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Electron transport chain activity during insect metamorphosis

The midgut of the caterpillar, Manduca sexta, is an obligatorily aerobic tissue that is fated to be destroyed at larval‐pupal metamorphosis. Top‐down metabolic control analysis has revealed that the substrate oxidation system (metabolite transporters, citric acid cycle, electron transport chain) confers most of the control over mitochondrial oxygen consumption and the activity of this system is greatly depressed during the early stages of metamorphosis (Am. J. Physiol. 287:R314–R321). In order to determine if there are metamorphic changes in the electron transport chain, the maximal activities of Complexes I, II, III and IV (cytochrome c oxidase) were determined in mitochondria isolated from midguts of feeding fifth instar larvae (day 2) and larvae committed to pupation (day 5 “wandering larvae”). The activity of Complex IV was determined polargraphically, whereas the activities of the other complexes were measured spectrophotometrically. There were no significant changes in the activities of Complexes I, II, and III during development. In contrast, cytochrome c oxidase activity was 38% higher in mitochondria isolated from pre‐commitment larvae compared to that of wandering larvae. If, however, these activities are normalized to cytochrome aa3 content, there is no difference in cytochome c oxidase activity. These results indicate that the loss of cytochrome aa3 from the mitochondria is responsible for the decline in cytochrome c oxidase activity and this loss may play a part in the inhibition of the substrate oxidation system that occurs early in metamorphosis.