Ecdysones plants

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. 

Ecdysone is the steroid hormone of the arthropods. It can be regarded as an early form of the steroid hormones. Steroid hormones with signaling functions also occur in plants. 

Osmunda japonica L. Zi Kee Guan Zhong (Japanese royal fern) (whole plant) Ponasterone A, ecdysterone, custeodysine, ecdysone.33 Anthelmintic, treat inflammation of the salivary glands. 

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. 

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. 

PE-free callus from Polypodium vulgare was shown to biotransform ecdysone fo 20-hydroxyecdysone, which is the last step in the biosynthetic pathway of the main plant PE. This hydroxylation is catalysed by a cytochrome P450 enzyme which was subsequently purified from that source (Canals et al, 2005). In another study, Reixach et al. (1999) have shown that 25-deoxy-20-hydroxyecdysone was transformed efficiently in both tissues into 20-hydroxyecdysone, but no 25-deoxyecdysteroids such as pterosterone and inokosterone were formed. Likewise, incubation of 2-deoxyecdysone produced exclusively ecdysone and 20E, indicating a high 2-hydroxylase activity in both tissues. 

The ability of insect sulfotransferases to catalyze the sulfation of plant and insect steroids may simply reflect the broad substrate specificity of the enzymes. On the other hand, it may be indicative of a more important physiological function of the enzymes. Sulfate esters of cholesterol, campesterol and e-sitosterol have been identified in the meconium of tobacco hornworm (M. sexta) pupae ( ), and these steroids are known precursors of a-and e-ecdysone and other molting hormones in this species ( ). Further, there is evidence that houseflies (M. domestica) and diapausing pupae of M. sexta convert 22,25 bisdeoxyecdysone, a-ecdysone and 20-hydroxyecdysone into sulfate and glucoside conjugates ( ). 

Sterols. A considerable amount of work was devoted to the study of backbone rearrangements. - While partial syntheses of an insect moulting hormone, ecdysone, were announced by several groups, evidence is accumulating on the widespread occurrence in natxire of its hydroxylated derivatives. This year saw the isolation of 20-hydroxy-ecdysone - - from silkworm (ecdysterone), crayfish - (crustecdysone), oak-silk moth, tobacco hornworm, and from plants such as Podocarpus nakaii, Podocarpus elatus, and Achyranthis. The ready isolation of insect-moulting hormones from plants in contrast to -the extremely poor... 

It is not too surprising that many plants produce ecdysteroids to defend themselves against potential insect pests. The so-called phyto-ecdysones have a potent molt-inducing effect, and some of them may be even more potent than p-ecdysone. Gymnosperms and ferns often contain much of phyto-ecdysones. Bracken (Pteridium aquilium) and the rhizomes of the well-known liquorice fern (Polypodium vulgare), for instance, have up to 1% of their dry weight of p-ecdysone, and the rhizomes of liquorice can be used as a commercial source of P-ecdysones. A Siberian medicinal plant (Leuzea carthamoides) was recently studied by Zeleny et al. (1997). The plant has as... 

The important insect hormone a-ecdysone (121) and related steroids are also present in plants (Section 9). Their biosynthesis from cholesterol probably proceeds via the A -diene to the triolone (119). The side chain is then hydroxylated to the (22/ )-hydroxy steroid (120), a-ecdysone (121), and ) -ecdysone (122) (crustecdysone). Finally, breakdown of )8-ecdysone gave... 

In many plants the capacity for cholesterol synthesis may be greater than it may seem at first glance because of the marked synthesis of steroid hormones, cardeno-lides and ecdysones which are all derived from cholesterol. 

Subsequent hydroxylation of a-ecdysone yields )8-ecdysone (Fig. 19), the most oxidized ecdysone normally encountered in plants. 

Insects, unlike most vertebrates and plants, lack the capacity for de novo sterol synthesis and require dietary sterol for their normal growth, development and reproduction. This sterol requirement is in most cases satisfied by cholesterol (86) which is one of the principal sterols in insects, serving as component of the cell membranes and as a precursor of ecdysone (107). The zoophagous species such as the house fly Mucosa domestica are unable to convert phytosterol to cholesterol. For this reason, cholesterol is an essential nutrient for these species. In phytophagous and omnivorous insects, sterols such as sitosterol (87), campesterol (88), and stigmasterol (89) are dealkylated to cholesterol. Thus, 24-dealkylation is one of the essential metabohc processes in phytophagous insects (Fig. 15). 

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