Possible inactivation of ingested ecdysteroids by conjugation with long-chain fatty acids in the female tick <i>Ornithodoros moubata</i> (acarina:argasidae)

<i>Ornithodoros moubata</i> females proved to be extremely sensitive to ingested 22,25-dideoxyecdysone; 15-20 ng provoked molting in all females and temporarily inhibited vitellogenesis. In contrast, this tick was very resistant to ingested ecdysteroids containing 22-OH groups, such as ecdysone, 20-hydroxyecdysone, ponasterone A, and makisterone A. Dosages about 500 times greater were necessary to produce supermolting and reduce fecundity [Connat et al: Z Ang Ent <i>96</i>, 520 (1983)]. Ingested tritiated ecdysone, 20-hydroxyecdysone, 2-deoxyecdysone, and ponasterone A were rapidly converted to apolar esterase-labile metabolites having approximately the same retention time as the AP2 identified as esters of ecdysteroids at C-22 with long-chain fatty acids (C16:0, C18:0, C18:1, C18:2) [Diehl et al: Int J Invert Reprod Dev <i>8</i>, 1 (1985)]. These products were then gradually transformed to the more polar apolar conjugates, AP1. A more detailed study with ingestion of large quantities of 20-hydroxyecdysone (10 <i>μ</i>/ml blood) demonstrated that only small amounts of free hormone were present in the hemolymph during the first day after the blood meal. The hormone was rapidly metabolized to AP2, then to AP1, in the intestinal cells and to a lesser extent in the peripheral tissues. Finally, AP1 accumulated in the intestinal cells and midgut content, probably because excretion outside the animal is impossible in this tick species. In contrast, ingested 22,25-dideoxyecdysone was not metabolized to apolar products. This could account for its high biological activity. This compound was converted to unidentified more polar products. Two of them comigrated with ecdysone and 20-hydroxyecdysone on RP-18 HPLC column, but not on silica column, and therefore cannot correspond to these compounds. <br> We hypothesize that esterification of ecdysteroids at the C-22 position with fatty acids represents a detoxification mechanism for ingested ecdysteroids that might be present in blood from herbivorous or parasite-infected hosts.