On the intracellular localization of the ecdysone 20‐monooxygenase in Musca domestica. L

Abstract

AbstractThe cytochrome P‐450‐dependent 20‐monooxygenation of ecdysone is catalyzed both by mitochondria and microsomes isolated from Musca domestica (L.) larvae; however, about 50% of the activity is associated with mitochondria, and 37% is associated with microsomes. Pretreatment of larvae with ecdysone results in an increase in Vmax and a decrease in Km values in mitochondria but not in microsomes. Phenobarbital, a known cytochrome P‐450 inducer, increases the cytochrome P‐450 levels in microsomes without affecting the 20‐monooxygenase activity, but both the cytochrome P‐450 levels and monooxygenase activity are depressed in mitochondria from phenobarbital‐pretreated larvae. The ecdysone 20‐monooxygenase activity is equally distributed between mitochondria and microsomes in adult insects. Pretreatment of the insects with ecdysone does not significantly modify the 20‐monooxygenase activity of either mitochondrial or microsomal fractions, but the cytochrome P‐450 levels are reduced in mitochondria. Phenobarbital also depresses the mitochondrial cytochrome P‐450 levels while markedly increasing the microsomal cytochrome P‐450 levels. However, no significant changes in ecdysone 20‐monooxygenase activity are produced by phenobarbital pretreatment. The effects of ecdysone on adult cytochrome P‐450 are mostly evidenced in mitochondria isolated from females, whereas in males the changes are not statistically significant. It is concluded that the mitochondrial ecdysone 20‐monooxygenase is under regulatory control by ecdysone in the larval stage, which suggests that only the mitochondrial activity has a physiological role during insect development in M. domestica. In adults, both the mitochondrial and microsomal ecdysone 20‐monooxygenase activities are not responsive to ecdysone, which, coupled to their high Km values, indicates that the reaction may not be of physiological importance in adult insects and that the mitochondrial cytochrome P‐450 species being depressed by ecdysone in females are possibly not involved in ecdysone metabolism.