Mass Spectrometric Studies of Sterol Metabolites to Elucidate the Biosynthetic pathway of Ecdysone in the Desert locust Schistocerca Gregaria

Abstract

Ecdysone is a prohormone for 20-hydroxyecdysone, in locusts it is biosynthesized in the prothoracic gland where dietary sterol, in particular cholesterol is sequestered from circulating hemolymph and converted into ecdysone or closely related ecdysteroids via two proposed synthetic pathways. The objective of this work was to a) identify by coupled gas chromatography-mass spectrometry sterols and their trimethylsilyl derivatives present in wheat seedlings, oral secretion, gut, frass, hemolymph, fat bodies and prothoracic glands of the desert locust after feeding on wheat plant, b) propose a pathway for biosynthesis of ecdysone in the locust based on the results. Sterols were identified by comparison of their mass spectral data and those of their trimethylsilyl derivatives with authentic standards if available. Metabolites of cholesterol in the gut were further confirmed through incubation of the gut with cholesterol-[4-13C]. Overall, the distribution of sterols was: wheat 10, oral secretion 15, foregut 15, midgut 15, hindgut 10, frass 10, hemolymph 7, fat body 7 and prothoracic gland 4. This is the first detailed profiling of phytosterols in the rearing diet, body tissues and excreta of locusts. The major discovery from this study is that it identified a metabolite of cholesterol, cholesta-5,7-dien-3-ol,(3β) in the oral secretion, foregut and midgut this compound was initially reported in literature to be synthesized in the prothoracic glands indicating that biosynthesis of ecdysone may start early in the gut system of the desert locust. The incubation studies also revealed other metabolites of cholesterol in the gut system. This study provides opportunities for further research into sterol profiling in other locust species and ecdysone biosynthesis a knowledge which can be exploited for the desert locust control.