Lipid thermotropic transitions in Triatoma infestans lipophorin

Rhodnius prolixus lipophorin: lipid composition and effect of high temperature on physiological role

Lipophorin is a major lipoprotein that transports lipids in insects. In Rhodnius prolixus, it transports lipids from midgut and fat body to the oocytes. Analysis by thin-layer chromatography and densitometry identified the major lipid classes present in the lipoprotein as diacylglycerol, hydrocarbons, cholesterol, and phospholipids (PLs), mainly phosphatidylethanolamine and phosphatidylcholine. The effect of preincubation at elevated temperatures on lipophorin capacity to deliver or receive lipids was studied. Transfer of PLs to the ovaries was only inhibited after preincubation of lipophorin at temperatures higher than 55 °C. When it was pretreated at 75 °C, maximal inhibition of phospholipid transfer was observed after 3-min heating and no difference was observed after longer times, up to 60 min. The same activity was also obtained when lipophorin was heated for 20 min at 75 °C at protein concentrations from 0.2 to 10 mg/ml. After preincubation at 55 °C, the same rate of lipophorin loading with PLs at the fat body was still present, and 30% of the activity was observed at 75 °C. The effect of temperature on lipophorin was also analyzed by turbidity and intrinsic fluorescence determinations. Turbidity of a lipophorin solution started to increase after preincubations at temperatures higher than 65 °C. Emission fluorescence spectra were obtained for lipophorin, and the spectral area decreased after preincubations at 85 °C or above. These data indicated no difference in the spectral center of mass at any tested temperature. Altogether, these results demonstrate that lipophorin from R. prolixus is very resistant to high temperatures.

Lipophorin density variation during oogenesis on Rhodnius prolixus

The density of lipophorin was determined in adult females of Rhodnius prolixus on different days after a meal. Several populations od lipoproteins, differing in density but always in the range of HDL, were found in the hemolymph. The density of the major population was analyzed and a complex profile of density variation was found associated with the principal metabolic events in these insects digestion and oogenesis. During the initial three days after the blood meal, with the onset of the digestive process, the density of lipophorin decreased from 1.1185 g/l to 1.1095 g/l, associated with the transfer of lipids from midgut to the lipophorin particles. During the period of intense vitellogenesis and lipid uptake by the ovary, the lipophorin density started to increase and reached the value, 1.1322 g/l, and remained stable up to the end of oogenesis. As soon as the requirement of lipids to build up the oocytes ceased, the density of lipophorin decreased to its initial value associated with the transfer of lipids from fat body to lipophorin. Soon after the blood meal the midgut was the main source of lipids capable of replenishing the lipophorin particles, while the fat body assumed this function during the succeeding days and reached its maximum capacity around day 10, as estimated by the rate of lipid transfer. The principal lipids transferred were phospholipids and diacylglycerols. Except in the protein/lipid ratio no major changes were observed among different lipids isolated from lipophorin of different densities.

Role of diacylglycerol and apolipophorin-III in regulation of physiochemical properties of the lipophorin surface: metabolic implications

Manduca Sexta adults insects have two defined lipophorin species of densities 1.09 g/mL, [high-density lipophorin (HDLp)] and 1.02 g/mL [low-density lipophorin (LDLp)], respectively, and a continuous broad range of lipophorin particles of intermediate size and density, intermediate-density lipophorin (IDLp). The transformation of HDLp into IDLp and LDLp is the result of the progressive loading of HDLp with diacylglycerol (DG) and an exchangeable apolipoprotein, apolipophorin-III (apoLp-III). In this paper, we describe the physiochemical changes which occur in the lipophorin surface as a result of the transformation of HDLp into LDLp. (1) The increase in apoLp-III content, from 0 to 16 molecules per particle, is accompanied by a gradual increase in the zeta-potential which, at pH 8.6 ranges from /1.02 mV for lipophorins without apoLp-III to -7.76 mV for lipophorins containing 16 molecules of apoLp-III. (2) As judged by the changes in the partition constant for trimethylammonium diphenylhexatriene and oleic acid, an average 2-fold increase in the size of the lipophorin lipid surface takes place when HDLp is loaded with Dg and transformed into LDLp. (3) These data, as well as the results obtained by end point lipolysis with a triacylglycerol (TG) lipase, indicated that the accessible DG content increases 4-7 times when HDLp is converted in LDLp. (4) Fluorescence polarization of the cationic and anionic lipid probes, trimethylammonium diphenylhexatriene and cis-parinaric acid, embedded in eight different subspecies of lipophorin, containing from 12 to 50% DG, showed a small decrease in the surface lipid order when going from HDLp (25% DG) to LDLp (50% DG). (5) Porcine pancreatic phospholipase A2 was used as a probe of the lipoprotein surface. As the DG content of the lipoprotein increased, a higher enzyme activity against the lipoprotein-phospholipids was observed, with a maximum activity 5-fold higher against LDLp than against HDLp. Overall, the changes observed as the lipoprotein particles are loaded with DG and apoLp-III provide a link between the structure and properties of the lipophorin surface and the physiological roles of HDLp and LDLp particles.

Transport and utilization of free fatty acids in Triatoma infestans

Triatoma infestans hemolymph has 0.31 mg/ml of free fatty acids and 2.8 mg/ml of diacylglycerols. Almost all the diacylglycerols are transported by lipophorin whereas free fatty acids are carried by lipophorin and a very high density lipoprotein. The binding of cis-parinaric acid to lipophorin was employed to specify the free fatty acid binding properties of lipophorin. Lipophorin has 10 binding sites of high affinity (3 x 10(7)) and approximately 45 binding sites of low affinity (1 x 10(6)). The relative rate of tissue incorporation of free fatty acids and diacylglycerols was measured by injecting insects with hemolymph previously labeled in both, free fatty acids and diacylglycerols. In this way, the half-life of the hemolymph free fatty acids was estimated to be about 4 min. Based on this result and taking into account the content of free fatty acids and diacylglycerols in hemolymph, the incorporation of free fatty acids, expressed in moles of fatty acids, seems to be 3.4 times higher than that of diacylglycerols. This finding can be applied to other insects.