The cytochrome system of the Cecropia silkworm. II. Spectrophotometric studies of oxidative enzyme systems in the wing epithelium

Abstract

As shown by spectroscopic studies, the wing epithelium of the Cecropia silkworm is typical of the insect''s non-muscular tissues in undergoing a virtual breakdown of the cytochrome system at the outset of pupal diapause, and a prominent resynthesis of this system when diapause is terminated. Further information on these marked changes has now been provided by spectrophotometric determinations of DPNH oxidase, succinate-cytochrome c reductase, DPNH-cytochrome c reductase, and cytochrome c oxidase activities in particulate preparations from wing homogenates. By the measurement of these activities, the changing concentrations of cytochromes c, b, b5 and a + a3, respectively, have been followed during the course of metamorphosis. In the freshly pupated insect, all the cytochrome components are present in high concentration. However, during the nest 24 hours the wing loses all but a trace of its cytochrome system; cytochromes b and c decline in concentration by more than 95% and, by present techniques, thus reach undetectable levels which then persist throughout diapause. Cytochromes b5 and a + a3 also decrease prominently, but to a lesser degree, and low concentrations of both components remain detectable in the diapausing wings. Several months later, when the insect''s prothoracic glands function to terminate diapause, cytochromes b and c reappear in detectable concentration. During the subsequent growth and development of the adult moth, b and c, together with a + a3 and b5, once again return to a high level. These findings confirm the results of the earlier, spectroscopic studies, afford a quantitative insight, and provide an explanation for several metabolic peculiarities of the diapausing tissues, particularly their extremely low respiration and the virtual insensitivity of this respiration to cyanide or carbon monoxide. In addition, the present findings strongly support the view that cytochrome synthesis in Cecropia and its relatives is closely linked to the action of the prothoracic gland hormone. According to the present analysis, withdrawal of this endocrine support for growth at the outset of diapause is causally related to the breakdown of the cytochrome system. And when the endocrine organs recover their function to bring about the termination of diapause, resynthesis of the cytochrome system appears to represent at least 1 type of reaction which intervenes between the primary action of the prothoracic gland hormone and the insect''s developmental response.