Nes W D, Lopez M, Zhou W, Guo D, Dowd P F, Norton R A
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock 79409, USA.
Lipids. 1997 Dec;32(12):1317-23. doi: 10.1007/s11745-006-0170-5.
Heliothis zea (corn earworm), an insect that fails to synthesize sterols de novo, was reared on an artificial diet treated with 18 different sterol supplements. Larvae did not develop on a sterol-less medium. delta 5-Sterols with a hydrogen atom, a methylene group, an E- or Z-ethylidene group, or an alpha- or beta-ethyl group (cholesterol, ostreasterol, isofucosterol, fucosterol, sitosterol, and clionasterol, respectively) at position C-24, and delta 5-sterols doubly substituted in the side chain at C-24 with an alpha-ethyl group and at C-22 with a double bond (stigmasterol) supported normal larval growth to late-sixth instar (prepupal: maturity). The major sterol isolated from each of these sterol treatments was cholesterol, suggesting that H. zea operates a typical 24-dealkylation pathway. The sterol requirement of H. zea could not be met satisfactorily by derivatives of 3 beta-cholestanol with a 9 beta, 19-cyclopropyl group, gem dimethyl group at C-4, a delta 5,7-bond or delta 8-bond, or by side chain modified sterols that possessed a delta 25(27)-24 beta-ethyl group, delta 23(24)-24-methyl group or 24-ethyl group, or delta 24(25)-24-methyl or 24-ethyl group. The major sterol recovered from the larvae (albeit developmentally arrested larvae) treated with a nonutilizable sterol was the test compound. Sterol absorption was related to the degree of sterol utilization. The most effective sterols absorbed by the insect ranged from 27 to 66 micrograms per insect, whereas the least effective sterols absorbed by the insect ranged from 0.6 to 6 micrograms per insect. Competition experiments using different proportions of cholesterol and 24-dihydrolanosterol (from 9:1 to 1:9 mixtures) indicated that abnormal development of H. zea may be induced on less than a 1 to 1 mixture of utilizable (cholesterol) to nonutilizable (24-dihydrolanosterol) sterols. The results demonstrate new structural requirements for sterol utilization and metabolism by insects, particularly with respect to the position of double bonds in the side chain and functionalization in the nucleus. The novel sterol specificities observed in this study appear to be associated with the dual role of sterols as membrane inserts (nonmetabolic) and as precursors to the ecdysteroids (metabolic).
棉铃虫是一种无法从头合成甾醇的昆虫，它被饲养在添加了18种不同甾醇补充剂的人工饲料上。幼虫在无甾醇培养基上无法发育。在C-24位带有氢原子、亚甲基、E-或Z-亚乙基或α-或β-乙基的δ5-甾醇（分别为胆固醇、骨甾醇、异岩藻甾醇、岩藻甾醇、谷甾醇和海胆甾醇），以及在C-24侧链上被α-乙基和C-22位被双键双重取代的δ5-甾醇（豆甾醇）支持幼虫正常生长至六龄后期（预蛹期：成熟期）。从这些甾醇处理的每一种中分离出的主要甾醇都是胆固醇，这表明棉铃虫具有典型的24-脱烷基化途径。棉铃虫对甾醇的需求不能通过具有9β,19-环丙基、C-4位偕二甲基、δ5,7-键或δ8-键的3β-胆甾烷醇衍生物，或具有δ25(27)-24β-乙基、δ23(24)-24-甲基或24-乙基，或δ24(25)-24-甲基或24-乙基的侧链修饰甾醇来满足。用不可利用的甾醇处理的幼虫（尽管发育停滞）中回收的主要甾醇是测试化合物。甾醇吸收与甾醇利用程度相关。昆虫吸收的最有效的甾醇范围为每只昆虫27至66微克，而吸收的最无效的甾醇范围为每只昆虫0.6至6微克。使用不同比例的胆固醇和24-二氢羊毛甾醇（从9:1到1:9混合物）的竞争实验表明，当可利用的（胆固醇）与不可利用的（24-二氢羊毛甾醇）甾醇的混合物比例小于1:1时，可能会诱导棉铃虫发育异常。结果证明了昆虫对甾醇利用和代谢的新结构要求，特别是关于侧链双键位置和核功能化方面。在本研究中观察到的新的甾醇特异性似乎与甾醇作为膜插入物（非代谢）和蜕皮甾类前体（代谢）的双重作用有关。
