In previous
studies, we determined that a combination of cysteine and serine proteinase
inhibitors in the diet of Tribolium
castaneum larvae was synergistic in growth inhibition. One hypothesis
for this synergism was that the combination of inhibitors prevented adaptive
responses of the larvae to either of the inhibitors. To test this
hypothesis, we fed cysteine and/or serine proteinase inhibitors to T. castaneum larvae, and then determined
the effect on the properties of digestive proteinases in vitro. As previously reported, cysteine proteinse activity
was major and serine proteinase activity was minor in control larvae.� Dietary serine proteinase inhibitors had
minimal effects on T. castaneum larvae.
However, when larvae ingested cysteine proteinase inhibitors, there was a
dramatic shift from primarily cysteine-type proteinases to serine proteinases
in the proteinase profile of the midgut. Moreover, a combination of
cysteine and serine proteinase inhibitors in the diet prevented this shift to serine
proteinase-based digestion, and there was a corresponding substantial
retardation in growth. Therefore, it appears that the synergistic effect
of a combination of cysteine and serine proteinase inhibitors in the diet of T. castaneum larvae is achieved through
the prevention of the adaptive response to overcome the activity of either
inhibitor.
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