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FIGURE 2 The structure of 20-hydroxyecdysone (20HE) and related ecdysteroids. The hydroxyl substituent of 20HE at position 20 confers biological activity to ecdysone. Many insects obtain phytoecdysones from plants and convert them to biologically active forms. These molecules differ from 20HE only in the number of carbons in the alkyl side chain, shown for (A) makisterone A (28 carbons), (B) makisterone C (29 carbons), and (C) ponasterone A, which lacks the hydroxyl group at position 25.

The first ecdysone structure soon was recognized as a precursor to the biologically active material. Upon release from the prothoracic glands, ecdysone, or its 3-dehydro form in some insects, is converted to the active form upon arriving at target tissues. The tissues capable of responding to the hormone produce the enzymes needed to attach a single hydroxyl group to the carbon at position 20, making it 20-hydroxyecdysone (20HE). This substance had already been identified in crustaceans as crustecdysone. Identical molecules function as the biologically active hormone in insects and in crustaceans.

Not long after the identification of ecdysone, phytoecdysones were discovered in plants. The first, ponasterone A, was discovered by Koji Nakanishi, and an identical molecule was later found in some crustaceans. Ponasterone A, which differs from ecdysone in lacking a single hydroxyl at position 25, eventually proved useful in radiolabeled form for the characterization of ecdysteroid receptors. Hundreds of phytoecdysones have been identified, including 20HE itself. Reasons for the presence of phytoecdysones in plants are unclear, but they may serve defensive roles by disrupting the growth of herbivorous insects.

As more insects were examined, additional configurations of the basic ecdysone structure were found. This group of molecules now is collectively known as ecdysteroids. Insects require cholesterol in the diet to synthesize ecdysteroids. Often phytosterols such as campesterol and p-stigmosterol are converted to produce hormonally active makisterones A and C, respectively (Fig. 2).

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