Possible inactivation of ingested ecdysteroids by conjugation with long‐chain fatty acids in the female tick Ornithodoros moubata (acarina:argasidae)

Abstract

Ornithodoros moubata 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 96, 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 8, 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 μ/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. 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.

Distribution and metabolism of exogenous ecdysteroids in the egyptian cotton leafwormSpodoptera littoralis (lepidoptera: noctuidae)

“…Although varying concentrations of ecdysteroid were not administered to the larvae by injection, literature precedent suggests that in other species an absence of the apolar pathway in the haemolymph and carcass results in more pronounced effects on growth and development (Connat et al, 1986). It appears that the timing of the injection relative to the developmental status of the insect also plays a part in determining the effect of the ecdysteroid.…”

Apolar conjugates of ecdysteroids are not used as a storage form of molting hormone in the argasid tick Ornithodoros moubata

“…*Abbreviations: AP = apolar products (with reference to compounds that are appreciably less polar than ecdysone) previously described [7,11] and specified as API and AP2 according to their retention times in reverse-phase HPLC; E = ecdysone; 20E = 20-hydroxyecdysone; RP = reverse phase.…”

Metabolism of [3H]‐ecdysone in embryos and larvae of the tick Ornithodoros moubata

“…The identity of these products is unknown. Product B appears to comigrate with 20,26-dihydroxyecdysone, the presence of which has also been suggested during [3H]-E metabolism in 0. moubnta fifth instar nymphs [MI, but products A and C do not correspond to any PP found in nymphs or in females either directly or after esterase hydrolysis [10,24]. As previously described in nymphs, the PP appear to be most actively produced during high or decreasing endogenous ES titers [9].…”

“…This detoxification mechanism is present and active in all the developmental stanes of 0. rnaubatlz from vounn ovocvtes to nvmphs 1181 and adults 110,131. [13,24] ES in the diet. Localization of this enzymatic system has not been completely defined but it is present in numerous tick organs: ovaries [lo], Malpighian tubules, intestine, and dorsal and ventral carcass (Vuilleme, unpublished results).…”