Studies on the Black Box: incorporation of 3-oxo-7-dehydrocholesterol into ecdysteroids by Drosophila melanogaster and Manduca sexta.
It has long been hypothesized that the oxidation of 7-dehydrocholesterol (7dC), made from dietary cholesterol (C), to 3-oxo-7dC (3-oxo-Delta(5,7)C) immediately precedes the unknown "Black Box" oxidations that lead to the formation of the first up-stream intermediate exhibiting the highly characteristic ecdysteroid structure of the steroid molting hormone of insects, crustaceans and some other arthropods. Perhaps rate-limiting and under the control of the prothoracicotropic hormone (PTTH), the biosynthesis of 3-oxo-7dC and its subsequent oxidative modifications have been difficult to study because of their apparent instability, i.e. no intermediates between 7dC and the diketol (3-oxo-25,22,2-trideoxyecdysone) have ever been observed or identified in insect prothoracic gland incubations with radiolabelled precursors. However, we show that 3-oxo-7dC can be converted into lipophilic, photosensitive, ketone-blocked (PSKB) ketal derivatives which will release 3-oxo-7dC when and where desired following brief irradiation with innocuous long-wave (365 nm) UV-light both in vivo and in vitro. In this manner, 3-oxo-7dC is quickly and efficiently incorporated into ecdysteroids by adult male and female Drosophila raised on a diet containing the PSKB ketals and in prothoracic glands of Manduca sexta incubated with the ketals emulsified into media. The instability of 3-oxo-7dC and its spontaneous transformation into extensively electron-delocalized intermediates will be discussed in relation to a possible mechanism of the Black Box oxidations eventually leading to the production of the active molting hormone 20-hydroxyecdysone (20E).
Warren
James T
JT
Department of Biology, Campus Box 3280, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3280, USA.
O'Connor
Michael B
MB
Gilbert
Lawrence I
LI
D000818:Animals; D003684:Dehydrocholesterols / Q000737:chemistry / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000737:chemistry / Q000378:metabolism*; D026461:Ecdysteroids / Q000737:chemistry / Q000378:metabolism*; D005260:Female; D008297:Male; D018402:Manduca / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction
D003684:Dehydrocholesterols; D026461:Ecdysteroids; C016705:7-dehydrocholesterol