Facilitated diacylglycerol exchange between insect hemolymph lipophorins. Properties of Manduca sexta lipid transfer particle

Different mechanisms for selective transport of fatty acids using a single class of lipoprotein in Drosophila

In mammals, lipids are selectively transported to specific sites using multiple classes of lipoproteins. However, in Drosophila, a single class of lipoproteins, lipophorin, carries more than 95% of the lipids in the hemolymph. Although a unique ability of the insect lipoprotein system for cargo transport has been demonstrated, it remains unclear how this single class of lipoproteins selectively transports lipids. In this study, we carried out a comparative analysis of the fatty-acid composition among lipophorin, the CNS, and CNS-derived cell lines and investigated the transport mechanism of fatty acids, particularly focusing on the transport of PUFAs in Drosophila We showed that PUFAs are selectively incorporated into the acyl chains of lipophorin phospholipids and effectively transported to CNS through lipophorin receptor-mediated endocytosis of lipophorin. In addition, we demonstrated that C14 fatty acids are selectively incorporated into the diacylglycerols (DAGs) of lipophorin and that C14 fatty-acid-containing DAGs are spontaneously transferred from lipophorin to the phospholipid bilayer. These results suggest that PUFA-containing phospholipids and C14 fatty-acid-containing DAGs in lipophorin could be transferred to different sites by different mechanisms to selectively transport fatty acids using a single class of lipoproteins.

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

Manduca sexta, known as the tobacco hornworm or Carolina sphinx moth, is a lepidopteran insect that is used extensively as a model system for research in insect biochemistry, physiology, neurobiology, development, and immunity. One important benefit of this species as an experimental model is its extremely large size, reaching more than 10 g in the larval stage. M. sexta larvae feed on solanaceous plants and thus must tolerate a substantial challenge from plant allelochemicals, including nicotine. We report the sequence and annotation of the M. sexta genome, and a survey of gene expression in various tissues and developmental stages. The Msex_1.0 genome assembly resulted in a total genome size of 419.4 Mbp. Repetitive sequences accounted for 25.8% of the assembled genome. The official gene set is comprised of 15,451 protein-coding genes, of which 2498 were manually curated. Extensive RNA-seq data from many tissues and developmental stages were used to improve gene models and for insights into gene expression patterns. Genome wide synteny analysis indicated a high level of macrosynteny in the Lepidoptera. Annotation and analyses were carried out for gene families involved in a wide spectrum of biological processes, including apoptosis, vacuole sorting, growth and development, structures of exoskeleton, egg shells, and muscle, vision, chemosensation, ion channels, signal transduction, neuropeptide signaling, neurotransmitter synthesis and transport, nicotine tolerance, lipid metabolism, and immunity. This genome sequence, annotation, and analysis provide an important new resource from a well-studied model insect species and will facilitate further biochemical and mechanistic experimental studies of many biological systems in insects.

Recent progress in molecular genetic studies on the carotenoid transport system using cocoon-color mutants of the silkworm

The existence of tissue-specific delivery for certain carotenoids is supported by genetic evidence from the silkworm Bombyx mori and the identification of cocoon color mutant genes, such as Yellow blood (Y), Yellow cocoon (C), and Flesh cocoon (F). Mutants with white cocoons are defective in one of the steps involved in transporting carotenoids from the midgut lumen to the middle silk gland via the hemolymph lipoprotein, lipophorin, and the different colored cocoons are caused by the accumulation of specific carotenoids into the middle silk gland. The Y gene encodes carotenoid-binding protein (CBP), which is expected to function as the cytosolic transporter of carotenoids across the enterocyte and epithelium of the middle silk gland. The C and F genes encode the C locus-associated membrane protein, which is homologous to a mammalian high-density lipoprotein receptor-2 (Cameo2) and scavenger receptor class B member 15 (SCRB15), respectively; these membrane proteins are expected to function as non-internalizing lipophorin receptors in the middle silk gland. Cameo2 and SCRB15 belong to the cluster determinant 36 (CD36) family, with Cameo2 exhibiting specificity not only for lutein, but also for zeaxanthin and astaxanthin, while SCRB15 seems to have specificity toward carotene substrates such as α-carotene and β-carotene. These findings suggest that Cameo2 and SCRB15 can discriminate the chemical structure of lutein and β-carotene from circulating lipophorin during uptake. These data provide the first evidence that CD36 family proteins can discriminate individual carotenoid molecules in lipophorin.

Lipid transfer particle from the silkworm, Bombyx mori, is a novel member of the apoB/large lipid transfer protein family

Lipid transfer particle (LTP) is a high-molecular-weight, very high-density lipoprotein known to catalyze the transfer of lipids between a variety of lipoproteins, including both insects and vertebrates. Studying the biosynthesis and regulation pathways of LTP in detail has not been possible due to a lack of information regarding the apoproteins. Here, we sequenced the cDNA and deduced amino acid sequences for three apoproteins of LTP from the silkworm (Bombyx mori). The three subunit proteins of the LTP are coded by two genes, apoLTP-II/I and apoLTP-III. ApoLTP-I and apoLTP-II are predicted to be generated by posttranslational cleavage of the precursor protein, apoLTP-II/I. Clusters of amphipathic secondary structure within apoLTP-II/I are similar to Homo sapiens apolipoprotein B (apoB) and insect lipophorins. The apoLTP-II/I gene is a novel member of the apoB/large lipid transfer protein gene family. ApoLTP-III has a putative conserved juvenile hormone-binding protein superfamily domain. Expression of apoLTP-II/I and apoLTP-III genes was synchronized and both genes were primarily expressed in the fat body at the stage corresponding to increased lipid transport needs. We are now in a position to study in detail the physiological role of LTP and its biosynthesis and assembly.

Lipoproteins in Drosophila melanogaster--assembly, function, and influence on tissue lipid composition

Interorgan lipid transport occurs via lipoproteins, and altered lipoprotein levels correlate with metabolic disease. However, precisely how lipoproteins affect tissue lipid composition has not been comprehensively analyzed. Here, we identify the major lipoproteins of Drosophila melanogaster and use genetics and mass spectrometry to study their assembly, interorgan trafficking, and influence on tissue lipids. The apoB-family lipoprotein Lipophorin (Lpp) is the major hemolymph lipid carrier. It is produced as a phospholipid-rich particle by the fat body, and its secretion requires Microsomal Triglyceride Transfer Protein (MTP). Lpp acquires sterols and most diacylglycerol (DAG) at the gut via Lipid Transfer Particle (LTP), another fat body-derived apoB-family lipoprotein. The gut, like the fat body, is a lipogenic organ, incorporating both de novo-synthesized and dietary fatty acids into DAG for export. We identify distinct requirements for LTP and Lpp-dependent lipid mobilization in contributing to the neutral and polar lipid composition of the brain and wing imaginal disc. These studies define major routes of interorgan lipid transport in Drosophila and uncover surprising tissue-specific differences in lipoprotein lipid utilization.

The helix bundle: a reversible lipid binding motif

Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.

Lipid transfer particle mediates the delivery of diacylglycerol from lipophorin to fat body in larval Manduca sexta

This work analyzed the process of lipid storage in fat body of larval Manduca sexta, focusing on the role of lipid transfer particle (LTP). Incubation of fat bodies with [(3)H]diacylglycerol-labeled lipophorin resulted in a significant accumulation of diacylglycerol (DAG) and triacylglycerol (TAG) in the tissue. Transfer of DAG to fat body and its storage as TAG was significantly inhibited (60%) by preincubating the tissue with anti-LTP antibody. Lipid transfer was restored to control values by adding LTP to fat body. Incubation of fat body with dual-labeled DAG lipophorin or its treatment with ammonium chloride showed that neither a membrane-bound lipoprotein lipase nor lipophorin endocytosis is a relevant pathway to transfer or to storage lipids into fat body, respectively. Treatment of fat body with suramin caused a 50% inhibition in [(3)H]DAG transfer from lipophorin. Treatment of [(3)H]DAG-labeled fat body with lipase significantly reduced the amount of [(3)H]DAG associated with the tissue, suggesting that the lipid is still on the external surface of the membrane. Whether this lipid represents irreversibly adsorbed lipophorin or a DAG lipase-sensitive pool is unknown. Nevertheless, these results indicate that the main pathway for DAG transfer from lipophorin to fat body is via LTP and receptor-mediated processes.

Characterization of cholesterol transport from midgut to fat body in Manduca sexta larvae

Using in vitro methods, we investigated the transfer of cholesterol from larval Manduca sexta midgut to the hemolymph lipoprotein, lipophorin, and the transfer of cholesterol from lipophorin to larval fat body. In the midgut, transfer of free cholesterol shows saturation kinetics, but the apparent Km is higher than the measured Kd for the midgut lipophorin-receptor complex. In addition, the transfer is unaffected by suramin, which binds to the receptor and inhibits lipophorin binding, and by antibodies to the lipid transfer particle, which is required for export of diacylglycerol from the midgut to lipophorin. In the fat body, transfer of free cholesterol also shows saturation kinetics, and the apparent Km is higher than the measured Kd for the fat body lipophorin-receptor complex. Suramin and anti-lipid transfer particle antibodies exert only a small (20%) inhibitory effect. In both tissues it seems that the most likely mode of cholesterol transfer is via aqueous diffusion, which is also an important mechanism in vertebrate cells. Based on these results, we propose that cholesterol homeostasis in larval M. sexta is maintained by a mass action mechanism in which cholesterol is freely transferred between lipophorin and tissues depending on the needs of the tissues. This simple mechanism is ideally suited to insects, which can neither make cholesterol nor internalize lipophorin, the two mechanisms that vertebrate cells use to control their cholesterol content.

Cholesterol efflux from larval Manduca sexta fat body in vitro: high-density lipophorin as the acceptor

The objective of this study was to characterize the transfer of cholesterol from Manduca sexta larvae fat body to high-density lipophorin. [(3)H]-Cholesterol-labeled fat body was incubated with lipophorin under different conditions and cholesterol transfer was determined. Transfer rate exhibited a hyperbolic dependency on lipophorin concentration with an apparent K(m) of 3.6 mg/ml, which is consistent with either an aqueous diffusion mechanism of cholesterol transfer or a receptor-mediated process. Several results, including the high K(m), the high activation energy, and the lack of Ca(2+) dependence favor aqueous diffusion model. In addition, anti-lipid transfer particle antibodies had only a small inhibitory effect, suggesting it is not involved in cholesterol transfer. However, the transfer was inhibited in the presence of suramin, which would be consistent with a receptor-mediated process. The effects of suramin may be complex because it can change membrane properties when bound to the lipophorin receptor and affect the rate of cholesterol desorption. The preponderance of data suggests that the export of cholesterol from fat body to lipophorin follows a simple aqueous diffusion pathway. Although we cannot completely exclude some contribution from a receptor-mediated pathway, it seems that if such a pathway were present, it represents a minor route.

Role of lipid transfer particle in delivery of diacylglycerol from midgut to lipophorin in larval Manduca sexta

The present work analyzed the function of lipid transfer particle (LTP) in the process of exporting diacylglycerol from larval Manduca sexta midgut cells to lipophorin. When midgut sacs, which had been prelabeled in vivo with [(3)H]oleic acid, were incubated in vitro with a lipophorin-containing medium, a significant amount of radiolabeled diacylglycerol was transferred to lipophorin. Negligible amounts of diacylglycerol were released into lipophorin-free medium. In contrast, lipid-labeled lipophorin did not transfer diacylglycerol to the midgut sacs. The transfer of diacylglycerol from the midgut sac to lipophorin was blocked by preincubation of midgut sacs with antibody against LTP. Diacylglycerol transfer was restored to control values by the addition of purified LTP to midgut sacs that had been treated with antibody against LTP. Under these conditions the amount of diacylglycerol transferred was a function of the LTP concentration. These are the first results showing that LTP is required to export diacylglycerol from the midgut to lipophorin.

Purification and characterization of a lipid transfer particle in Rhodnius prolixus: phospholipid transfer

In this study we report the purification and characterization of a lipid transfer particle (LTP) from Rhodnius prolixus hemolymph, and its participation in phospholipid and diacylglycerol transfer processes. (3)H-diacylglycerol labeled low density lipophorin from Manduca sexta ((3)H-LDLp) was incubated with R. prolixus lipophorin (Lp) in the presence of Rhodnius hemolymph. Following incubation and isolation, both lipoproteins showed equivalent amounts of (3)H-labeled lipids. Hemolymph was subjected to KBr gradient ultracentrifugation. SDS-PAGE analysis of gradient fractions showed the enrichment of bands with molecular masses similar to the M. sexta LTP standard. LTP containing fractions were assayed and lipid transfer activity was observed. Purification of LTP was accomplished by (i) KBr density gradient ultracentrifugation, (ii) size exclusion, (iii) Cu(++) affinity and (iv) ion exchange chromatographies. LTP molecular mass was estimated approximately 770 kDa, comprising three apoproteins, apoLTP-I (315 kDa), apoLTP-II (85 kDa) and apoLTP-III (58 kDa). Phospolipid content of (32)P-LTP was determined after two-dimensional TLC. (32)P-phospholipid-labeled and unlabeled lipophorins, purified from R. prolixus were incubated in the presence of LTP resulting in the time-dependent transfer of phospholipids. LTP-mediated phospholipid transfer was not a selective process.

Lipid transport biochemistry and its role in energy production

Recent advances on the biochemistry of flight-related lipid mobilization, transport, and metabolism are reviewed. The synthesis and release of adipokinetic hormones and their function in activation of fat body triacylglycerol lipase to produce diacylglycerol is discussed. The dynamics of reversible lipoprotein conversions and the structural properties and role of the exchangeable apolipoprotein, apolipophorin III, in this process is presented. The nature and structure of hemolymph lipid transfer particle and the potential role of a recently discovered lipoprotein receptor of the low-density lipoprotein receptor family, in lipophorin metabolism and lipid transport is reviewed.

Purification and properties of a small lipid-binding protein from the hemolymph of Triatoma infestans

A small lipid-binding protein (sLBP) was purified from the hemolymph of the blood-sucking bug Triatoma infestans. Its isolation involved size exclusion-high performance liquid chromatography (HPLC) followed by anion exchange chromatography-HPLC. The molecular weight of the protein, as determined by gel permeation chromatography, was 20 kDa. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) resolved the protein into a single polypeptide with M(r) approximately equal to 16 kDa. The sLBP contains 6% lipids. Diacylglycerols represent the major lipid class, whereas phosphatidyl-choline, phosphatidyl-ethanolamine, free fatty acids and hydrocarbons were found in minor amounts. The amino acid composition indicated a high content of aspartic and glutamic acids and non-polar aliphatic amino acids. The N-terminal sequence did not resemble the sequence of any other previously reported insect hemolymph protein. Far-UV circular dichroism suggested that sLBP adopts a conformation rich in beta-sheet structure. The presence of this protein in hemolymph, fat body and unfertilized eggs was explored throughout the last nymphal and adult stages of the insect by Western blot assays. These assays indicated that sLBP is particularly abundant in hemolymph. A high concentration of sLBP was also detected in the fat body of the nymphs.

Lipid transfer particle catalyzes transfer of carotenoids between lipophorins of Bombyx mori

The yellow color of Bombyx mori hemolymph is due to the presence of carotenoids, which are primarily associated with lipophorin particles. Carotenoids were extracted from high density lipophorin (HDLp) of B. mori and analyzed by HPLC. HDLp contained 33 micrograms of carotenoids per mg protein. Over 90% of carotenoids were lutein while alpha-carotene and beta-carotene were minor components. When larval hemolymph was subjected to density gradient ultracentrifugation, a second minor yellow band was present, which was identified as B. mori lipid transfer particle (LTP). During other life stages examined however, this second band was not visible. To determine if coloration of LTP may fluctuate during development, we determined its concentration in hemolymph and compared it to that of lipophorin. Both proteins were present during all life stages and their concentrations gradually increased. The ratio of lipophorin: LTP was 10-15:1 during the fourth and fifth instar larval stages, and 20-30:1 during the pupal and adult stages. Thus, there was no correlation between the yellow color attributed to LTP and its hemolymph concentration. It is possible that yellow coloration of the LTP fraction corresponds to developmental stages when the particle is active in carotene transport. To determine if LTP is capable of facilitating carotene transfer, we took advantage of a white hemolymph B. mori strain which, when fed artificial diet containing a low carotene content, gives rise to a lipophorin that is nearly colorless. A spectrophotometric, carotene specific, transfer assay was developed which employed wild type, carotene-rich HDLp as donor particle and colorless low density lipophorin, derived from the white hemolymph strain animals, as acceptor particle. In incubations lacking LTP carotenes remained associated with HDLp while inclusion of LTP induced a redistribution of carotenes between the donor and acceptor in a time and concentration dependent manner. Time course studies suggested the rate of LTP-mediated carotene transfer was relatively slow, requiring up to 4 h to reach equilibrium. By contrast, studies employing 3H-diacylglycerol labeled HDLp as donor particle in lipid transfer assays revealed a rapid equilibration of label between the particles. Thus, it is plausible that the slower rate of LTP-mediated carotene transfer is due to its probable sequestration in the core of HDLp.

Purification and properties of a lipid transfer particle from Bombyx mori: comparison to the lipid transfer particle from Manduca sexta

A lipid transfer particle (LTP) was purified from the hemolymph of the silkworm Bombyx mori. Like other insect LTPs, the B. mori LTP is a very high density lipoprotein containing 21% lipid and three apoproteins of mass approximately 350 kDa, approximately 85 kDa, and approximately 60 kDa. B. mori LTP catalyzes the exchange of lipids between different density class lipoproteins found in adult hemolymph and between adult lipoproteins and vitellogenin. However, in no case was net lipid transfer observed. Manduca sexta LTP also catalyzed exchange of lipids, but not net transfer of lipids, between different density class lipoproteins found in adult hemolymph.

Disulfide bond engineering to monitor conformational opening of apolipophorin III during lipid binding

Apolipophorin III (apoLp-III) from the Sphinx moth, Manduca sexta, is an exchangeable, amphipathic apolipoprotein that alternately exists in water-soluble and lipid-bound forms. It is organized as a five-helix bundle in solution, which has been postulated to open at putative hinge domains to expose the hydrophobic interior, thereby facilitating interaction with the lipoprotein surface (Breiter, D. R. , Kanost, M. R., Benning, M. M., Wesenberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, H. M. (1991) Biochemistry 30, 603-608). To test this hypothesis, we engineered two cysteine residues in apoLp-III, which otherwise lacks cysteine, by site-directed mutagenesis at Asn-40 and Leu-90. Under oxidizing conditions the two cysteines spontaneously form a disulfide bond, which should tether the helix bundle and thereby prevent opening and concomitant lipid interaction. N40C/L90C apoLp-III was overexpressed in Escherichia coli and characterized for disulfide bond formation, secondary structure content, and stability, under both oxidizing and reducing conditions. Functional characterization was carried out by comparing the abilities of the oxidized and reduced protein to associate with modified lipoproteins in vitro. While the reduced form behaved like wild type apoLp-III, the oxidized form was unable to associate with lipoproteins. These results suggest that opening of the helix bundle is required for interaction with lipoproteins and provide a molecular basis for the dual existence of water-soluble and lipid-bound forms of apoLp-III. However, in phospholipid bilayer association assays, wild type, reduced, and oxidized N40C/L90C apoLp-III exhibited similar abilities to transform dimyristoylphosphatidylcholine multilamellar vesicles to disc-like complexes, as judged by electron microscopy. These data emphasize that underlying differences exist in initiating or maintaining a stable interaction of apoLp-III with phospholipid disc complexes versus spherical lipoprotein surfaces.

Manduca sexta lipid transfer particle: synthesis by fat body and occurrence in hemolymph

Lipid transfer particle (LTP) is present in hemolymph of the tobacco hornworm Manduca sexta. Biosynthesis of LTP, occurrence in hemolymph, and the role of LTP-apoproteins in the lipid transfer reaction were investigated using antibodies specific for LTP or for each of the apoproteins. In vitro protein synthesis followed by immunoprecipitation demonstrated that LTP is synthesized by the fat body and secreted into the medium. In contrast to apolipophorin III, an exchangeable apoprotein of lipophorin (the major lipid transport protein in hemolymph), apoLTP-III could not be detected free in hemolymph. LTP concentrations in the hemolymph were measured by a sandwich ELISA using a mouse monoclonal antibody against apoLTP-III as capturing antibody and rabbit polyclonal antibody against apoLTP-I as detecting antibody. LTP concentration increased during the late fifth instar larval stage, followed by a decrease in the wandering stage. Subsequently, LTP concentrations were strongly increased in hemolymph of adult moths. The role of the three apoproteins of LTP in the lipid transfer reaction was analyzed using apoprotein-specific antibodies. All three, apoLTP-I, -II, and -III, appeared to be important for lipid transfer activity, as shown by inhibition of lipid transfer by antibodies specific for each of the three apoproteins.

Biosynthetic route of locust apolipophorin III isoforms

Insect apolipophorin III (apoLp-III) plays a key role in the enhanced diacylglycerol transport during insect flight. For apoLp-III of the migratory locust, two different isoforms have been described (apoLp-IIIa and -b), displaying different N-termini and isoelectric points; each of the isoforms is however equally well capable to perform its function in lipid transport. In the present report the biosynthetic route of the apoLp-III isoforms is elucidated. Immunoprecipitation of media from in vitro fat body incubations and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that the locust fat body synthesized and secreted apoLp-III. ApoLp-III levels in the hemolymph showed that in young adults, apoLp-III concentrations were only very low (1-3 mg/ml). During adult maturation, however, the apoLp-III concentration increased rapidly to approximately 17 mg/ml. During apoLp-III elevation, the apoLp-IIIa:-b ratio remained equal or in the favour of the a-isoform, while in adults from approximately 12 days after adult ecdysis apoLp-IIIb was the most abundant isoform. Analysis of the protein by native polyacrylamide gel electrophoresis showed that only the apoLp-IIIa form was secreted. Injection of radiolabeled apoLp-IIIa into the hemolymph of adult locusts resulted in a slow conversion into apoLp-IIIb.

A turbidimetric assay of lipid transfer activity

A novel method to assay insect plasma lipid transfer particle (LTP) activity has been developed that employs insect high density lipophorin (HDLp) and human low density lipoprotein (LDL) as donor/acceptor substrate particles. At a 3:1 or greater HDLp:LDL protein ratio, LTP-mediated net vectorial transfer of diacylglycerol from lipophorin to LDL produces destabilized LDL particles that aggregate, causing sample turbidity. Turbidity was measured spectrophotometrically as a function of absorbance at 340 nm. After an initial lag phase, lipoprotein sample turbidity increased as a function of reaction time and LTP concentration. Saturation was observed at longer times or higher LTP concentrations, indicating that a reaction end point had been reached. As the substrate HDLp concentration was increased relative to LDL, a saturable increase in LTP-induced lipoprotein sample turbidity was observed. When the LDL concentration was increased relative to HDLp, however, there was an initial production of turbidity but at higher concentrations the sample did not develop turbidity. Reaction progress was also dependent on temperature over the range 0-37 degrees C. Taken together the results are consistent with the concept that LTP-mediated diacylglycerol transfer from HDLp to LDL creates unstable product LDL particles that aggregate. The assay method is advantageous because it employs relatively abundant, natural lipoprotein substrates, does not require prelabeling of donor lipid particles with radioactive or fluorescent lipids, and does not require separation of donor and acceptor after incubation. This is the first description of a lipid transfer assay that can be measured spectrophotometrically.

Monolayer-bilayer equilibrium of phospholipid: stabilization of neutral lipid droplets in aqueous medium and catabolism of plasma lipoproteins

Phospholipid spreads as monolayer at air/water and oil/water interfaces. Interfacial pressure of the monolayer in equilibrium with a definite bulk phase of the lipid, (equilibrium) spreading pressure, depends on lyotropic and thermotropic polymorphic state of the lipid bulk phase. Phosphatidylcholine (PC) in hydrated liquid crystalline state (bilayers in L alpha state) gives an appreciably large value of spreading pressure, 45-46 mN/m. Monolayer-bilayer equilibrium of a neutral lipid-phospholipid mixture is determined by miscibilities of the lipids in mixed monolayer, PC bilayer and bulk (liquid or solid) phase of the neutral lipid. A neutral lipid of limited solubility in PC bilayer forms a separate phase in aqueous medium. The phase is stabilized as small particles in the medium by PC monolayer at the particle surface. The monolayer is in equilibrium with the bilayer. This sort of equilibrium plays important roles in formation and catabolism of triglyceride- and cholesteryl ester-rich lipoprotein particles in animal plasma. The equilibrium is a critical factor also in stabilization of aqueous dispersion of lipophilic vitamin (neutral lipid). Coexistence of emulsion particles (neutral lipid core covered with PC monolayer) and vesicles made of PC bilayer are observed in a stable dispersion.

Role of lipid transfer particle in transformation of lipophorin in insect oocytes

Oocyte development in Manduca sexta involves the deposition of large amounts of lipoprotein-derived lipid. One source of this lipid is hemolymph high-density lipophorin-adult (HDLp-A) which, upon entry into the oocyte, is transformed into a lipid and apolipophorin III deficient product particle, egg very-high- density lipophorin (VHDLp-E; density = 1.24 g/ml; Kawooya et al. (1988) J. Biol. Chem. 263, 8740-8747). An in vitro model of this transformation has been established using human low-density lipoprotein (LDL) as acceptor of HDLp-A associated lipid in a reaction catalyzed by isolated M. sexta hemolymph lipid transfer particle (LTP). Facilitated vectorial net transfer of lipid from HDLp-A to LDL resulted in formation of a very-high-density lipophorin (VHDLp) product with a density and apolipoprotein content similar to that of VHDLp-E. Lipid was found to comprise 25% of the VHDLp particle mass, whereas over 50% of HDLp-A mass is lipid. Based on these observations it was hypothesized that a lipid transfer factor may be present in M. sexta oocytes and function in the transformation of HDLp-A to VHDLp-E in vivo. Transfer activity was present in the buffer soluble fraction of oocyte homogenates and purification of the active material revealed a catalyst with electrophoretic and immunological properties identical to hemolymph LTP. Incubation of 125I-HDLp-A with an M. sexta oocyte homogenate resulted in transformation of the radiolabeled lipoprotein to a density corresponding to that of VHDLp-E. When the incubation media was preincubated with anti-LTP IgG this conversion was inhibited to a large extent. Inhibition was relieved, however, by addition of exogenous LTP. The results provide the first demonstration of M. sexta LTP in a tissue other than hemolymph and support the concept that LTP-catalyzed lipid transfer plays an integral role in the conversion of HDLp-A to VHDLp-E in vivo.

Lipid transfer particle-catalyzed transfer of lipoprotein-associated diacylglycerol and long chain aliphatic hydrocarbons

Following injection of [1-14C]acetate into the sphinx moth, Manduca sexta, radiolabel was incorporated into lipid components of the major hemolymph lipoprotein, adult high density lipophorin (HDLp-A). Analysis of the labeled lipids by thin layer chromatography and radiochromatogram scanning revealed incorporation of radioactivity into the diacylglycerol (DAG) and hydrocarbon components as well as a third lipid fraction of unknown identity. Lipid transfer experiments were carried out using 14C-lipid HDLp-A and human low density lipoprotein (LDL) as donor/acceptor substrates and M. sexta lipid transfer particle (LTP) as catalyst. In control incubations lacking LTP, nearly all of the radiolabeled lipid remained associated with HDLp-A. LTP, however, induced a time-dependent vectorial transfer of radiolabeled lipid from HDLp-A to LDL. Lipid analysis of the LDL fraction, reisolated following the transfer reaction, revealed that labeled lipid components originally associated with HDLp-A were present in the acceptor LDL particles. The recovery of radiolabeled hydrocarbon associated with LDL demonstrates the capacity of LTP to facilitate transfer of these long chain, extremely hydrophobic, lipids and suggests LTP may function as a mediator of hydrocarbon transport and metabolism in vivo. When acceptor LDL particles were analyzed prior to complete transfer of HDLp-A-associated lipid it was observed that DAG was transferred preferentially during the initial stages of the reaction after which hydrocarbon transfer increased. This result suggests that LTP may have a lipid substrate preference for DAG versus hydrocarbon. Alternatively the observed preference for DAG may be a function of the relative accessibility of the substrates within the donor lipoprotein. In other experiments it was demonstrated that, unlike other lipids associated with HDLp-A, free fatty acid spontaneously transfers to LDL in the absence of lipid transfer catalyst.

Lipid transfer proteins

Translocations of various lipid species between membranes have been extensively studied. The transport of water-insoluble lipids is thought to require the participation of lipid transfer proteins (LTP). Several LTP, differing in their physiochemical properties and substrate specificities, have been purified to homogeneity from blood plasma, eucaryotic and procaryotic cells. Depending on their site of activity, they can be classified as extracellular and intracellular LTP. Extracellular LTP are found in the blood plasma and intracellular LTP, which were originally characterized as phospholipid exchange proteins, are ubiquitous in nature. Despite the enormous knowledge about their physicochemical properties and their function in vitro their physiological role has not been clearly demonstrated. However, their ubiquitous occurrence indicates an important role in cellular events. This review gives an overview of this interesting category of proteins, which are able to catalyze inter-membrane transfer and exchange of lipids.

Lipid transfer between human plasma low-density lipoprotein and a triolein/phospholipid microemulsion catalyzed by insect hemolymph lipid transfer particle

Lipid transfer between human plasma low-density lipoprotein (LDL) and an LDL-size microemulsion of triolein and phosphatidylcholine stabilized with human apolipoprotein A-I was catalyzed by the lipid transfer particle from hemolymph of the tobacco hornworm (Manduca sexta). Net transfer of phospholipid and triacylglycerol from the emulsion to LDL was observed and the apparent initial rates of transfer were dependent on the amount of catalyst. Net transfer of phospholipid mass was twice as much as that of triacylglycerol with respect to both the initial rate and the final equilibrium state. The final amount of net transfer of both lipids was dependent upon the initial ratio of LDL: microemulsion present in the incubation mixture up to 1:1 on the basis of phospholipid. The microemulsion lipid composition was maximally altered from an initial weight ratio of 1.09 +/- 0.08 (phospholipid/triolein) to 0.90 +/- 0.03 by this reaction. Further increase of LDL in the incubation caused neither further net transfer nor further change in the lipid composition of the microemulsion. The catalyst neither affected spontaneous transfer of free cholesterol between the emulsion and LDL nor enhanced cholesteryl ester transfer in this reaction system. As a result of the facilitated reaction, LDL gained a significant amount of phospholipid and triacylglycerol causing up to an 8% increase in core lipids and 14% in phospholipid. Some free cholesterol is recovered in the emulsions via spontaneous exchange. Transfer or exchange of apolipoproteins during the course of facilitated lipid transfer did not occur.