The alteration of protoxin activation and degradation of activated toxin was considered as one of the main mechanisms of insect esistance to Bacillus thuringiensis. The activities of proteinases in midgut juice and BBMV from Helicoverpa armigera Hübner and Spodoptera exigua Hübner were assayed and the processes of protoxin activation and degradation of activated Cry1Ac toxin were analysed by SDS-PAGE. Serine proteases of midgut juice from cotton bollworm and beet armyworm were more active than those of BBMV. Midgut juice and BBMV could activate Cry1Ac protoxin and thereafter breakdown the activated Cry1Ac toxin. However the process of protoxin activation and toxin degradation by BBMV is much slower than by midgut juice, and the protoxin activation and toxin breakdown by cotton bollworm is much faster than by beet armyworm. Moreover, the inhibition of PMSF, TLCK, TPCK over midgut protease activities were also measured. Three inhibitors gave the same inhibition pattern over proteases from cotton bollworm and beet armyworm. TLCK gave strong inhibition on trysin activity, TPCK had weak inhibition over chymotrypsin.

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