Ecdysone metabolism inLocusta migratorialarvae and adults

Abstract

The metabolism of ecdysone and 20-hydroxyecdysone has been investigated in fifth instar larvae and male adults of Locusta migratoria (Insecta, Orthoptera). The major pathways have been elucidated by labelling experiments, followed by the isolation and the characterization of the different metabolites. These ecdysteroids are converted by locusts through three major reactions: (1) C-26 hydroxylation followed by conversion to ecdysonoic acids, (2) C-3 acetylation, and (3) C-2 phosphate ester formation. The combination of (2) and (3) gives rise to acetylphosphate derivatives, which represent the major conjugates in insect faeces. Minor metabolites are also formed, corresponding to 3-dehydroecdysteroids and phosphate conjugates of various metabolites which have not yet been fully characterized. The reactions are identical in larvae and adults and are similar to those described for embryos of the same species and the related Schistocerca gregaria. In vitro organ incubations have allowed us to determine the respective roles of gut, Malpighian tubules, fat body and epidermis in these reactions. Ingested ecdysteroids undergo a rapid conversion to acetylphosphate derivatives in the gut, and this could represent an efficient mechanism of inactivating phytoecdysteroids present in the diet.