Measurement and prediction of the rates of spontaneous transfer of phospholipids between plasma lipoproteins

The purpose of this report is to develop a correlation between the hydrophobicity of a phospholipid as measured by reversed-phase high-performance liquid chromatography and its rate of spontaneous transfer and to use this correlation to predict the rate of transfer of any homologous lipid from any lipoprotein. We have studied the mechanism of transfer of a series of fluorescent or radiolabeled phospholipids among natural and reassembled serum lipoproteins. Fluorescent phosphatidylcholines included those with 9-(1-pyrenyl)nonanoic acid in the sn-2 position and lauric, myristic, palmitic, stearic, oleic or linoleic acid at sn-1. The radioactive phosphatidylcholines contained [3H]oleic acid in the sn-2 position and lauric, myristic, or palmitic acid at sn-1. The kinetics of transfer of the pyrene-labeled lipid were followed by changes in the excimer fluorescence, and that of the radioactive lipids by separation of the donor (lipid-apolipoprotein recombinant) from the acceptor (single bilayer vesicles) on a column of Sephacryl S-200. The retention time of each lipid was measured by high-performance hydrophobic chromatography through a Waters radially compressed C18 column eluted with 75% isopropanol and 25% triethylammonium phosphate (0.15 M). A linear relationship was observed between the rate-constant of transfer and the retention time which suggest that the rate of desorption of phosphatidylcholines from lipoproteins and vesicles is controlled predominately by the hydrophobic effect. For a homologous series of lipids, the rate of transfer can be predicted from retention times obtained from hydrophobic chromatography. The kinetics of transfer of 1-lauroyl-2-[9-(1-pyrenyl)nonanoyl] phosphatidylcholine between isolated human serum lipoproteins exhibits a linear correlation between the transfer half-time and the size of the donor lipoproteins. As a consequence, transfer from very-low-density lipoprotein is 10-times slower than that observed from high-density lipoproteins. The observed correlations between phospholipid transfer rates and both the Stokes radius of the donor and the retention time of the phospholipid on a hydrophobic column permit one to calculate the rate of transfer of homologous molecules between lipid-protein complexes. The results predict that the spontaneous transfer of phospholipids between plasma lipoproteins would be too slow to be a physiologically important phenomena.     

Effects of hydrophobicity on turnover of plasma high density lipoproteins labeled with phosphatidylcholine ethers in the rat

Rat high density lipoproteins (HDL) were labeled with a series of phosphatidylcholines and ether analogs of phosphatidylcholine. The rates of turnover of the phosphatidylcholine ethers in the rat decreased as a function of increasing hydrophobicity and were more than five times faster than those of apolipoprotein A-I turnover and spontaneous lipid transfer. The major tissue sites for uptake were the liver, adrenals, and ovaries. The rate of turnover of a phosphatidylcholine was faster than that of the corresponding ether analog due to the action of lecithin:cholesterol acyltransferase, although this activity was slow compared to the turnover of high density lipoprotein-phosphatidylcholine. Injection of a purified human phosphatidylcholine transfer protein increased the turnover rate of a phosphatidylcholine and its ether analog. We conclude that a major route for the turnover of plasma high density lipoprotein-phosphatidylcholine in the rat is independent of spontaneous lipid transfer, hydrolysis, and HDL particle uptake, and that it involves the activity of a plasma phosphatidylcholine transfer protein.     

Spontaneous and plasma factor-mediated transfer of pyrenyl cerebrosides between model and native lipoproteins

A series of pyrenyl glucocerebrosides was synthesized by reacylation of psychosine with pyrene-labeled fatty acids having 3-11 methylene units. When incorporated into model high-density lipoproteins consisting of dimyristoylphosphatidylcholine-apolipoprotein A-II complexes and incubated with unlabeled complexes, these lipids exhibited spontaneous transfer. Half times of transfer varied from 1.5 min to 365 min at 37 degrees C. The logarithm of the rate of transfer was linearly related to the number of fatty acyl methylene units and HPLC retention time. Transfer occurred by passage of lipid monomers through the aqueous phase. Spontaneous transfer of the glycolipids also occurred when they were incorporated into native high-density lipoproteins. Rates of transfer between native high-density lipoprotein particles were higher than those observed between model high-density lipoprotein particles. A partially purified lipid exchange protein from plasma, as well as unfractionated lipoprotein-deficient serum, stimulated the transfer of fluorescent glycolipid between model high-density lipoprotein or native high-density lipoprotein and low-density lipoprotein 2-24 fold. The protein also stimulated the transfer of tritiated ganglioside GM3 between native low-density lipoprotein and high-density lipoprotein. This protein may play a role in glycolipid exchange in vivo.     

Molecular volumes of phospholipids and glycolipids in membranes

Data on the molecular volumes of phospholipids and glycolipids in membranes are collected together in order to determine the contributions from the component groups, for as wide a range of lipids as possible, including sphingolipids. Wherever possible, the volumes of the methylene groups in the lipid chains are established from the dependence on chain length at fixed temperature in a given phase. In this way, it is also possible to determine the constant contribution from cis double bonds in the chains of monoenoic unsaturated phosphatidylcholines, and the volume of the branched methyl groups in isoacyl phosphatidylcholines. Issues concerning separation of contributions from the polar head groups from those of the chain terminal methyl groups are discussed. Molecular volumes of lipids in crystals are also analysed to provide information on head-group packing that can be compared with the situation in membranes, and used to set limits on the relative contributions from polar groups and terminal methyl groups. Comparisons are made with volumetric analyses based on diffraction studies of bilayers of single lipids. The parameters derived can be used to estimate molecular volumes of lipids for which dilatometric or densitometric data are lacking. Lipid volumes are determining parameters for lipid dynamics, membrane partitioning and permeation of solutes, and are essential quantities for the structural analysis of lipid membranes.     

Fluorescence assay of the specificity of human plasma and bovine liver phospholipid transfer proteins

The specificities of a human plasma and bovine liver phospholipid transfer protein were studied using a fluorescence assay based on the transfer of pyrenyl phospholipids. This method was used previously to determine the mechanism of spontaneous transfer of phospholipids between model lipoproteins (Massey, J.B., Gotto, A.M., Jr. and Pownall, H.J. (1982) Biochemistry 21, 3630-3636). The pyrenyl phospholipids varied in the headgroup moiety; pyrenyl phosphatidylcholines contained different fatty acyl chains in the sn-1 position. Model high-density lipoproteins (R-HDL) consisting of apolipoprotein A-I and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) were used as donor and acceptor particles. As previously shown, the bovine liver protein mediated the transfer of only phosphatidylcholine. In contrast, the human plasma protein transferred all species studied which included a phosphatidylserine, phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, phosphatidic acid, sphingomyelin, galactosylcerebroside, and a diacylglycerol. The activity of these transfer proteins was only slightly affected by changes in the acyl chain composition of the transferring lipid. Pyrenyl and radioactive ([3H]POPC) phospholipids were transferred with equal rates by the human transfer protein, suggesting that this protein has similar binding characteristics for pyrenyl and natural phospholipids. Spontaneous phospholipid transfer occurs by the aqueous diffusion of monomeric lipid where the rate is highly dependent on fatty acyl chain composition. In this study, no correlation between the rate of spontaneous transfer and protein-mediated transfer was found. The apparent Km values for R-HDL and low-density lipoprotein (LDL), when used as acceptors, were similar when based on the number of acceptor particles. The apparent Vmax for the bovine liver protein was identical for R-HDL and LDL but for the plasma protein Vmax was slightly higher for R-HDL. These results suggest that, like the bovine liver protein, the plasma protein functions as a phospholipid-binding carrier that exchanges phospholipids between membrane surfaces. The assay of lipid transfer proteins by pyrenyl-labeled lipids is faster and easier to perform than other current methods, which require separation of donor and acceptor particles, and is suitable for studies on the function and mechanism of action of lipid transfer proteins.     

Parinaroyl and pyrenyl phospholipids as probes for the lipid surface layer of human low density lipoproteins

A simple protocol employing lipid transfer proteins was developed to label human low density lipoprotein (LDL) in a controlled manner with parinaroyl and pyrenyl phosphatidylcholines. In order to study the lipid fluidity in the surface lipid layer of LDL, the temperature-dependence of both polarization (parinaroyl probes) and excimer to monomer (E/M) intensity ratio (pyrenyl probes) were analyzed. A series of pyrenyl phosphatidylcholines containing a pyrenyl fatty acid varying from 6 to 14 carbons in length at the sn-2 position were inserted into LDL to investigate the lateral distribution of different phosphatidylcholines in the lipoprotein surface at 37 degrees C. Both polarization and E/M vs. temperature plots displayed discontinuities in the region of 22-32 degrees C, which coincides with the melting of the neutral lipid core, indicating that the latter induces an ordered to more disordered phase transition in the surface lipid layer. Determination of the E/M intensity ratio as a function of pyrene lipid concentration in LDL showed a linear relationship for the pyrenyl hexanoate and octanoate species, whereas a slope discontinuity was observed for the lipids containing a longer pyrenyl chain. These data suggest that two lipid domains with distinct properties exist in the surface layer and secondly, pyrenyl lipids partition between these domains in a chainlength-dependent manner. This is consistent with measurement of the tryptophan to pyrene energy transfer efficiency vs. pyrenyl lipid concentration, which showed a biphasic relationship for the long-chain pyrenyl lipids. These measurements further indicate that two surface lipid domains correspond to the protein-lipid boundary and the bulk lipid phase, respectively. The fact that relatively small changes in chainlength have a marked influence on the partitioning of pyrenyl lipids between the boundary and the bulk phase suggests also that native phospholipid species may not be randomly distributed in the surface lipid layer of LDL.     

Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces.

The kinetics of transfer of natural and fluorescent nonesterified fatty acids (NEFA) and lysolecithins (lysoPC) from phospholipid and protein surfaces were measured. The kinetics of transfer of 12-(1-pyrenyl)dodecanoic acid, from liquid crystalline and gel phase single unilamellar phospholipid vesicles, very low, low, and high density lipoproteins, human serum albumin, and rat liver fatty acid-binding protein, were first-order and characterized by similar rate constants. The halftimes (t1/2) of NEFA transfer from lipids and proteins were dependent on the acyl chain structure according to log t1/2 = -0.62n + 0.59m + 12.0, where n and m, respectively, are the numbers of carbon atoms and double bonds. The structure of the donor surface had a measurable but smaller effect on transfer rates. The kinetics of NEFA and lysoPC transfer are slow relative to the lipolytic processes that liberate them. Therefore, one would predict a transient accumulation of NEFA and lysoPC during lipolysis and an attendant modulation of many metabolic processes within living cells and within the plasma compartment of blood. These data will be useful in the refinement of current models of membrane and lipoprotein function and in the selection of fluorescent NEFA analogs for studying transport in living cells.     

Kinetics of transfer of alpha-tocopherol between model and native plasma lipoproteins

The rate of spontaneous transfer of alpha-tocopherol, cholesterol and beta-carotene between model and native lipoproteins was measured to determine the mechanism and kinetics of equilibration of these lipids in plasma. Cholesterol and alpha-tocopherol transfer from apolipoprotein A-I/1-palmityl-2- oleoylphosphatidylcholine ( POPC ) recombinants to bovine brain ganglioside/ POPC single bilage vesicles with half-times of approximately 20 min and 70 min, respectively. Under identical conditions, there is no significant transfer of beta-carotene even after an 18-h incubation period. alpha-Tocopherol transfers from apolipoprotein A-II/dimyristoylphosphatidylcholine recombinants with a half-time of 40 min and an activation energy of 17.2 kcal/mol. Incubation of high-density lipoproteins containing alpha-[3H]tocopherol with low-density lipoproteins or very-low-density lipoproteins results in the equilibration of the labelled lipid between the lipoprotein classes in 1 h. A comparison of the rates of transfer indicates that alpha-tocopherol equilibrates 2-3-times more slowly than cholesterol but on a time scale much shorter than the lifetime of lipoproteins in the circulation. Thus, the distribution of alpha-tocopherol is not kinetically controlled but determined thermodynamically by the partitioning between the total amount of lipid in each compartment. The spontaneous transfer of beta-carotene is too slow for this equilibration to occur.     

Conformational study of the acetyl group and cyclohexenone in progesterone interacting with phosphatidyllipid by means of circular dichroism

The interaction between the A-ring and the 17-acetyl groups of progesterone (PROG) and various concentrations of distearoyl-, dipalmitoyl-, dioleoyl- and diarachidoyl-L-alpha-phosphatidylcholines, and dipalmitoyl-L-alpha-phosphatidyl-DL glycerol in methanol and chloroform solutions and its preferred conformational assignments in the presence of those lipids were examined qualitatively by circular dichroism on the basis of PROG spectra in the wavelength regions of 260-400 nm. PROG did not interact with saturated distearoyl and dipalmitoyl phosphatidylcholines, but did with unsaturated dioleoyl and diarachidoyl phosphatidylcholines, and saturated dipalmitoylphosphatidylglycerol. The interacting moieties of PROG were an alpha,beta-unsaturated cyclohexenone of the A-ring for oleoyl and glycerol lipids, and the 17-acetyl group for unsaturated and glycerol lipids. The interaction with these lipids, the rotational conformations of the 17-acetyl group, and invertible conformations of the cyclohexenone of PROG were discussed on the basis of the elliptical strength of the Cotton effect and energy estimation of the preferred conformers. Oleoylphosphatidylcholine caused an increase in slightly energetically unstable conformers of the acetyl group and stable conformers of the alpha,beta-unsaturated cyclohexenone. Glycerol lipid, on the other hand, caused an increase in energetically unstable conformers of cyclohexenone, but it was similar to the effect of oleoyl lipid on the 17-acetyl group. Diarachidoyl-L-alpha-phosphatidylcholine, with eight double bonds, other hand, increased the number of energetically stable conformers of the 17-acetyl group, but had no effect on the conformation of cyclohexenone. It became apparent that the double bond of hydrocarbon moiety as well as the head group of choline and glycerol in lipids were closely related to the conformational populations of both groups of the PROG molecule. The specific effect on the conformations of the acetyl and alpha,beta-unsaturated cyclohexenone of PROG of various lipids with different substitutions in their heads or hydrocarbon moieties might in part explain the nongenomic action of the steroid.     

Phospholipid species containing long and very long polyenoic fatty acids remain with rhodopsin after hexane extraction of photoreceptor membranes

About one-fourth the phosphatidylcholines (PCs) from bovine disk photoreceptor membranes contain very long chain (24-36 carbons) polyunsaturated (4, 5, and 6 double bonds) fatty acids of the n-3 and n-6 series (VLCPUFA). Such fatty acids, exclusively occurring in dipolyunsaturated species, are esterified to the sn-1 position of their glycerol backbone, docosahexaenoate being the major fatty acid at sn-2. Chromatographically, such PCs display a weakly polar character relative to other species, ascribable to their exceedingly large number of carbons. After hexane extraction of lyophilized disks, PC is the major component of the fraction of lipids that remains associated with rhodopsin, followed by phosphatidylserine, while a large proportion of the phosphatidylethanolamine is removed. The fatty acid composition of the hexane-removable and protein-bound lipid fractions markedly differs, the latter being enriched in lipid species containing long-chain and very long chain polyenes. This is observed for all lipid classes except free fatty acids. VLCPUFA-containing PCs are the most highly concentrated species in the rhodopsin-associated lipid fraction. The very long chain polyenes these PCs have at sn-1 may account for their resistance to being separated from the protein. It is hypothesized that their unusually long polyenoic fatty acids could be well suited to partially surround alpha-helical segments of rhodopsin.     

Domain formation in model membranes studied by pulsed-field gradient-NMR: the role of lipid polyunsaturation

The effects of increased unsaturation in the sn-2 fatty acyl chain of phosphatidylcholines (PCs) on the lipid lateral diffusion have been investigated by pulsed-field gradient NMR. Macroscopically oriented bilayers containing a monosaturated PC, egg sphingomyelin, and cholesterol (CHOL) have been studied at temperatures between 0 degrees C and 60 degrees C, and the number of double bonds in the PC was one, two, four, or six. For PC bilayers, with and without the incorporation of egg sphingomyelin and CHOL, the lateral diffusion increased with increasing number of double bonds, as a consequence of the increased headgroup area caused by the unsaturation. Addition of CHOL caused a decrease in lipid diffusion due to the condensing effect of CHOL on the headgroup area. Phase separation into large domains of liquid-disordered and liquid-ordered phases were observed in the ternary systems with PCs containing four and six double bonds, as evidenced by the occurrence of two lipid diffusion coefficients. PC bilayers with one or two double bonds appear homogeneous on the length scales probed by the experiment, but the temperature dependence of the diffusion suggests that small domains may be present also in these ternary systems.     

2H nuclear magnetic resonance order parameter profiles suggest a change of molecular shape for phosphatidylcholines containing a polyunsaturated acyl chain.

Solid-state 2H nuclear magnetic resonance spectroscopy was used to determine the orientational order parameter profiles for a series of phosphatidylcholines with perdeuterated stearic acid, 18:0d35, in position sn-1 and 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 in position sn-2. The main phase transition temperatures were derived from a first moment analysis, and order parameter profiles of sn-1 chains were calculated from dePaked nuclear magnetic resonance powder patterns. Comparison of the profiles at 37 degrees C showed that unsaturation causes an inhomogenous disordering along the sn-1 chain. Increasing sn-2 chain unsaturation from one to six double bonds resulted in a 1.6-kHz decrease in quadrupolar splittings of the sn-1 chain in the upper half of the chain (or plateau region) and maximum splitting difference of 4.4 kHz at methylene carbon 14. The change in chain order corresponds to a decrease in the 18:0 chain length of 0.4 +/- 0.2 A with 18:2 omega 6 versus 18:1 omega 9 in position sn-2. Fatty acids containing three or more double bonds in sn-2 showed a decrease in sn-1 chain length of 0.7 +/- 0.2 A compared with 18:1 omega 9. The chain length of all lipids decreased with increasing temperature. Highly unsaturated phosphatidylcholines (three or more double bonds in sn-2) had shorter sn-1 chains, but the chain length was somewhat less sensitive to temperature.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of plasma-derived lipid transfer protein with macrophages in culture

This study investigates the ability of human plasma-derived lipid transfer protein to facilitate lipid transfer to and from intact viable cells in culture. Mouse peritoneal macrophages or J774 macrophages were preincubated with acetylated low density lipoprotein and [3H]oleate/albumin to promote the intracellular synthesis and accumulation of cholesteryl [3H]oleate and 3H-labeled triglyceride. The addition of partially purified lipid transfer protein to cultures of lipid-loaded macrophages resulted in a time and concentration-dependent transfer of radiolabeled cholesteryl ester and triglyceride from macrophages to the medium. At 48 hr, lipid transfer protein facilitated the net transfer of 16 and 11% of cellular cholesteryl ester and triglyceride radioactivity, respectively, to the medium; transfer in the absence of the lipid transfer protein was less than 2%. The transfer of cholesteryl ester radioactivity was accompanied by a similar decrease in cellular cholesteryl ester mass indicating a net transfer event. Lipid transfer from cells was not dependent on the presence of a lipoprotein acceptor in the medium; however, low and high density lipoproteins present at 200 micrograms cholesterol/ml did significantly stimulate the transfer protein-facilitated efflux of these lipids. Lipid transfer protein did not appear capable of transferring radiolabeled lipid from low density or high density lipoprotein to macrophages. Radiolabeled cholesteryl ester and triglyceride transferred from cells to the medium by lipid transfer protein were associated with large molecular weight (greater than 2 x 10(6)) components in the medium with an average density greater than 1.21 g/ml; these lipids were not associated with lipid transfer protein itself. However, these radiolabeled lipids were readily incorporated into low or high density lipoproteins when these lipoproteins were added to the medium either during or after its incubation with cells. It is concluded that lipid transfer protein can facilitate the net efflux of cholesteryl esters from intact, living macrophages. These studies suggest a novel and potentially antiatherogenic role for lipid transfer protein.     

In vitro lipid transfer between lipoproteins and midgut-diverticula in the spider Polybetes pythagoricus

It has been already reported that most hemolymphatic lipids in the spider Polybetes pythagoricus are transported by HDL1 and VHDL lipoproteins. We studied in vitro the lipid transfer among midgut-diverticula (M-diverticula), and either hemolymph or purified lipoproteins as well as between hemolymphatic lipoproteins. M-diverticula and hemolymph were labeled by in vivo ¹⁴C-palmitic acid injection. In vitro incubations were performed between M-diverticula and either hemolymph or isolated lipoproteins. Hemolymph lipid uptake was associated to HDL1 (67%) and VHDL (32%). Release from hemolymph towards M-diverticula showed the opposite trend, VHDL 75% and HDL1 45%. Isolated lipoproteins showed a similar behavior to that observed with whole hemolymph. Lipid transfer between lipoproteins showed that HDL1 transfer more ¹⁴C-lipids to VHDL than vice versa. Only 38% FFA and 18% TAG were transferred from M-diverticula to lipoproteins, while on the contrary 75% and 73% of these lipids, respectively, were taken up from hemolymph. A similar trend was observed regarding lipoprotein phospholipids. This study supports the hypothesis that HDL1 and hemocyanin-containing VHDL are involved in the uptake and release of FFA, phospholipids and triacylglycerols in the spider P. pythagoricus. The data support a directional flow of lipids from HDL1 and VHDL suggesting a mode of lipid transport between lipoproteins and M-diverticula.     

Free cholesterol is a potent regulator of lipid transfer protein function

This study investigates the effect of altered lipoprotein free cholesterol (FC) content on the transfer of cholesteryl ester (CE) and triglyceride (TG) from very low- (VLDL), low- (LDL), and high-(HDL) density lipoproteins by the plasma-derived lipid transfer protein (LTP). The FC content of VLDL and HDL was selectively altered by incubating these lipoproteins with FC/phospholipid dispersions of varying composition. FC-modified lipoproteins were then equilibrated with [3H] TG, [14C]CE-labeled lipoproteins of another class to facilitate the subsequent modification of the radiolabeled donor lipoproteins. LTP was added and the extent of radiolabeled TG and CE transfer determined after 1 h. With either LDL or VLDL as lipid donor, an increase in the FC content of these lipoproteins caused a concentration-dependent inhibition (up to 50%) of CE transfer from these particles, without any significant effect on TG transfer. In contrast, with HDL as donor, increasing the HDL FC content had little effect on CE transfer from HDL, but markedly stimulated (up to 2.5-fold) the transfer of TG. This differential effect of FC on the unidirectional transfer of radiolabeled lipids from VLDL and HDL led to marked effects on LTP-facilitated net mass transfer of lipids. During long-term incubation of a constant amount of LTP with FC-modified VLDL and HDL, the extent of net mass transfer was linearly related to lipoprotein FC content; a 4-fold increase in FC content resulted in a 3-fold stimulation of the CE mass transferred to VLDL, which was coupled to an equimolar, reciprocal transfer of TG mass to HDL. Since lipid transfer between lipoproteins is integral to the process of reverse cholesterol transport, we conclude that lipoprotein FC levels are a potent, positive regulator of the pathways involved in sterol clearance. FC may modulate lipid transfer by altering the availability of CE and TG to LTP at the lipoprotein surface.     

Interfacial properties of model membranes and plasma lipoproteins containing ether lipids

The interfacial properties of synthetic ester and ether phosphatidylcholines (PCs) were investigated by using the polarity-sensitive fluorescent probes 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and pyrene. The physical state of the phospholipid matrix was determined by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH). Single-bilayer phospholipid vesicles formed by sonication and model high-density lipoproteins were studied. On the basis of a number of spectroscopic and thermodynamic criteria, the interfacial regions of PCs and their ether analogues are similar. The fluorescence properties of Prodan in model lipoproteins or single-bilayer vesicles were independent of the phospholipid fatty acyl chain length and polar head group, as well as the substitution of ether linkage for ester bonds in the phospholipid. The spectral shifts correlated mainly with the physical state of the phospholipid. The emission spectrum of Prodan appeared at shorter wavelengths upon transfer from water to liquid-crystalline phospholipid and blue shifted further when the lipid was cooled to its gel phase. The effect of cholesterol in model high-density lipoproteins on the emission spectrum of Prodan was dose dependent and, at 18 mol % cholesterol, the spectrum was similar to that observed in a pure gel-phase lipid and was independent of temperature. The quantum yield of Prodan fluorescence in an ether-PC matrix was similar to that observed in water, whereas in an ester-PC matrix it was enhanced by a factor of about 5. Phospholipid-water partition coefficients of Prodan were independent of the physical state of 1,2-dimyristoyl-sn-glycero-3-phosphocholine or 1,2-tetradecyl-sn-glycero-3-phosphocholine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Circular dichroism for the determination of amphotericin B binding to liposomes

Circular dichroism (CD) of the antifungal antibiotic amphotericin B (AmB) can be used to characterize the liposomal preparations of the drug with regard to the levels of drug bound to the lipids. The very intense dichroic doublet centered around 340 nm of free amphotericin B in water or the dichroism observed above 435 nm can be used to determine the percentages of bound AmB and free AmB in preparations containing high antibiotic/lipid ratios (ranging from 10(-2) to 10(-1] used in these carrier systems. Examples are given for AmB in the presence of small unilamellar vesicles prepared from four saturated fatty acyl chain phosphatidylcholines of different chain lengths, with or without cholesterol. The transfer of AmB from vesicles to two blood components, serum albumin, and lipoproteins can also be monitored by CD under particular conditions.     

Interaction of bovine serum high density lipoprotein with mixed vesicles of phosphatidylcholine and cholesterol.

The interaction of sonicated, small vesicles of egg phosphatidylcholine and cholesterol (2:1, mol/mol) with bovine high density serum lipoproteins was examined in terms of lipid transfer between both types of particles and the resulting changes in lipoprotein structure. Saturation of high density lipoprotein preparations with vesicle lipids gave final lipoprotein particles with essentially unchanged protein content and composition, unchanged cholesterylester and nonpolar lipid content, but with markedly increased phospholipid content (59% increas by weight) and moderately increased cholesterol content (20% increase by weight). The lipoproteins enriched in lipid were relatively uniform, spherical particles, 110 +/- 3.6 A in diameter (6 A larger than the original lipoproteins); they had a markedly decreased intrinsic protein fluorescence, a red-shifted fluorescence wavelength maximum, and more fluid lipid domains. These results indicate that the direct addition of excess lipids from membranes or other lipoproteins is a possible mechanism for lipid transfer to high density lipoproteins. Also they suggest a structural flexibility of high density lipoproteins that allows the addition of significant amounts of surface components.     

Inter-relationship of lipids transferred by the lipid-transfer protein isolated from human lipoprotein-deficient plasma

In a previous study we demonstrated that highly purified lipid-transfer protein facilitated the transfer of triglyceride, cholesteryl ester, and phosphatidylcholine between plasma lipoproteins. It remained unclear, however, whether these lipids were transferred by independent sites on the lipid-transfer protein. To address this point, we have studied the protein-mediated transfer of triglyceride, cholesteryl ester, and phosphatidylcholine as a function of the concentration and lipid composition of donor and acceptor lipoproteins. Lipoproteins labeled in vitro, reconstituted lipoproteins of defined lipid composition, and phosphatidylcholine liposomes with or without triglyceride and/or cholesteryl ester have been used to investigate the inter-relationships of lipids transferred by the lipid-transfer protein. In studies of initial (less than or equal to 10-13%) transfer, we found that, although absolute transfer rates were affected, the ratio of cholesteryl ester to triglyceride transferred was independent of donor and acceptor lipoprotein concentrations and acceptor lipoprotein lipid composition. With reconstituted lipoproteins as donor, we demonstrated that this ratio was linearly related to the ratio of cholesteryl ester to triglyceride in the donor particle; the sum of triglyceride and cholesteryl ester transferred remained constant and independent of the lipid composition of the donor. Experiments with intact lipoproteins labeled in vitro and with small unilamellar vesicles in the presence and absence of p-chloromercuriphenylsulfonate, confirmed the interdependence of triglyceride and cholesteryl ester transfer. In contrast, under all assay conditions, no correlation was found between the amount of phosphatidylcholine transferred and the transfer of triglyceride and/or cholesteryl ester. We conclude that triglyceride and cholesteryl ester compete for transfer and that the extent of transfer for each lipid is determined by its relative concentration in the donor particle, whereas phosphatidylcholine transfer is independent of triglyceride and cholesteryl ester transfer. The data also strongly support the conclusion that lipid transfer protein promotes both the exchange and net transfer of triglyceride and cholesteryl ester and that the net transfer process proceeds by a reciprocal exchange of triglyceride and cholesteryl ester without net transfer of core lipid between lipoproteins.