Ecdysones insects

From cholesterol 4.18) biosynthesis leads through truncation of the side-chain and other modifications to mammalian steroid hormones, e.g. oestrone 4.22) [11, 33]. Vitamin D3 4.24) is obtainable by photochemically mediated electrocyclic ring-opening of 7-dehydrocholesterol 4.23), followed by a thermal 1,7-sigmatropic hydrogen shift (Scheme 4.7) [34, 35]. The ecdysone insect moulting... 

Insects, crustaceans, platehelminthes, nematodes and annelids use homoses-quiterpenoid epoxides (juvenile hormones) and ecdysteroids (ecdysone, 20-... 

The A -6-ketone functionality in polyhydroxy cholestanes is common to all the insect moulting hormones of the ecdysone type. Therefore, there has recently been considerable interest in the introduction of A -double bonds into 6-ketones. The different approaches that follow illustrate the utility of isomerization, rearrangement, and blocking groups. 

Caterpillars and other moulting insects excrete the hormone a-ecdysone which at moulting time becomes hydroxylated to 20-hydroxyecdysone (20-E), which in turn triggers the moulting process enabling the insect to shed its exoskeleton and resume feeding. Rohm and Hass have developed a novel insecticide, tebufenozide (Formula 9.5) which mimics 20-E, binding to the same site. The consequence of this is that the insect stops... 

Most of the more recently described allenic steroids bear an allene group at the 17-position, which was usually formed by an SN2 substitution [106] or reduction [86d] process of a suitable propargylic electrophile. Thus, reduction of the pro-pargylic ether 109 with lithium aluminum hydride followed by deprotection of the silyl ether resulted in the formation of the allenic steroid 110, which irreversibly inhibits the biosynthesis of the insect moulting hormone ecdysone (Scheme 18.35) [107]. 

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. 

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. 

Ecdysone, a highly hydroxylated steroid (Fig. 22-12), is a molting hormone for insects.331,332 Several molecules with ecdysone activity are known, and some of these are produced by certain plants. Although ecdysones are needed by insects for larval molting, they are toxic in excess. Perhaps plants protect themselves from insects by synthesizing these substances. 

Eukaryote organisms primarily respond to external signals by an initial signal perception by receptors. In general, such receptors can be either cytosolic or located on the plasma membrane [13-15]. The former mechanism applies to thyroid hormones (triiodothyronine and tetraiodothyronine or thyroxine), retinoids (e.g. retinoic acid), the insect developmental hormones such as ecdysone, steroid hormones (such as... 

Juvenile hormone plays a critical role in maintaining the juvenile or larval stage of insects, and if its secretion is not controlled, normal development to the adult stage is prevented. Use of hormones or substances with hormonelike activity to control insect populations is an area of intense research interest and activity.2 The secretion of juvenile hormone is controlled by other hormones originating in the brain (brain hormone) and the phthoracic gland (moulting hormone, ecdysone see Table 30-2). 

Another common mechanism for modulating hormonal response involves two (or more) hormonal inputs with both positive and negative effects (see fig. 24.21). The hypothalamic peptides, somatostatin and GRF, have opposite effects on GH synthesis and secretion. Similarly, glucagon and insulin have opposite effects on gluconeogenesis in the liver (see the discussion earlier in this chapter), and some of the effects of ecdysone on gene expression in insects are blocked by juvenile hormone (a terpene derivative fig. 24.22). 

Structure of /3-ecdysone and juvenile hormone. These hormones play major roles in the growth and maturation of insects by controlling the timing for molting of the insect exoskeleton. 

Ecdysone. A hormone that stimulates the molting process in insects. 

Antoniewski C., Laval M. and Lepesant J. A. (1993) Structural features critical to the activity of an ecdysone receptor binding site. Insect Biochem. Mol. Biol. 23, 105-14. 

Tebufenozide acts by mimicking the effects of the insect hormone ecdysone, which along with juvenile hormone, controls the initiation of a molt. Exposure to tebufenozide induces a premature molt that traps the insect in its old cuticle. This compound is especially effective against caterpillars. 

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