Ecdysone metabolism

At the physiological level it is well established that vital dyes such as nile blue, neutral red and methylene blue retard larval development under normal lighting conditions (12L/12D with source unspecified) (25 27). Female but not male pupal weights are also reduced. Unfortunately experiments were conducted without dark controls so that it is difficult to evaluate the role of photosensitization in these effects. As house flies and fire ants succumb to photosensitization, they lose motor control and become more excitable (28). This suggested a neurotoxic effect and investigation of fire ant acetylcholinesterase vitro revealed that this enzyme was sensitive to photo-oxidation. vivo results, however, revealed no effect on the enzyme which suggests another mode of action. Epoxldatlon of cholesterol and membrane lysis may be alternative primary sites. If this were the case ecdysone metabolism of insects would probably also be effected. 

Figure 2 Principal reaction paths of metabolism of ecdysone. The larger the print, the more common the reaction in known examples. Modified from R. Lafont C. Dauphin-Villemant J. T. Warren H. H. Rees, Insect Hormones. In Comprehensive Molecular Insect Science-, Elsevier Oxford, 2005 Vol. 3, pp 125-195, Fig. 15, Copyright Elsevier, 2004, in turn Modified from R. Lafont J.-L. Connat, Pathways to Ecdysone Metabolism. In Ecdysone From Chemistry to Mode of Action-, J. Koolman, Ed. Georg Thieme Stuttgart, 1989 Chapter 14, Fig. 14.1.

Yu, S.J. and Terriere, L.C., Ecdysone metabolism by soluble enzymes from three species of Diptera and its inhibition by the insect growth regulator TH-6040, Pestic. Biochem. Physiol., 7,48,1977. 

Yu and Terriere (1977) concluded from their experiments on housefly larvae that the disruption of chitin formation by diflubenzuron is due to the inhibition of enzymes responsible for ecdysone metabolism. 

Insect steroid metabolism has two biochemically distinctive components dealkylation of phytosterols to cholesterol and polyhydroxylation of cholesterol to ecdysone. We will focus on the first of these. Lacking the ability to synthesize sterols de novo, insects instead have evolved a dealkylation pathway to convert plant sterols to cholesterol(7-10). The dealkylation pathways are apparently absent in most other higher and lower organisms, which can convert mevalonate to squalene and thence into sterols( ). Specific insecticides are possible based on these biochemical differences. 

As reviewed by Williams (43), ecdysone has been isolated from more than 10 species of conifers, 20 ferns, and 30 flowering plants (out of 1000 species surveyed). A total of 28 different plant ecdysones are known, the most ubiquitous being /3-ecdysone. The ecological significance of /3-ecdysone in plants is unclear. It is not toxic when orally ingested (as feeding larvae would obtain it from a food plant), but there is some evidence that it could be a feeding deterrent in concentrations as low as 1 ppb. Perhaps it serves as a steroid base for other compounds once it is in an insect s metabolic system. 

Scheme 3). A clone of cells from the midge Cbironomus tentans was found to be resistant to the effects of ecdysteroids because they metabolized 20-hydroxyecdysone rapidly. The initial oxidation product was 20,26-dihydroxyecdysone, but this was oxidized further to 20-hydroxy-26-oxo-ecdysone (21). This aldehyde (21) then formed a tautomeric equilibrium mixture of two cyclic hemiacetals (22) and (23), which were separable, isolated, and their structures determined (Scheme 3) with the use of acetonides (Section 4.03.3.6).32 These are the first examples of ecdysteroids with side-chain hemiacetals. Although 20,26-dihydroxyecdysterone still... 

The detailed mechanism of the formation of ecdysones in insects continues to be explored. [3a- H]-3/3,I4o-Dihydroxy-5/8-cholest-7-en-6-one is metabolized in Calliphora stygea at puparium formation via the keto-triol (92) to a- and /8-ecdysones and so the diol (93) is a likely precursor in vivo. In another Calliphora species a study of the interconversion of cholest-5-en-3/8-ol into cholesta-... 

Our laboratory is concerned with targeting potential insecticides that disrupt normal development and metamorphosis in insects. Juvenile hormones (JHs), acting in concert with the steroid hormone ecdysone, are believed to control the timing of the larval-larval molts, larval-pupal and pupal-adult transformations of the insects. It has been demonstrated that the events leading to pupation are initiated by reduction of the JH titer in the hemolymph. In addition to a cessation of biosynthesis, this reduction in JH titer is controlled by degradative metabolism (16,17). Hydrolysis of the epoxide and ester functionalities present in active JH are two routes of degradation and subsequent inactivation of JH (18). The primary route of JH metabolism in the hemolymph of last stadium lepidopterous larvae is ester hydrolysis, and it is catalyzed by the enzyme juvenile hormone esterase (JHE). JHE has been shown to... 

The effect of 22-dehydrocholesterols (97) on the growth of Drosophila was investigated by Kirchner et al. [180]. cis- and /ran5-22-dehydrocholesterols were added to media of diet for 10 species of Drosophila. The cis isomer prevented normal maturation of 4 species and the trans isomer was toxic to 9. These findings were corroborated by tests with 4 representative species on a sterol-deficient medium under axenic conditions. Addition of cholesterol to the latter overcame the toxicity of the trans isomer. trans-22-Dehydrocholesterol may be acting as a competitive inhibitor in the metabolism of phytosterols to cholesterol or ecdysone by the insects. 

The biosynthesis and metabolism of the insect moulting hormone, ecdysone have been the subject of recent reviews [184-186]. Ecdysone (107) was first isolated in a crystalline form from the silkworm, Bombyx mori, by Butenandt and Karlson in 1954, and the structure was determined in 1965. Soon after, the second moulting hormone was isolated and the structure was elucidated as 20-hydroxyecdysone (108). Ibe same hormone was also isolated from the sea-water crayfish, Jasus lalandei by Horn in 1966. Many ecdysone analogues were isolated from arthropods and certain... 

The effect of ecdysones and ecdysoids has been studied most extensively by Karlson et al. (Karlson, 1960 1966 Karlson and Schmied, 1955 Karlson and Sekeris, 1962). They found that besides exerting their action by other routes ecdysone hormones act by interfering with tyrosine metabolism. Tyrosine in the insect haemolymph is converted in several steps to quinone , as shown in Scheme 1.29. 

The sterol requirements of invertebrates are frequently satisfied by modification of dietary steroids. Thus, cholesterol is formed from 24-alkylated steroids, such as ergosterol and /5-sitosterol, by Crustaceans and insects. The mechanism of this process seems to be the reverse of their mode of formation. The 24-ethyl group of -sitosterol is converted into a 24-ethylidene group with fucosterol, and cholesta-5,24-dienol is formed on loss of the alkyl group. Cholesterol is required in insects for metabolism to the hormone ecdysone (84). However, plants also produce ecdysone and both organisms metabolize cholesterol to ecdysone. which is then further metabolized to ecdysterone (85)." ... 

Perhaps, [ H]ecdysone should be injected in tobacco hornworms at an earlier stage of the life cycle (5th instar) to determine whether it is incorporated and metabolized to 26-hydro) yecdysone, the major ecdysteroid of ovaries and eggs of Manduca. 

Snyder MJ, Chang ES (1991c) Metabolism and excretion of injected [3H]-ecdysone by female lobsters, Homarus americanus. Biol Bull 180 475 184... 

While the very high affinities of tebufenozide, methoxyfenozide and chromafenozide for lepidopteran ecdysone receptors help explain the basis for their selective lepidopteran toxicity, the same does not apply to the fourth BAH insecticide, halofenozide, which is toxic to both lepidopteran and coleopteran larvae. Halofe-nozide has significantly reduced affinity for ecdysone receptors from the two insect orders. It seems that the relatively weak affinity of halofenozide to the ecdysone receptor of the target susceptible insect may be compensated by its increased metabolic stability in the same insect. 

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