Lipophorins apolipoproteins

When lipophorin, containing labeled DG, was injected into M. sexta larvae, the DG disappeared from the hemolymph with a half-life of about 50 min, and after 4 hr about 60% of the injected label was found in fat body as TG (Tsuchida and Wells, 1988). When DG-labeled lipophorin was incubated in vitro with fat body, more than 95% of the label was taken up by fat body within 4 hr the half-life of labeled DG in the incubation medium was about 60 min, a value comparable to the in vivo measurement. During the incubation, after 95% of the DG had entered the fat body, there was no detectable loss of lipophorin apolipoproteins from the incubation medium. However, the density of the lipophorin in the incubation medium increased substantially, consistent with the loss of DG. These results are compatible with the hypothesis that lipophorin functions as a reusable, noninternalized shuttle. 

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... 

There is one molecule each of apoLp-I and apoLp-lI in each lipophorin molecule (Shapiro et al., 1984 Surholt et al., 1992). A similar amino acid composition, as well as the presence of oligosaccharide chains of the high-mannose type, has been observed for apoLp-I and apoLp-II from different insect species (Pattnaik et al., 1979 Ryan et al., 1984 Shapiro et al., 1984 Kashiwazaki and Ikai, 1985 Dillwith et al., 1986 Nagao and Chino, 1987 Rimoldi o/., 1991). Both apolipoproteins are water insoluble when separated from lipophorin, and in this regard resemble vertebrate apoB. No amino acid sequence data, either from direct or cDNA... 

One of the most intriguing features of lipophorin composition is the large variation in lipid content and composition that can be accommodated without modifications in the apolipoprotein composition of the particles (Table I). This feature makes lipophorin a good system in which to analyze the structure of lipoproteins and the physicochemical factors that govern their structure and properties. In addition to the previously discussed data on the size and shape of lipophorins, several studies on other aspects of lipophorin structure have been performed and need to be discussed before describing models for lipophorin structure. 

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... 

Alcohol oxidase Lipoproteins, apolipoproteins, lipophorins, vitellogenins, yolk proteins, chylomicrons... 

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