INSECT PROTEASES, PLANT PROTEASE INHIBITORS, AND POSSIBLE PEST CONTROL

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Abstract
Since the first observation that plants contained protease inhibitors, as identified by their ability to inhibit vertebrate enzymes, it has been postulated that the presence of these substances was related to their phytoprotective abilities. However the following assumptions (1) that phytophagous insects use trypsin, and (2) that ingested inhibitors disrupt digestive proteolysis in insects, have not been adequately tested. Identification of non-tryptic enzymes, cathepsin B, D, and H in phytophagous Coleoptera and unique trypsin-like enzymes in Lepidoptera, indicates insect proteases may differ from their vertebrate counterparts. Putative inhibitor proteins inhibited vertebrate trypsin and chymotrypsin in vitro but had no effect on the trypsin- or chymotrypsin-like activity from the insect midgut. Feeding experiments with the European corn borer,Ostrinia nubilalis(Hübner), indicate that ingestion of inhibitors may not disrupt digestive proteolysis in vivo and the vertebrate trypsin inhibitor in corn may be ineffective as a phytoprotective strategy for this insect. Limitations and implications of ingested inhibitors for future pest control may depend on the origin of the inhibitor, as well as the insect's response.