JH esterase

Figure 3. Plot of JH-hydrophobicity and JH-esterase hydrolytic activity of JH I, JH II and JH III.

Figure 6. Relationship between the inhibitory activity of trlfluoromethylketones against JH-esterase from Trlchoplusla nl and the molar equivalents of these inhibitors.

Figure 7. Proposed scheme for the reactions of JH-esterase with JH homologs (A) and with trifluoromethylketones (B).

Hammock and co-workers (Hammock et ai, 1982 Abdel-Aal, 1984 Prestwich et ai, 1984) prepared several 3-alkylthio-l,l,l-trifluoro-2-propanones with juvenile hormone-like side chains. These compounds were designed as possible transition-state analogue inhibitors of JH esterases. The most active analogue (78) showed a dose-dependent delay in pupation and a selective inhibition of JH esterase of the cabbage looper, Trichopltisia ni. 

Levels of JH III in suspense superparasitized by B. longicaudatus were considerably higher than in unparasitized A. suspense (53) JH III was also found in first instars of the parasitoid. These results suggested that the elevated JH III levels in the superparasitized host may result from a decrease in JH esterase activity with a continued or elevated synthesis of the hormone in the host or from a secretion of JH III by the parasitoids. 

An endocrine role has also been postulated in some species (84). Injection of teratocytes from the braconid parasitoid M. croceipes into larvae of the tobacco budworm, Heliothis virescens, caused an elevation of the ecdysteroids, an increase in JH levels and a decrease in the JH esterase activity in the host (85, Dahlman, D. L. Arch. Insect Biochem. Physiol., in press). 

The rhabdoviruses were proposed to affect migration of vesicles to the cell apices and the activation of the molting fluid. The function of the pox virus is not yet known but could relate to the changes in the hemolymph levels of ecdysteroids, JH, and JH esterases (53). 

Mueller RF, Nielsen PH (1996) Characterization of thermophilic consortia from tTwo souring oil reservoirs. Appl Environ Mcrobiol 62 3083-3087 Niazi JH, Prasad DT, Karegoudar TB (2001) Initial degradation of dimethylphtha-late by esterases from Bacillus species. FEMS Microbiol Lett 196 201-205 Obst M, Krug A, Luftmann H, Steinbuchel A (2005) Degradation of cyanophycin by Sedimentibacter hongkongensis strain KI and Citrobacter amalonaticus... 

There emerges a very complex picture of three hormones synthesized and secreted at variable rates, competing for carrier binding proteins, presumed receptor proteins, epoxide hydratase and carboxyl esterase enzymes (35,36). It is possible experimentally to measure tEe timing of critical periods for larval determination and to measure total levels of JH at these critical periods although both measurements involve extreme difficulty. Approaches to this were described recently by G.B. Staal (3 7) using third instar larvae of the tobacco hornworm moth, Manduca sexta, which were allatectomized and raised on JH impregnated diets as an experimentally reproducible method of JH therapy. 

A CA epoxidase perhaps identical to the precocene epoxidase biosynthesizes Insect juvenile hormones (JH) from the analogous inactive oleflnic precursor, and the enzyme activity appears higher in precocene-sensitive species (32). Subsequent detoxification of JH occurs primarily by EHs and esterases in peripheral tissues, and preliminary information does not indicate major differences for JH degradation routes between chewing and sucking herbivores, or insect carnivores (33,34). More study of the role of detoxification in regulating the action of JH in target tissues is required. 

A good example of substrate specificity in enzyme action is the metabolism of juvenile hormone (JH) analogs by insects. In a series of experiments summarized in Table 9.7 to 9.10, the activity of esterases and microsomal monooxygenases from three dipterans against 12... 

Table 9.10 Metabolism of JH analogs by microsomal esterases from CSMA houseflies...

Niazi JH, Prasad DT, Karegoudar TB. Initial degradation of dimethyl phthalate by esterases from Bacillus species. FEMS Microbiol Lett 2001 196(2) 201-205. 

Our laboratory is concerned with targeting potential insecticides that disrupt normal development and metamorphosis in insects. Juvenile hormones (JHs), acting in concert with the steroid hormone ecdysone, are believed to control the timing of the larval-larval molts, larval-pupal and pupal-adult transformations of the insects. It has been demonstrated that the events leading to pupation are initiated by reduction of the JH titer in the hemolymph. In addition to a cessation of biosynthesis, this reduction in JH titer is controlled by degradative metabolism (16,17). Hydrolysis of the epoxide and ester functionalities present in active JH are two routes of degradation and subsequent inactivation of JH (18). The primary route of JH metabolism in the hemolymph of last stadium lepidopterous larvae is ester hydrolysis, and it is catalyzed by the enzyme juvenile hormone esterase (JHE). JHE has been shown to... 

Introducing a sulfur atom to the carbonyl significantly increased the activity of the resulting compounds (B) on JHE and some but not all other esterases, possibly by bioisosterically mimicking Q5) the a,ft double bond of the natural JHs. Based on this finding, a series of aliphatic, aromatic (2,8.) and terpenoid (9) derivatives were synthesized. The most active compound of these series was 3-octylthio-l,1,l-trifluoro-2-propanone (OTFP). 

Similar reduction of a prochiral diketone C with another yeast, Pichia terricola afforded D, which was converted to JH I,28 JH II,28 JH 029 and 4-methyl JH I.30 In the case of (+)-4-methyl JH I, asymmetric hydrolysis of dimethyl 3-methylglutarate with pig-liver esterase (PLE) was also employed for its... 

The juvenile hormones are easily degraded by membrane-bound epoxide hydrolases and carboxyl esterases. The degradation products have no hormone activity and are excreted as sulfates. A carrier protein protects it from degradation, and its concentration is important for the regulation of the amount of active JH. 

The two main steps of JH metabolism are the hydrolysis of the ester bond by esterase and the microsomal oxidation of the epoxy group into a dihydrodiol group effected by the enzyme epoxyhydrase. Either of these metabolic processes brings about complete disappearance of activity, and 5 and 6, the proposed metabolic intermediates are inactive (Slade and Zibitt, 1971 Slade and Wilkinson, 1973 Ajami and Riddiford, 1973 Yu and Terriere, 1978). 

Ethyl-4-[2-(t-butylcarbonyloxy)butoxy] benzoate (ETB, ZR-2646, 71) occupies an intermediate position between the juvenile hormone mimics and the antijuvenile hormones to be discussed later. It acts as an anti-JH at low doses and as a JH mimic at high doses. The nature of this JH agonist/antagonist action is not understood, yet its interference with the induction of juvenile hormone esterase seems to elucidate at least partly its action on insects (Staal, 1977 Sparks et al., 1979). 

In D. punctata, the haemolymph titre of JH is determined by the rate of biosynthesis and release from the CA and its degradation and clearance from the haemolymph by tissue uptake and excretion [13, 76]. The two primary routes of metabolism are through the cleavage of the methyl ester group by haemolymph esterases or the hydration of the epoxide by tissue-associated epoxide hydrolases. In the CA of final instars and in adult females of D. punctata, the low JH III titre in the haemolymph corresponds to low rates of biosynthesis by the CA and high esterase activity [43, 57, 77],... 

Originally, it was thought that these compounds could provide the structural basis for the design of pesticides because it is unlikely that insects would develop resistance to their own or closely related hormones [120]. However, this has proven not to be the case. Most insects possess enzymes such as mixed function oxidases and esterases in their alimentary tract and haemolymph which are, among other functions, important for the metabolism of plant toxins [121, 122]. For example, insects which have developed resistance to insecticides may also show resistance to JH... 

Trifluoromethyl ketones (TFKs) have been found to inhibit various hydrolytic enzymes (1-6 ). Series of aliphatic and aromatic trifluoromethyl ketone sulfides (7-10 ) proved to be exceptionally powerful inhibitors of insect juvenile hormone esterase (JHE), an enzyme of key importance in insect metamorphosis. The trifluoroketone moiety is believed to behave as a transition state mimic (11,12) of juvenile hormones (JHs), substrates of the enzyme. The /3 sulfur atom is anticipated to mimic the a-/3 double bond present in all natural JH substrates. In earlier structure-activity relationship (SAR) studies (7,11) clear correlation was found among the molar I50 values of these compounds against JHE and the calculated molar refractivlty of the inhibitors. 

Anti-juvenile hormone activity, and the resulting cessation of feeding, has long been considered by the agricultural chemical industry as a means for insect control. Juvenile hormone esterase (JHE) contributes to the decline in JH titer at certain times during larval development. 

The failure of the artificially high titers of JHE to disrupt larval development may be due to several factors. Since the reduction in JHE is mediated by a variety of events, one can predict that for recombinant JHE to be effective, levels in the hemolymph must greatly exceed those normally present in the last larval instar. In normal larvae JH metabolism by esterase and epoxide hydrolase is high in most tissues while in infected larvae JHE levels increase in the hemolymph and infected, but presumably not uninfected tissues. Thus tissue levels of JH may not decrease sufficiently to induce metamorphosis. Since epoxide hydrolase levels are not affected there is also the possibility that JH acid may be remethylated to an active form in some tissues. The JH binding protein is hypothesized to increase the degradation of JH in the presence of JHE by keeping the JH in a pool accessible to the enzyme. However, the titers of JH binding protein in infected insects are not known. 

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