Main groups of these proteins are protease inhibitors and lectins. The first group is constituted by substances that interfere with the digestion of proteins. Lectins may disturb the absorption of nutrients.
Protease inhibitors are proteins that inhibit proteolytic enzymes. They occur mainly in plants. Well-known are the trypsin inhibitors in legumes (e.g., soybeans), vegetables (e.g., alfalfa), cereals, and potatoes. Protease inhibitors have been reported to inhibit the growth of rats, chickens, and other monogastric animals. Apart from that, an important finding is enlargement of the pancreas in rats, chickens, mice, and young guinea pigs following the administration of trypsin inhibitors in feeding experiments. This effect is attributed to an increase in trypsin synthesis by the pancreas in response to enzyme inhibition. In the pancreas of rats fed on diets rich in trypsin inhibitors during long periods of time, nodular hyperplasia and adenomas have been observed. These symptoms did not show themselves in pigs, dogs, calves, and Cebus monkeys. Mechanistic studies revealed that the so-called Kunitz trypsin inhibitor forms stable one-to-one complexes with the protease. The inhibitor binds to the acitve site of the protein substrate, followed by hydrolysis of a peptide bond between two amino acids, viz. arginine and isoleucine. A disulfide bridge prevents dissociation of the inhibitor. As a result, it remains bound to the enzyme.
Lectins are proteins of plant origin. They especially occur in legumes such as peanuts, soybeans, kidney beans, and peas. Initially, they were called hemagglutinins, as they can agglutinate the red blood cells. After the finding that lectins can bind to specific receptors on a variety of cells including epithelial cells lining the small intestine and lymphocytes, the name lectins has found general acceptance.
Bean lectins have been shown to affect the morphology of the small intestine (proliferation of the intestinal epithelium, Figure 11.7), to inhibit absorption of nutrients and to disturb the immune function of the gut. These effects are attributed to a sequence of events: binding to receptors on epithelial cells of the small intestine, uptake by the cells through endocytosis, and stimulation of the protein synthesis. Some members of this group of proteins have been structurally identified. The lectin concanavalin A, occurring in jack beans, was found to be a lipoprotein, the protein part being composed of 4 identical polypeptides of 237 amino acids. Each subunit appeared to contain binding sites for Ca, Mn, and sugar moieties.
Apart from inhibition of absorption, lectins can also be toxic. A highly toxic example is ricin, present in castor beans. Many poisonings leading to death have been reported, such as children dying after ingestion of raw castor beans. Feeding insufficiently heated raw material proved to be fatal for animals. Ricin consists of two polypeptide chains. One chain is used for binding to the cell, the other inactivates the ribosomal subunits involved in protein synthesis (after uptake through endocytosis). Inhibition of the protein synthesis results in the disappearance of essential enzymes, and ultimately in death.
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