Adult lipophorin

Insects use camouflage coloration as a means of avoiding predation. The green color of the tobacco hornworm larvae, (Manduca sexta) can be separated into constituent blue and yellow components. The water soluble blue component is the biliprotein, insecticyanin. The yellow color is derived from lipoprotein bound carotenes. This lipoprotein, lipophorin, is the major lipid transport vehicle in insect hemolymph. In addition to transporting dietary lipid, lipophorin is also involved in the transport of lipophilic insecticides. Nearly all the recovered radioactivity in hemolymph from topically applied [14c] ddt is associated with lipophorin. Lipophorin of adult M. sexta is larger, less dense and is associated with small amounts of a third, adult specific, apoprotein. Alterations in adult lipophorin density, lipid content and apoprotein stoichiometry can be caused by injection of the decapeptide, adipokinetic hormone. 

Lipophorin biosynthesis in adult insects has been studied in the house fly, Musca domestka (Capurro and de Bianchi, 1990b), in L. migratoria (Weers et ai, 1992), and in M. sexta (S. V. Prasad and M. A. Wells, unpublished). In each of these cases in vitro incubations of fat body resulted in the release of a lipophorin whose density and lipid composition closely resembled that of mature lipophorin. An important difference between larvae and adults is the rate of lipophorin biosynthesis. For example, in M. sexta larvae, which are rapidly growing, the amount of lipophorin per animal can increase up to 10-fold in 3 days (Prasad et ai, 1987), requiring a prodigious rate of lipophorin synthesis. In adults, lipophorin synthesis need only replace that which is lost from the hemo-lymph due to turnover the half-life of lipophorin in adult L. migratoria is... 

Drosophila melanogaster is another dipteran where pheromone biosynthesis has been studied [92]. Adult sexually mature female D. melanogaster utilizes primarily Z7,Z11-27 H as a contact sex pheromone. The biosynthesis of this compound follows the biosynthesis of other hydrocarbon-derived pheromones (Fig. 3). It is biosynthesized in oenocytes [93], transported through the hemo-lymph by lipophorin [94], and deposited on the cuticle surface. Biosynthesis in the oenocytes follows a similar pathway [95] as that described for the house fly... 

Pho D. B., Pennanec h M. and Jallon J. M. (1996) Purification of adult Drosophila melanogaster lipophorin and its role in hydrocarbon transport. Arch. Insect Biochem. Physiol. 31, 289-303. 

When the larva undergoes metamorphosis to an adult moth, a somewhat different lipophorin is found in the blood. The density of this adult form is 1.08 g/ml, and a new apoprotein, apoLp—III, 17,000 daltons (19), associates with the lipophorin, in addition to apoLp-I an apoLp-II. Analysis shows that the particle has twice the lipid content as the larval form and an apoprotein ratio of 1 apoLp-I to 1 apoLp-II to 2 apoLp-III. 

ApoLp—111 is one of the most abundant proteins of adult hemolymph, reaching a concentration of 17 mg/ml. In hemolymph apoLp-III can be found free or associated with lipophorin. Only a small part of the total apoLp-III is associated with lipophorin when the animal is resting. 

Lipophorins from several orders of adult insects have been examined, and so far only in L. migratoria (22), M. sexta and a hemipteran, Leptoglossus zonatus (25), has apoLp-TTl been observed. Efforts are now underway to determine if apoLp-III is invariably associated with flight metabolism fueled by fat. 

To finish this duscussion on lipophorin biosynthesis we will mention studies on the origins of PLs, hydrocarbons, sterols, and carotenoids. It has been reported that in adult M. sexta and Rhodnius prolixm PL can be transferred from fat body to lipophorin (Van Heusden et al., 1991 Correa et al., 1992). This transfer of PL is independent of de novo synthesis of lipophorin however, the mechanism by which it occurs is unknown. Hydrocarbon transport by lipophorin has been studied only in P. ameri-cana. Katase and Chino (1982) have shown, in in vitro incubations, that a fat body rich in oenocytes, one type of cell in the hemolymph, which is the major site of hydrocarbon biosynthesis (Diehl, 1975), can release labeled hydrocarbon to lipophorin. It was also shown, using in vitro incubations, that the labeled hydrocarbon in lipophorin was delivered to the epidermis, the normal site of hydrocarbon deposition in insects. The sterols and carotenoids that are present in lipophorin must arise from the diet, because insects cannot biosynthesize either sterols or carotenoids de novo. Chino and Gilbert (1971) have shown that sterol can be transferred from the midgut to lipophorin, and the same is most likely true for carotenoids. The mechanism by which hydrocarbons, sterols, and carotenoids are transferred from either oenocytes or midgut epithelial cells to lipophorin is unknown. 

---