Ecdysones biosynthetic pathway

Figure 4. Possible biosynthetic pathway from dietary steroids to a-ecdysone.

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. 

Following a possible biosynthetic pathway to 3/8,14-dihydroxycholest-7-en-6-one (355), a precursor of ecdysones, 3/8-acetoxycholesta-5,7-diene (352)was isomerized to the 6,8(14)-diene (353) epoxidation and acidic hydrolysis then gave the mixed 6,14a-diols (354), which were oxidized by MnOa to give the required 14a-hydroxy-7-en-6-one system (355). Several cholest-7-en-6-one derivatives related to ecdysones, but lacking side-chain hydroxylation, have been prepared from cholesterol by efficient routes based largely on known individual steps. 

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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