Insect-specific enzymes

Harnessing Insect-Specific Enzymes to Activate Novel Proinsecticides... 

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

Post-translational modifications, such as phosphorylation, complex glycosylation, and lipidation, typically occur in eukaryotic organisms. Therefore, their expression in prokaryotic systems like Escherichia coli is difficult. However, it should be noted that via clever engineering and coexpression of specific enzymes, access can be granted to specific lipidated proteins via expression in bacteria, for example, via the expression of A -myristoyltransferase in E. coli Eukaryotic systems that can be used for the expression of post-translationally modified proteins are yeast and Dictyostelium discoidum. Furthermore, lipidated proteins, such as the Rah proteins, can be obtained via purification from tissue sources or from membrane fractions of insect cells that had been infected with baculovirus bearing a Rah gene. ... 

The three hormones that regulate pheromone production in insects are shown in Figure 1.1. PBAN alters enzyme activity through second messengers at one or more steps during or subsequent to fatty acid synthesis during pheromone production (Rafaeli and Jurenka, Chapter 5). In contrast, 20-hydroxyecdysone and JH induce or repress the synthesis of specific enzymes at the transcription level (Tittiger, Chapter 7 Blomquist, Chapter 9). 

GSH-S-transferase activity has been found in all insect species examined although the level of activity varies with the species and strain of insect and with the substrate employed (3,6,41). It is evident that, as in manmals, the insect enzymes are present as a group of structurally similar isozymes with varying degrees of overlapping substrate specificity the enzymes occur in the 100,000g supernatants of a variety of insect tissues. Enzyme purification studies indicate that all GSH-S-transferases have a similar molecular weight ( 50,000) and consist of two approximately equal subunits (40). 

Fig. 15. Fate of PA AT-oxides ingested by vertebrates and insects. Following reduction of the N-oxide in the gut and passive uptake of the alkaloid free base, bioactivation occurs in all organisms which possess cytochrome P450 enzymes. Detoxification by N-oxidation is possible in vertebrates with an efficient multisubstrate flavin monooxygenase (FMO). Specialized herbivorous lepidopterans apply the same strategy they developed a substrate specific enzyme, senecionine AT-oxygenase (SNO) and keep the non-toxic PA-AT-oxides for their own benefit...

Robinia pseudacacia resulted in accumulation of free fructose and oligosaccharides containing principally glucose residues. Transfructosidases have also been found in both plant nectar and insect viscera. A trans-glucosidase with specificity different from the other enzymes described occurs in honeydew and manna formed by certain insects. This enzyme is produced by the insects, not the plant, and transfers glucosyl groups to the 3-position of the fructose of sucrose. ... 

Further metabolism of ecdysones in insects can occur by conversion into both less and more polar metabolites as well as into glycoside and sulphate conjugates. ° Ecdysone (143) is metabolized by a soluble enzyme from blowfly pupae into 2p,14o,22,25-tetrahydroxy-5p-cholest-7-ene-3,6-dione(145 3-dehydroecdysone). A non-specific enzyme system from larval gut tissues... 

Monofluoroacetic acid offers little promise as a mosquito larvicide, but 2-ethylhexyl monofluoroacetate is a very powerful aphicide. The high toxicity of the fluorinated lower aliphatic acids and their esters to vertebrates probably precludes their use as insecticides except under highly controlled conditions. The toxicity of the fluorinated acids to the vertebrates is attributed to their interference with an enzyme system, but the possibility of finding one with a high specificity for insects is not excluded. 

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