Enzyme depletion in insect feeding experiments: Fact or artifact?

Abstract

1. 1. Musca domestica larvae were fed polyacrylamide gels to induce starvation and subsequently were fed starch gels. 2. 2. Amylase activity decreased in midgut cells during starvation and increased on refeeding, whereas in midgut contents it decreased along the whole experiment. 3. 3. The majority of the luminal amylase is soluble, although in refed larvae there is a significant amount which is pelleted on centrifuging. 4. 4. The calculated values for amylase activity remaining in midgut content of starved and refed larvae, taking into account literature data for amylase excretion in M. domestica larvae, agreed with the experimental values. 5. 5. The results suggest that amylase excretion in the absence of amylase secretion is the main event causing the decrease of amylase activity on refeeding. 6. 6. Another important event is the co-sedimentation of amylase with starch during homogenate clearing by centrifugation. 7. 7. These explanations probably also hold for other luminal hydrolases, making unnecessary the misleading concept of “enzyme depletion by the substrate”.