They are substances that interfere with the digestion, absorption, or utilization of proteins. Antiproteins occur in many plants and some animals. Various protease inhibitors inhibit proteolytic enzymes of the gut. Lectins are antiproteins also called hemaggulutinins, can agglutinate red blood cells.
Ovomucoids and ovoinhibitors are found in eggs and inhibit proteolytic enzymes. Egg whites contain chymotrypsin inhibitors. Many are sensitive to heat treatment and are inactivated at boiling temperatures for 15 min. Trypsin and chymotrypsin inhibitors are found in legumes, vegetables, milk, wheat, and potatoes. The trypsin inhibitor found in milk is only destroyed after heating 850 C for at least 1 h. Soybean has pancreatic enzyme inhibitors that have been shown to reduce growth.
Lectins have been isolated from the legumes soybean, peanut, lima beans, kidney beans, fava beans, lentils, and pea, and potatoes, banana, mango, and wheat germ. These compounds interfere with amino acids, thyroxine, and fat absorption; therefore, such lectins are goitrogenic. Ricin is a toxic lectin isolated from castor bean, and is notorious for causing deaths of children who eat raw castor beans as well for being a substance of concern as an instrument of bioterrorism. Most lectins are inactivated by moist heat; hence, steaming is an effective processing technique used to inactivate ricin in castor oil. However, dry heat is largely ineffective to deactivate lectins.