Transfer of phospholipids

The fate of injected liposomes is drastically altered by administration route, dose and size, lipid composition, surface modification, and encapsulated drugs. Liposomes encapsulating drugs are often administered iv, therefore, the stability of liposomes in plasma is important. When liposomes composed of PC with unsaturated fatty acyl chains are incubated in the presence of serum, an efflux of internal solute from the liposomes is observed. This increase in permeability is caused by the transfer of phospholipids to high density lipoprotein (HDL) in serum (55). To reduce the efflux of liposomal contents, cholesterol is added as a liposomal component... 

Fig. 5.2.1 The major metabolic pathways of the lipoprotein metabolism are shown. Chylomicrons (Chylo) are secreted from the intestine and are metabolized by lipoprotein lipase (LPL) before the remnants are taken up by the liver. The liver secretes very-low-density lipoproteins (VLDL) to distribute lipids to the periphery. These VLDL are hydrolyzed by LPL and hepatic lipase (HL) to result in intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL), respectively, which then is cleared from the blood by the LDL receptor (LDLR). The liver and the intestine secrete apolipoprotein AI, which forms pre-jS-high-density lipoproteins (pre-jl-HDL) in blood. These pre-/ -HDL accept phospholipids and cholesterol from hepatic and peripheral cells through the activity of the ATP binding cassette transporter Al. Subsequent cholesterol esterification by lecithinxholesterol acyltransferase (LCAT) and transfer of phospholipids by phospholipid transfer protein (PLTP) transform the nascent discoidal high-density lipoproteins (HDL disc) into a spherical particle and increase the size to HDL2. For the elimination of cholesterol from HDL, two possible pathways exist (1) direct hepatic uptake of lipids through scavenger receptor B1 (SR-BI) and HL, and (2) cholesteryl ester transfer protein (CfiTP)-mediated transfer of cholesterol-esters from HDL2 to chylomicrons, and VLDL and hepatic uptake of the lipids via the LDLR pathway...

The transfer of CE from HDL to TRL and LDL and the transfer of TG back to HDL is facilitated by cholesteryl ester transfer protein (CETP) or lipid transfer protein 1 (LTP 1). CETP also catalyzes the transfer of phospholipids... 

C22. Crain, R. C., and Zilversmit, D. B., Net transfer of phospholipid by the non-specific phospholipid transfer proteins from bovine liver. Biochim. Biophys. Acta 620, 37-48 (1980). 

For example, flip-flop transfer of phospholipids in membranes is usually also very slow, from hours to days. However, there are exceptions for example, phosphatidylethanol undergoes rapid and reversible transbilayer distribution in unilamellar PC vesicles in the presence of multivalent cations, including calcium. ... 

After synthesis in the various compartments of endoplasmic reticulum of alveolar type II cells, surfactant components are assembled in the cytosol into lamellar bodies. In the process of formation of lamellar bodies, the transfer of phospholipids between membranes is facilitated by phospholipid transfer proteins, which are nonenzymatic proteins found in all eukaryotic cells and which play an important role in lipid metabolism. There are three well-characterized phospholipid transfer proteins ... 

Because membranes are dynamic structures, the mechanism by which they are synthesized is complex. Currently, little is known about the synthesis of the membrane bilayer except for the following features phospholipid translocation across membranes and the intracellular transfer of phospholipids between membranes. 

If recently synthesized phospholipid molecules remained only on the cytoplasmic surface of the ER, a monolayer would form. Unassisted bilayer transfer of phospholipid, however, is extremely slow. (For example, half-lives of 8 days have been measured across artificial membrane.) A process known as phospholipid translocation is now believed to be responsible for maintaining the bilayer in membranes (Figure 12F). Transmembrane movement of phospholipid molecules (or flip-flop), which may occur in as little as 15 seconds, appears to be mediated by phospholipid translocator proteins. One protein (sometimes referred to as flippase) that transfers choline-containing phospholipids across the ER membrane has been identified. Because the hydrophilic polar head group of a phospholipid molecule is probably responsible for the low rate of spontaneous translocation, an interaction between flippase and polar head groups is believed to be involved in phosphatidylcholine transfer. Translocation results in a higher concentration of phosphatidylcholine on the lumenal side of the ER membrane than that... 

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. 

Several assays measure the transfer of phospholipids between artificial vesicles. These assays are particularly appealing because they have well-defined acceptor and donor membranes that are easily prepared and, in some cases, can be modified. 

The transfer of radiolabeled phospholipids between vesicles and erythrocyte membranes could be used to assay lipid transfer activity. Intact erythrocytes are not an ideal substrate for routine measurements of transfer activity because some transfer proteins do not readily accelerate the transfer of phospholipids from these membranes. Van Meer et al. (1980) found that a very high concentration of the phosphatidylcholine-specific transfer protein was necessary to exchange the phosphatidylcholine of intact red blood cells. Erythrocyte ghosts are a more active substrate for this protein (Bloj and Zilversmit, 1976). However, the nonspecific transfer protein from bovine liver accelerates the exchange of phospholipid between intact erythrocytes and phosphatidylcholine vesicles (Crain and Zilversmit, 1980c). 

This experimental approach has been used previously to study the spontaneous transfer of phospholipids between artificial membranes (Martin and MacDonald, 1976 Duckwitz-Peterlein et al., 1977). Xti et al. (1982) used the fluorescence anisotropy of diphenylhexatriene (DPH) in the phosphatidylcholine bilayer to measure the change in the physical state of DPPC and DMPC vesicles upon mixing in the presence of transfer protein. The fluorescent measurements were recorded at a temperature intermediate between the phase transition of the two initially pure vesicles. By using flow cytometry, it was possible to measure the fluorescence... 

The effect of different phospholipid head groups on the protein-stimulated transfer by phosphatidylcholine- and phosphatidylinositol-specific proteins has been studied. Contradictory results were obtained for the effect of acidic phospholipids on the transfer of phospholipid by the phosphatidylcholine exchange protein from beef liver. DiCorleto et al. (1977) used small unilamellar vesicle-mitochondria and small unilamellar vesicle-multilamellar vesicles to study the effect of varying amounts of acidic phospholipids incorporated into phosphatidylcholine donor vesicles. Up to 20 mol% phosphatidic acid or phosphatidylinositol in the donor was found to stimulate the transfer of phosphatidylcholine in both assay systems. Wirtz et al. (1979) and Hellings et al. (1974) found different results for the phosphatidylcholine exchange protein with unilamellar and multilamellar vesicles. In these assays, the incorporation of acidic phospholipids (phosphatidic acid or phosphatidylinositol) into the donor particles had an inhibitory effect on the rate of phosphatidylcholine transfer. 

Phospholipid transfer protein is a plasma protein that promotes the transfer of phospholipids down a phospholipid concentration gradient, probably by a shuttle mechanism... 

Vesicle-mediated transfer of phospholipids to plasma membrane during cell aggregation of Dictyostelium discoideum. J. Biol. Chem. 256, 5845-5850. 

The mode of action of lipid transfer proteins is not entirely elucidated. However, it is assumed that a complex between phospholipid and protein is formed. This complex interacts with the various membranes the transfer of the phospholipid molecule from the complex to the membrane leads to a net transfer of phospholipid. However, an exchange of the phospholipid molecule bound to the complex with the phospholipids of the membranes leads to an overall-processes of exchange of phospholipids this explains why these proteins were previously called phospholipid exchange proteins". 

Yamada M., Tanaka T., Kader J.C. and Mazliak P. (1978) - Transfer of phospholipids from microsomes to mitochondria in germinating castor bean endosperm. Plant Cell Physiol., 173-176. 

Phospholipid transfer proteins (PLTP) have been isolated to homogeneity from various plant sources spinach leaves, maize seedlings and castor bean endosperm These proteins are able to facilitate an in vitro transfer of phospholipids between membranes. This led to the hypothesis that these proteins participate In vivo in the turnover and biogenesis of membranes. In order to study the function of these proteins, it is essential to know their biochemical properties as well as their biogenesis. In this paper, we compare the properties of the transfer proteins isolated from the three tissues and we indicate new informations concerning the vitro synthesis of the protein Isolated from maize seedlings. 

Kader JC. Intracellular transfer of phospholipids, galactolipids and fatty acids in plant cells. In Hilderson HJ, editor. Subcellular biochemistry. New York Plenum Publishing Corporation, 1990 69-111. 

Sturbois B, Moreau P, Maneta-Peyret L, Morre DJ, Cassagne C. Cell-free transfer of phospholipids between the endoplasmic reticulum and the Golgi apparatus of leek seedlings. Biochim Biophys Acta 1994 1189 31-37. 

In our laboratory, studies of lipid transfer in leek seedlings in vivo, have already shown the existence of a vesicular process for the transfer of phospholipids and particularly of very long chain fatty acid-containing lipids [6]. This process follows the vesicular endoplasmic reticulum- Golgi apparatus- plasma membrane pathway. Using the cell-free system developed by Morre and coworkers, we have reconstituted in vitro the vesicular transfer of some phospholipids between the ER and the GA. This transfer is ATP and cytosol-dependent, is N Ethyl Maleimide and temperature sensitive and specific for the ER as donor and the GA as acceptor. The phospholipids transferred via an ATP-dependent manner in vitro between the ER and the GA were phosphatidylcholine (PC +79%), phosphatidylethanolamine (PE +67%) and phosphatidylserine (PS +123%) [7]. All those results are in favour of a vesicular transport of phospholipids between the ER and the GA of leek seedlings, and brought us to purify these transition vesicles issued from the ER. 

Therefore, it is likely that the transition vesicles isolated correspond, at least to some extent, to those that are operative in the transfer of phospholipids between the ER and the Golgi apparatus. 

Wirtz, K. W. A., 1974, Transfer of phospholipids between membranes, Biochim. Biophys. Acta 344 95. 

---