Warren James T, O'Connor Michael B, Gilbert Lawrence I
Department of Biology, Campus Box 3280, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3280, USA.
Insect Biochem Mol Biol. 2009 Oct;39(10):677-87. doi: 10.1016/j.ibmb.2009.08.004. Epub 2009 Aug 20.
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).
长期以来，人们一直推测，由膳食胆固醇（C）生成的7-脱氢胆固醇（7dC）氧化为3-氧代-7dC（3-氧代-Δ⁵,⁷C），紧接着是未知的“黑匣子”氧化过程，该过程导致形成第一种上游中间体，其具有昆虫、甲壳类动物和其他一些节肢动物的类固醇蜕皮激素的高度特征性蜕皮甾类结构。3-氧代-7dC的生物合成及其随后的氧化修饰可能是限速步骤，并受促前胸腺激素（PTTH）的控制，由于其明显的不稳定性，即在用放射性标记前体进行昆虫前胸腺孵育时，从未观察到或鉴定出7dC与双酮醇（3-氧代-25,22,2-三脱氧蜕皮激素）之间的中间体，因此一直难以研究。然而，我们表明，3-氧代-7dC可以转化为亲脂性、光敏性、酮保护（PSKB）缩酮衍生物，在用无害的长波（365nm）紫外光进行短暂照射后，无论在体内还是体外，该衍生物都能在需要的时间和地点释放3-氧代-7dC。通过这种方式，3-氧代-7dC能够快速有效地整合到蜕皮甾类中，这些蜕皮甾类来自于以含有PSKB缩酮的饲料饲养的成年雄性和雌性果蝇，以及与乳化在培养基中的缩酮一起孵育的烟草天蛾前胸腺。3-氧代-7dC的不稳定性及其自发转化为广泛电子离域的中间体将与“黑匣子”氧化的可能机制相关进行讨论，该氧化最终导致活性蜕皮激素20-羟基蜕皮酮（20E）的产生。
