Lipophorins phospholipids

P NMR studies on locust lipophorin indicate that a large proportion of the PLs reside on the surface of lipophorin (Katagiri et al., 1985). In addition to this study, the susceptibility of lipophorin phospholipids to hydrolysis by phospholipase A2 is consistent with a surface localization of PLs (Katagiri et al., 1985 Kawooya et al., 1991). Although the amphi-pathic nature of PLs makes this a logical conclusion, it has to be pointed out that the use of hydrolytic enzymes to demonstrate the location of any component would be valid only if a rearrangement of the lipid components in lipophorin does not occur on the time scale of the experiment. Because this condition is never met, a cautious interpretation of such data is necessary. 

The major lipoproteins of insect hemolymph, the lipophorins, transport diacylglycerols. The apolipo-phorins have molecular masses of -250, 80, and sometimes 18 kDa.34-37a The three-dimensional structure of a small 166-residue lipophorin (apolipophorin-III) is that of a four-helix bundle. It has been suggested that it may partially unfold into an extended form, whose amphipathic helices may bind to a phospholipid surface of the lipid micelle of the lipophorin 35 A similar behavior may be involved in binding of mammalian apolipoproteins. Four-helix lipid-binding proteins have also been isolated from plants.38 See also Box 21-A. Specialized lipoproteins known as lipovitellins... 

The only common major lipid component of vertebrate and insect lipoproteins is phospholipid (PL). Lipophorin PLs have been characterized in only a few insect species (Table II). The major PLs are phosphati-... 

Fig. 5. Dependence of lipophorin surface area on the mole of phospholipid per particle. The data are from 15 lipophorins without apoLp-IIl, and calculations were done according to Soulages and Brenner (1991). The surface area of the lipophorin was related to the surface area occupied by apoprotein and lipid by the equation...

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... 

These results show two distinct features of lipophorin biosynthesis during the larval stage. First, the nascent lipophorin produced in the fat body by de novo synthesis is an apolipoprotein-phospholipid complex that derives its transported lipids from the midgut. Second, lipophorin biosynthesis is not coupled to fat intake, as is the case with vertebrates. These processes are illustrated in Fig. 6 and fit observations made on lipid storage in larvae. Thus, it has been shown that more than 70% of the fatty acids in the diet are stored as TG in the larval fat body (Tsuchida and Wells, 1988). Although the fat body can convert carbohydrates to fatty... 

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