Evidence for an α-ecdysone cytochrome P-450 mixed function oxidase in insect fat body mitochondria

The functional roles of cytochrome P-450 mediated systems: present knowledge and future areas of investigations

Research in recent years has considerably enhanced our understanding of the membrane-bound cytochrome P-450-catalyzed biotransformations in organisms. A remarkable similarity has become apparent between diversified animal groups with respect to the components and functional roles of the cytochrome P-450 systems. For example, the steroidal hormone metabolism role for this cytochrome in mammals was recently extended to insect species. In addition, this cytochrome functions importantly in mammalian species for the metabolism of a variety of other endogenous substrates such as bile acids, anthranilic acid, fatty acids, bilirubin and heme, indoles, amines, and purines. Regarding the exogenous role, the system has been known to convert toxic lipophilic substances such as drugs and insecticides to more hydrophilic metabolites which can be easily eliminated by the water-based excretory system in organisms. This detoxication ability apparently arose in organisms, particularly the herbivores, in response to the need to detoxify plant-defensive toxins such as tannins, phenols, quinones, alkaloids, steroids, lecithins, and lignins. The interface between plants and insects is enormous, and therefore it appears probable that cytochrome P-450 will one day be shown to be important in the conversion/utilization of other plant secondary substances, e.g., synthesis of cholesterol from phytosterols, and sex pheromones from olefins, terpenes, and alkaloids.

Ecdysone oxidase: reaction and specificity

Ecdysone oxidase oxidizes 3-hydroxysteroids of the excysteroid type irreversibly to 3-dehydro derivatives. The hydrogen of the steroid is transferred by the enzyme to oxygen which is reduced to hydrogen peroxide. Ecdysone oxidase is relatively specific for ecdysone and ecdysterone. Apparent Michaelis constants for these two physiological substrates are 98 and 31 micron, respectively. Cholesterol oxidase and 3alpha-hydroxysteroid and 3beta-hydroxysteroid dehydrogenases which catalyze similar reactions are unable to oxidize ecdysteroids.

Ecdysterone biosynthesis: a microsomal cytochrome-P-450-linked ecdysone 20-monooxygenase from tissues of the African migratory locust

Ecdysone 20-monooxygenase, an enzyme which converts ecdysone to ecdysterone (the major moulting hormone of insects) has been characterized in cell-free preparations of tissues from African migratory locust. The product of the reaction has been identified as ecdysterone on the basis of several microchemical derivatization and chromatographic methods. Ecdysone 20-monooxygenase activity is located primarily in the microsomal fraction which also carries NADPH cytochrome c reductase and cytochrome P-450, as shown by sucrose density gradient centrifugation. Optimal conditions for the ecdysone 20-monooxygenase assay have been determined. The enzyme has a Km for ecdysone of 2.7 x 10(-7) M and is competitvely inhibited by ecdysterone (Ki = 7.5 x 10(-7) M). Ecdysone 20-monooxygenase is a typical cytochrome P-450 linked monooxygenase: the reaction requires O2 and is inhibited by CO, an effect partially reversed by white light. The enzyme is effectively inhibited by several specific monooxygenase inhibitors and by sulfhydryl reagents, but not by cyanide ions. Ecdysone elicits a type I difference spectrum when added to oxidized microsomes. NADPH acts as preferential electron donor. The transfer of reducing equivalents proceeds through NADPH cytochrome c (P-450) reductase: ecdysone 20-monooxygenase is inhibited by cytochrome c. Both NADPH cytochrome c reductase and ecdysone 20-monooxygenase are inhibited by NADP+ and show a similar Km for NADPH. The Malpighian tubules have the highest specific activity of ecdysone 20-monooxygenase, while fat body contain most of the cytochrome P-450 and NADPH cytochrome c reductase.