Examination of midgut luminal proteinase activities in six economically important insects

Abstract

A biochemical survey of insect midgut fluid proteinase activities was conducted to determine which proteinase inhibitors could play roles in insect control strategies. In this study, midgut juices from six insects of economic importance have been isolated and their proteolytic activities classified. Three lepidopterans (black cutworm, corn earworm, and tobacco budworm) and the boll weevil have maximal midgut fluid proteolytic activity in vitro at pH 10–11. Soybean trypsin inhibitor inhibits lepidopteran and boll weevil activities by 63–72%, indicating that their major proteinases are “trypsin-like” serine proteinases. Varying levels of inhibition were seen against the midgut fluid proteinase activities whereas bovine trypsin was inhibited 95–98% by each of four different trypsin inhibitors. Southern corn rootworm and Colorado potato beetle have maximal midgut fluid proteolytic activity in vitro at pH 6–7. E-64 inhibits southern corn rootworm (69%) and Colorado potato beetle (100%) midgut fluid proteinase activity, demonstrating that their major activities are provided by cysteine proteinases. Feeding studies found no mortality nor stunting associated with feeding high levels of trypsin inhibitors to boll weevil or tobacco budworm larvae despite the fact that these proteins were very good inhibitors 60–78% inhibition) of midgut proteolytic activity in vitro.