ABSTRACT
Plants have evolved general insect defense mechanisms that are sufficient for plant survival but not always effective enough to keep the damage to a level that would be acceptable for crop plants. One such defense mechanism is the use of plant proteinaceous inhibitors. The three important enzymes required for the digestion of food materials by insects are proteinases, amylases, and lipases. Polypeptides and proteins that are inhibitors of proteolytic enzymes are present in cells of almost all life forms and may be ubiquitous in nature. Natural proteinase or protease inhibitors are often found as major components in plants and are concentrated in seeds and tubers, particularly in those of the Graminae, Leguminosae (Fabaceae), and Solanaceae families. Proteinases that are inhibited by plant inhibitors are primarily found in animal, bacterial, and fungal species and only occasionally in plants. Most plant proteinase inhibitors are specific for serine endopeptidases. A few inhibitors from plants that inhibit metallocarboxypeptidases, acid proteinases, and sulfhydryl proteinases are known, but inhibitors of plant aminopeptidases have not yet been identified. In 1946, Kunitz crystallized an inhibitor of trypsin from soybeans (1) that still bears his name, the Kunitz soybean trypsin inhibitor (STI). The role of plant proteinase inhibitors in plant metabolism, their possible contribution to the natural protective system of plants, and their evolutionary relationship have been reviewed (2-12).
