Ecdysteroid metabolism in leeches

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Abstract
The presence of pathways of ecdysteroid metabolism in the leech Hirudo medicinalis has been investigated with in vivo injections of 20-hydroxyecdysone and several potential precursors. 20-Hydroxylation and 3-epimerization have been demonstrated, the latter being restricted to 2-deoxy-compounds. Conjugation to polar ionizable metabolites was also observed for ecdysone and 20-hydroxyecdysone, but not significantly for 2-deoxycompounds, and this suggests that the 2-OH could be involved in the conjugating reaction. By contrast, neither 22-hydroxylation nor 2-hydroxylation, which represent the two last steps in the biosynthetic pathway of insects could be detected. This raises a serious doubt regarding the endogenous origin of 20-hydroxyecdysone in leeches, although the presence of an alternative biosynthetic pathway cannot be excluded. The significance of these results is discussed with respect to the phylogeny of endocrine systems in invertebrates.