Lipophorins biosynthesis

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... 

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(Capurro and de Bianchi, 1990b), in L. migratoria (Weers et ai, 1992), and in M. sexta (S. V. Prasad and M. A. Wells, unpublished).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... 

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. 

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... 

The biosynthesis of Me3,Mel 1-29 H takes place in oenocyte cells, released into the hemolymph and transported by lipophorin to peripheral tissues (Fig. 7) [71, 231, 232]. Direct evidence for oenocytes biosynthesizing hydrocarbon has come recently with the dissociation of oenocytes from epidermal cells and in vitro incubation with labeled propionate [233]. Differential uptake of some hydrocarbons in different tissues has also been documented although the exact mechanism behind the differential placement of hydrocarbons is unknown [20,128,230,232,234]. Although the biosynthesis of hydrocarbons may not be under direct endocrine regulation supply of precursor hydrocarbon that is converted to the sex pheromone is a requirement. 

Gu, X., Quilici, D., Juarez, P, Blomquist, G. J. and Schal, C. (1995). Biosynthesis of hydrocarbons and contact sex pheromone and their transport by lipophorin in females of the German cockroach (Blattella germanica). Journal of Insect Physiology 41 257-267. 

Moths in the families Geometridae, Arctiidae, and some Noctuidae utilize hydrocarbons or epoxides of hydrocarbons as their sex pheromones. Hydrocarbon biosynthesis occurs in oenocyte cells that are associated with either epidermal cells or fat body cells (Wigglesworth, 1970). Once the hydrocarbons are biosynthesized, they are transported to the sex pheromone gland by lipophorin (Schal et al., 1998). The hydrocarbons can be released directly in the case of some moths or they are transformed into epoxides by addition of oxygen across one of the double bonds. 

Figure 5.3 Schematic drawing showing transport of hydrocarbons (and other lipids) from site of synthesis (oenocytes) to cuticle surface (epicuticle) and various tissues and glands. Arrows represent hypothetical transport of hydrocarbons (and/or precursors) [legends e epicuticule p procuticule h hydrocarbons (and/or precursors) d epidermal cell c canal issuing from an epidermal cell o oenocytes 1 lipophorins fm microsome fraction (reticulum endoplasmic of oenocytes, site of hydrocarbon biosynthesis) hi hemolymph pg pheromone glands ot other tissues (ovaries)] (updated from Bagnhres, 1996).

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