The injury metabolism of the cecropia silkworm—II. Injury-induced alterations in oxidative enzyme systems and respiratory metabolism of the pupal wing epidermis

Abstract

It is known that localized injury to the integument of diapausing silkmoth pupae causes an increase in the respiratory metabolism of an insect as a whole. In the present study, this ‘injury metabolism’ has been analysed in an individual tissue, the wing epidermis, both in terms of the oxygen consumption of the isolated intact wing, and through measurement of the activities of several oxidative enzyme systems in mitochondrial-microsomal preparations derived from homogenates of wing. The participation of the wing in the respiratory response of the pupa is clearly evident from both types of experiment. The oxygen consumption of isolated wings is a linear function of that of the pupa and shows a corresponding increase after injury. Moreover, an increase of several fold is observed in the activities of cytochrome c oxidase, DPNH-, TPNH-, and succinatecytochrome c reductase, and DPNH oxidase systems. The enzymatic changes appear to be associated with an increase in concentration of respiratory enzymes, similar in character and magnitude to that observed in the uninjured silkmoth when diapause is terminated. The oxygen consumption of the isolated wing also typifies the behaviour of the pupa as a whole in showing a substantial resistance to inhibition by carbon monoxide. In the intact pupa and in the isolated wing, the degree of sensitivity to this agent increases as a function of the prior respiratory rate in air. Thus the wings of injured pupae, whose respiration is augmented, exhibit greater sensitivity to carbon monoxide than do those isolated from uninjured diapausing pupae. These findings are considered in relation to the ‘excessoxidase’ hypothesis whereby cytochrome oxidase has previously been proposed to function as a carbon monoxide-resistant terminal oxidase in pupal tissues. The present findings are in accord with the hypothesis but do not rule out other explanations for the mechanism of resistance to carbon monoxide. The alterations in respiratory rate and enzyme concentration are of interest in relation to general problems of metabolic control in animal cells and, in particular, the control of specific protein synthesis. The increase in concentration and activity of respiratory enzymes after injury is correlated functionally with the enhanced biosynthetic activity that prevails at this time. Synthesis of respiratory enzymes is viewed as the action of an intracellular regulatory device to maintain a balance of exergonic and endergonic functions under the altered conditions of biological maintenance imposed by injury.