A new class of apolar ecdysteroid conjugate: esters of 20-hydroxy-ecdysone with long-chain fatty acids in ticks

Nymphs of the argasid tick <i>Ornithodoros moubata</i> rapidly transform most of the ingested molting hormone 20-hydroxy-ecdysone (20-hydroxy-E) into a family of 4 major apolar conjugates which are hydrolyzable by carboxylic-ester hydrolase. These conjugates - new among zoo- or phytoecdysteroids - are composed of 20-hydroxy-E esterified at C22 with the common long-chain fatty acids C_16:0, C_18:0, Cig_18:1, or C_18:2;. The chemical structure of the purified metabolites was identified by fatty acid analysis, by CI/D mass spectrometry and by comparison with a chemically prepared 20-hydroxy-E-22-palmitate. <br> The significance of the apolar metabolic pathway is discussed. It may serve to inactivate molting hormones which could be ingested with blood from hosts that feed on plants containing phytoecdysteroids. In addition, the conjugates may act as a storage form of hormones for the developing embryo.