in varieties of peas and species of beans.