Insects lipid metabolism

Fatty acyl-CoA desaturases are terminal oxidases of a membrane-bound enzyme complex that also includes cytochrome b5 and cytochrome b5 reductase (Bloomfield and Bloch, 1960). They remove substrate hydrogen atoms at a position determined by the specificity of the enzyme. They play essential roles in regulating membrane fluidity and are also involved in insect lipid and pheromone metabolism. They share the presence of three highly conserved histidine-rich sequences (H-boxes) that coordinate the diiron-oxo structure at the active sites (Shanklin and Cahoon, 1998) and four hydrophobic a helices that appear to anchor the protein into the lipid bilayer and situate the H-boxes in their correct position in the active site. 

Lipid metabolism in insects represents a system with many unique characteristics, which although complex, sdll appears simpler than that of vertebrates. Insects offer an especially attractive system in which to study... 

As the only unicellular parasite to live free in the host s bloodstream the African, or salivarian, trypanosomes are unique. Unfortunately, there is little information on African trypanosome lipids and lipid metabolism. The life cycle of the African trypanosome involves bloodstream forms in the mammalian host and various insect forms in the tsetse fly vector. Of the insect forms, only the in vitro propagated procyclic form, equivalent to those in the tsetse fly midgut, has been examined. Early studies used complex culture medium that contained significant exogenous lipid. Later studies used... 

The symposium from which this book was developed was designed to review and present current research on the biosynthesis and metabolism of lipids in plants together with the chemistry and biochemistry of lipid interactions however, some presentations reported input from disciplines other than chemistry. This 22-chapter volume presents a loose classification of the papers presented at the symposium into four sections (1) Introduction, (2) Plant Lipid Metabolism and Plant-Plant Interactions, (3) Plant-Insect and Plant-Nematode Interactions, and (4) Plant-Microbial Interactions. 

LIPID METABOLISM DURING DEVELOPMENT IN INSECTS A. M. Municio... 

Some information concerning differences in lipid metabolism involved in the development of the insect C(lHjOitAXXM capXJjOtJjOi is given in this paper. 

The final two categories listed in Table 1 are xenobiotics and other factors. Xenobiotics, which include pesticides (herbicides, insecticides, fungicides), are discussed later but there are many cases of accidental spillages of chemicals (e.g. hydrocarbons) which often have disasterous effects on vegetation. Damage to plants by physical insults or pest attack may alter lipid metabolism. For example, wounding stimulates the plant to repair the cut surface, especially by suberin whose precursors are fatty acids. Insect attack allows microbial entry into... 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

While the hydrocarbon fraction of insect cuticular lipids is certainly the most studied and has been shown to play a key role in a wide range of chemical communication, other lipids are often present on the surface of insects. The most common cuticular lipids in addition to hydrocarbons include a variety of types of esters, free fatty acids, primary and secondary alcohols, ketones and sterols. Triacylglycerols and the more polar phospholipids are not common components of insect cuticular lipids. In some cases, hydrocarbons are hydroxylated and metabolized to oxygenated components, and these products include some of the short range and contact pheromones of the housefly (Blomquist, 2003) and the German cockroach (Schal et al., 2003). The oxygenated cuticular lipids are discussed in Chapter 9 (Buckner, this book). 

Propionate serves several unique and important roles in insects. It is used by some insects, in very small amounts, as a precursor to homomevalonate which is an intermediate in the biosynthesis of juvenile hormone (JH) II (1,2) and probably JH I and JH 0 as well. Much larger amounts of propionate and methylmalonate are needed for the biosynthesis of methyl branched hydrocarbons which are major cuticular components in most of the approximately 100 insect species whose cuticular lipids have been examined (3-7). Until recently, there was little information available on either the source of propionate or its metabolism in insects. In mammals vitamin B 2 Is a key cofactor in propionate and methylmalonate metabolism (B—9). Recent observations that some insect species lack or contain low levels of vitamin B 2 (10)... 

The most widely recognized physiological action of the insect AKH/HGHs is to mobilize lipids or carbohydrates from fat body reserves for use as energy substrates by peripheral tissues. In Leptinotarsa decemlineata, proline is the main substrate for flight muscle metabolism (16), and Lom-AKH-I promotes fat body proline synthesis... 

Vertebrate, especially mammalian, lipoproteins have been extensively studied. In the invertebrate world, only insect lipoproteins have received serious attention. Whereas vertebrates rely on a battery of lipoproteins (chylomicrons, very low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins) to effect lipid transport, insects use primarily a single type of lipoprotein, lipophorin, for lipid transport. Lipophorin is both more versatile than vertebrate lipoproteins in terms of the diverse lipids it transports and more efficient than vertebrate lipoproteins in that, for the most part, it delivers lipids to tissues without being internalized and destroyed. We believe that new insights can be obtained from an understanding of insect lipoproteins, and in this article we review the current state of knowledge about the structure and metabolism of lipophorins. 

The key to understanding lipoprotein metabolism in insects was the discovery that lipophorin functions as a reusable shuttle (reviewed by Chino, 1985). Thus, lipophorin can be described as a basic apoli-poprotein-phospholipid complex, which can carry a variety of lipids in... 

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