Lipid transfer particle

Lipophorin acts as a reusable shuttle between the membrane-bound lipophorin receptors in tissues (Tsuchida and Wells 1990, Gopalapillai et al. 2006) and is not generally endocytosed in the cells (Law and Wells 1989, Arrese et al. 2001, Canavoso et al. 2001). Thus, the intracellular CBP alone seems not to be able to pick up carotenoid from the lipophorin that resides outside of the cell. Cell surface components are thought to be necessary to allow intracellular delivery of carotenoids (Figure 24.6, magnification) (Arrese et al. 2001). The lipid transfer particle (LTP) (Blacklock and Ryan 1994, Tsuchida et al. 1997) on the outer surface of membranes and unknown membrane-spanning factors that specifically transfer carotenoids might be candidates. 

Tsuchida, K., Soulages, J. L., Moribayashi, A., Suzuki, K., Maekawa, H., and Wells, M. A. 1997. Purification and properties of a lipid transfer particle from Bombyx mori Comparison to the lipid transfer particle from Manduca sexta. Biochim. Biophys. Acta, 1337(l) 57-65. 

Tsuchida, K., Arai, M., Tanaka, Y. et al. 1998. Lipid transfer particle catalyzes transfer of carotenoids between lipophorins of Bombyx mori. Insect Biochem. Mol. Biol., 28(12) 927-934. 

Lipophorin receptors Q Lipid transfer particle = Cellular junctions y Non-native hydrocarbons... 

Canavoso L. E. and Wells M. A. (2001) Role of lipid transfer particle in delivery of diacylglycerol from midgut to lipophorin in larval Manduca sexta. Insect Biochem. Mol. Biol. 31, 783-790. 

Liu H. and Ryan R. O. (1991) Role of lipid transfer particle in transformation of lipophorin in insect oocytes. Biochim. Biophys. Acta 1085, 112-118. 

Takeuchi N. and Chino H. (1993) Lipid transfer particle in the hemolymph of the American cockroach evidence for its capacity to transfer hydrocarbons between lipophorin particles. J. Lipid Res. 34, 543-551. 

Van Heusden M. C. and Law J. H. (1989) An insect lipid transfer particle promotes lipid loading from fat body to lipoprotein../. Lipid Res. 32, 1789-1794. 

Canavoso, L. E Yun, H.K., Jouni, Z.E. and Wells, M. A. (2004). Lipid transfer particle mediates the delivery of diacylglycerol from lipophorin to fat body in larval Manduca sexta. J. Lipid Res., 45,456-465. 

Singh, T.K.A. and Ryan, R.O. (1991). Lipid transfer particle-catalyzed transfer of lipoprotein-associated diacylglycerol and long chain aliphatic hydrocarbons. Arch. Biochem. Biophys., 286, 376-382. 

In this section we describe what is known about lipophorin metabolism. First we discuss the lipid transfer particle, which may play an important role in transferring lipids to and from lipophorin, then we describe the different roles of lipophorin in lipid delivery. 

The lipid transfer particle (LTP) is a very high-density lipoprotein having a molecular mass greater than 650 kDa. LTP contains about 15% lipid and three apolipoproteins with = 320,000, 85,000, and 55,000. First discovered in M. sexta (Ryan et al., 1986a), LTP has been purified... 

Fig. 8. Role of lipophorin in DG delivery to flight muscle. Adipokinetic hormone (AKH) is released from the corpus cardiacum and binds to the fat body, where it cause production of cAMP and entry of Ca . These second messengers activate lipolysis of triacylglycerol (TG) and production of diacylglycerol (DG). The DG leaves the fat body with the assistance of a lipid transfer particle (LTP) and is taken up by HDLp. The capacity of HDLp to carry DG is increased by binding of apoLp-HI to the surface. Ultimately, LDLp is formed and moves to the flight muscle, where a lipoprotein lipase hydrolyzes the DG to produce fatty acid (FA) and regenerate HDLp and apoLp-IIl. The FA enters the flight muscle, where it is oxidized to produce the ATP required to power flight. HDLp and apoLp-HI circulate back to the fat body to complete the cycle.

MTP is responsible for the transfer of TGs and cholesteryl esters from the endoplasmic reticulum (ER) to lipoprotein particles (VLDL in hepatocytes in the liver and chylomicrons in endocytes in the intestine) for secretion [52]. It is a heterodimer consisting of a unique large subunit essential for lipid transfer encoded by the mttp gene and a smaller subunit, the ubiquitous ER enzyme protein disulfide isomerase [53]. 

Reaction catalyzed by lecithin cholesterol acyltransferase (LCAT). The resulting cholesteryl ester is transferred to VLDL and LDL particles by a lipid transfer protein. 

In hepatocytes, vitamin E can take two routes. A fraction of it is packaged as VLDL and reenters the circulation, while excess is excreted in the bile. Plasma lipolysis of the VLDL particle again results in release not only of lipids, but also of vitamin E, with the remainder left with the LDL particles. This fraction can be further distributed to tissues via LDL receptor-mediated endocytosis or transferred between lipoproteins, mainly to HDL, by plasma lipid transfer proteins. Thus, mobilization of vitamin E from intestinal and liver... 

Appraisals of the optical properties of lipid solutions and dispersions will provide information on concentrations, aggregation and stability, phase transitions, densities, and repeating structuresJ Measurements of refractive index, scattered light intensity (polarized and depolarized), and birefringence are relatively easy laboratory methods on which certain product specifications may be based. Also, fluorescent techniques can readily provide information on lipid movements and transfer of lipid between particles. ... 

Lipid/DNA particles represent a nonliposomal but lipid-based delivery system for gene transfer. Monomeric or micellar lipids are allowed to interact with DNA in the presence of detergent or some other surface-active agent that is then removed by dialysis. As the surface-active agent diffuses out, solid, condensed particles of lipid and DNA form (17). These can be prepared such that they are smaller and more homogeneous than liposome/DNA complexes yet transfect cells equally well (F. Wong, unpublished observations). 

To quantitate the lipid transfer activity of a protein, one measures the movement of labeled lipids from one membrane, the donor, to a second membrane, the acceptor. Typically, the donor and acceptor membranes are incubated in the presence and absence of transfer protein. After the incubation, the particles are separated and either the loss of radiolabeled lipids from the donor particles or the appearance of radiolabeled lipids in the acceptor particles is quantitated. The rate of lipid transfer in the presence of protein minus the transfer that occurs in the absence of protein is a measure of the lipid transfer activity of the protein. The transfer activity is expressed as a percent of the donor lipid transferred or the number of nmols lipid transferred per unit of time. To determine if the rate of lipid transfer also represents the rate of exchange, it must first be established that lipid exchange occurs between donors and acceptors. Exchange occurs when the rate of lipid transfer from donor to acceptor equals the rate of transfer from acceptor to donor or when the chemical composition of the donor and acceptor membranes does not change during the transfer reaction. 

A back-transfer of unlabeled and radiolabeled lipids from the acceptor to donor particles must be considered when calculating the lipid transfer rate. When transfer reactions beyond the initial rates are examined, it is necessary to account for the dilution of radioactive lipids in the donor particles by back-transfer of unlabeled lipid from the acceptor particles. Unless this is done, the apparent transfer rate will decrease with time. Equation (1) can be used to determine the transfer rate when back-transfer of unlabeled lipid is significant. 

Measuring the rate of lipid transfer by an assay that includes the separation of donor and acceptor particles is complicated by the fact that rarely,... 

The transfer of phospholipids between mitochondria and microsomes in vitro was first used to measure the activity of lipid transfer proteins (Wirtz and Zilversmit, 1968). In this assay, isolated mitochondria and microsomes are incubated with an appropriate amount of transfer protein. Either particle may be radiolabeled and serve as the donor particle. The exchange reaction is terminated by sedimenting the mitochondria by centrifugation. The change in the radioactivity of either the donor or acceptor particles can be used to calculate the lipid transfer activity. 

Lipid transfer activities are generally determined by assays involving separation of donor and acceptor particles. These techniques have some distinct disadvantages. The time-consuming process of separating donors and acceptors is especially troublesome in a kinetic analysis of a transfer reaction when it is necessary to measure the time dependence of lipid transfer reactions. In addition, the separation of donor and acceptor membranes requires that the two particles differ in some respect. 

A complex relationship exists between the initial rate of lipid transfer and the concentration of acceptor and donor particles (Van den Besselaar et al., 1975 Wirtz et al., 1979). An example of this is illustrated in Figure 1. At a given concentration of acceptor, a low concentration of donor results in a low rate of lipid transfer. When the concentration of donor is increased, so is the transfer rate until a maximum in the transfer rate is reached. A further increase in the donor concentration results in a decline in the transfer rate. In a typical assay, acceptor lipid is present in excess of donor lipid. For example, about four times as much exchangeable acceptor as donor lipid is used in the small unilamellar vesicle-multilamellar vesicle assay (Crain and Zilversmit, 1980b). This minimizes the back-transfer of labeled lipid from the acceptor to the donor particle during the exchange reactions while still maintaining rapid rates of lipid transfer. 

Figure 1. Effect of donor particle concentration on the initial rate of lipid transfer. The computer-drawn curves are based on the kinetic model by Van den Besselaar et al. (1975) for protein-catalyzed phospholipid exchange. The kinetic constants are for, the transfer of phosphatidylcholine from liposomes containing 12 mol% phosphatidic acid to liposomes containing 2 mol% phosphatidic acid. The three curves represent three different acceptor concentrations indicated above the appropriate curves.

An alteration in the composition of the acceptor or donor particles may alter the rate at which lipids are transported. The change may be the result of a change in membrane properties, such as fluidity or charge, or a competitive effect. In transfer reactions, lipid molecules compete with each other. In addition, transfer proteins preferentially transfer particular classes of lipids. When the lipid composition of a substrate is modified, this competition between molecules is altered and as a consequence, the lipid transfer rate may change. 

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