Insects convert ecdysone and 20-hydroxyecdysone into their corresponding 26-oic derivatives, named ecdysonoic acid and 20-hydroxyecdysonoic acid respectively. The conversion takes place in several tissues and can either be the only pathway for converting ecdysone into highly polar ecdysteroids, or coexist with various conjugating mechanisms. 20-Hydroxyecdysonoic acid was isolated from Pieris brassicae pupae as its methyl ester derivative. Its chemical structure was identified by CUD mass spectrometry and compared with a synthetic compound (20-hydroxy-25-deoxyecdysonoic acid) chemically prepared by oxidation of inokosterone (20,26-di hydroxy-25-deoxyecdysone). Natural ecdysonoic acids appear to exist as a mixture of 25R and 25s isomers. The significance of this pathway is discussed in comparison with similar reactions occuring in the metabolism of steroid hormones in vertebrates.

Endogenous ecdysteroids as well as radiolabelled exogenous ecdysone were crosslinked to a protein when haemolymph from blowfly larvae was irradiated with UV-light under optimized conditions. This indicates that larval haemolymph of blowflies contains an ecdysteroid-binding protein.The crosslinked ecdysteroid-protein complex was shown to form a ternary complex when mixed with ecdysteroid-specific antibodies. Formation of the ternary complex was due to a specific interaction of the binding sites of the antibodies with the steroid. This specific interaction was used to isolate the ecdysteroid-protein complex from irradiated crude larval haemolymph by immunoadsorption. The method described here represents a rapid approach for the isolation of steroid-binding proteins by a combination of photoaffinity labelling and immunoadsorption.

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