Ecdysteroids isolated from insects and their biosynthesis

Karlson and Hoffmeister first demonstrated the conversion of [ H]cholesterol to ecdysone in Calliphora erythrocephala. Subsequently, the conversion was reported in other species. Direct evidence for the secretion of ecdysone from prothoracic glands has been provided by in vitro culture of such organs in appropriate media. The incorporation of radioactive cholesterol into ecdysone by such glands has been reported for B. mori [190], Manduca sexta [191], and Leucophaea manderae [192]. 

Since the elucidation of the structure of ecdysone, a number of synthetic cholesterol derivatives have been demonstrated as possible intermediates in the conversion of cholesterol into ecdysone. However, few attempts have been made to isolate the intermediates of ecdysone synthesis in the PG. Only 3j8-hydroxy-5a-cholestan-6-one (111) was isolated from the organ culture medium of the silkworm PG as a possible intermediate [193]. The PG possesses a comparatively large percentage of 7-dehydro-cholesterol (110) in several insects. Thus, 7-dehydrocholesterol may also be a possible early intermediate in the transformation of cholesterol into ecdysone [194]. 

The mode of formation of 7-dehydrocholesterol by larvae of C. erythrocephala was demonstrated by the Liverpool group. Recently the same group reported that 

Many endogenous ecdysteroids which can be considered as the intermediates of ecdysone biosynthesis were isolated from the ovaries of L. migratoria by Hoffmann and Horn [204]. From the results of the bioconversion of labelled precursors, the following sequence in the biosynthetic pathway of ecdysteroids in vitellogenic ovaries of the insect was proposed conversion of cholesterol to 3)3-hydroxy-5)8-cholest-7-en-6-one followed by 14a-hydroxylation to 3/S,14-dihydroxy-5/6-cholest-7-en-6-one (112) hydroxylation at C-25 and C-22 (in this order) to 2-deoxyecdysone (114) hydroxylation at C-2 to ecdysone (107). 

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Ecdysteroids isolated from insects and their biosynthesis... 

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