Transbilayer movement of various phosphatidylcholine species in intact human erythrocytes

Dynamic transbilayer lipid asymmetry

Cells have thousands of different lipids. In the plasma membrane, and in membranes of the late secretory and endocytotic pathways, these lipids are not evenly distributed over the two leaflets of the lipid bilayer. The basis for this transmembrane lipid asymmetry lies in the fact that glycerolipids are primarily synthesized on the cytosolic and sphingolipids on the noncytosolic surface of cellular membranes, that cholesterol has a higher affinity for sphingolipids than for glycerolipids. In addition, P4-ATPases, "flippases," actively translocate the aminophospholipids phosphatidylserine and phosphatidylethanolamine to the cytosolic surface. ABC transporters translocate lipids in the opposite direction but they generally act as exporters rather than "floppases." The steady state asymmetry of the lipids can be disrupted within seconds by the activation of phospholipases and scramblases. The asymmetric lipid distribution has multiple implications for physiological events at the membrane surface. Moreover, the active translocation also contributes to the generation of curvature in the budding of transport vesicles.

Natural phosphatidylcholine is actively translocated across the plasma membrane to the surface of mammalian cells

The cell surface of eukaryotic cells is enriched in choline phospholipids, whereas the aminophospholipids are concentrated at the cytosolic side of the plasma membrane by the activity of one or more P-type ATPases. Lipid translocation has been investigated mostly by using short chain lipid analogs because assays for endogenous lipids are inherently complicated. In the present paper, we optimized two independent assays for the translocation of natural phosphatidylcholine (PC) to the cell surface based on the hydrolysis of outer leaflet phosphoglycerolipids by exogenous phospholipase A2 and the exchange of outer leaflet PC by a transfer protein. We report that PC reached the cell surface in the absence of vesicular traffic by a pathway that involved translocation across the plasma membrane. In erythrocytes, PC that was labeled at the inside of the plasma membrane was translocated to the cell surface with a half-time of 30 min. This translocation was probably mediated by an ATPase, because it required ATP and was vanadate-sensitive. The inhibition of PC translocation by glibenclamide, an inhibitor of various ATP binding cassette transporters, and its reduction in erythrocytes from both Abcb1a/1b and Abcb4 knockout mice, suggest the involvement of ATP binding cassette transporters in natural PC cell surface translocation. The relative importance of the outward translocation of PC as compared with the well characterized fast inward translocation of phosphatidylserine for the overall asymmetric phospholipid organization in plasma membranes remains to be established.

Lipid rafts are necessary for tonic inhibition of cellular tissue factor procoagulant activity

A fraction of total cellular tissue factor procoagulant activity remains masked or "encrypted" in intact cells. Decryption of this activity partly involves the extracellular exposure of anionic phospholipids such as phosphatidylserine. Because of the potential association of tissue factor and phospholipid scramblase activity with lipid rafts, we have explored the role of lipid rafts in regulating factor VIIa/tissue factor activity. In HEK293 cells, tissue factor antigen was not stably associated with lipid rafts, yet disruption of rafts with methyl-beta-cyclodextrin resulted in a 3-fold stimulation of tissue factor procoagulant activity. Treatment with methyl-beta-cyclodextrin was not associated with cytotoxicity and did not result in the exposure of additional tissue factor antigen. Factor VIIa/tissue factor activity decrypted with methyl-beta-cyclodextrin was quantitatively similar to that obtained by using lytic concentrations of octyl glucoside but more sensitive to inhibition by cell surface tissue factor pathway inhibitor and the phospholipid binding protein, annexin V. Partial decryption of tissue factor was achieved with methyl-beta-cyclodextrin prior to complete disruption of lipid rafts, suggesting the role of an enzyme localized to lipid rafts in the transbilayer transport of phosphatidylserine. We conclude that lipid rafts are required for the maintenance of cellular tissue factor in an encrypted state.

Regulation of transbilayer plasma membrane phospholipid asymmetry

Lipids in biological membranes are asymmetrically distributed across the bilayer; the amine-containing phospholipids are enriched on the cytoplasmic surface of the plasma membrane, while the choline-containing and sphingolipids are enriched on the outer surface. The maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and disruption of this asymmetry is associated with cell activation or pathologic conditions. Lipid asymmetry is generated primarily by selective synthesis of lipids on one side of the membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells. It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry.

Multidrug-resistance P-glycoprotein (MDR1) secretes platelet-activating factor

The human multidrug-resistance (MDR1) P-glycoprotein (Pgp) is an ATP-binding-cassette transporter (ABCB1) that is ubiquitously expressed. Often its concentration is high in the plasma membrane of cancer cells, where it causes multidrug resistance by pumping lipophilic drugs out of the cell. In addition, MDR1 Pgp can transport analogues of membrane lipids with shortened acyl chains across the plasma membrane. We studied a role for MDR1 Pgp in transport to the cell surface of the signal-transduction molecule platelet-activating factor (PAF). PAF is the natural short-chain phospholipid 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. [(14)C]PAF synthesized intracellularly from exogenous alkylacetylglycerol and [(14)C]choline became accessible to albumin in the extracellular medium of pig kidney epithelial LLC-PK1 cells in the absence of vesicular transport. Its translocation across the apical membrane was greatly stimulated by the expression of MDR1 Pgp, and inhibited by the MDR1 inhibitors PSC833 and cyclosporin A. Basolateral translocation was not stimulated by expression of the basolateral drug transporter MRP1 (ABCC1). It was insensitive to the MRP1 inhibitor indomethacin and to depletion of GSH which is required for MRP1 activity. While efficient transport of PAF across the apical plasma membrane may be physiologically relevant in MDR1-expressing epithelia, PAF secretion in multidrug-resistant tumours may stimulate angiogenesis and thereby tumour growth.

Multidrug-resistance P-glycoprotein (MDR1) secretes platelet-activating factor

The human multidrug-resistance (MDR1) P-glycoprotein (Pgp) is an ATP-binding-cassette transporter (ABCB1) that is ubiquitously expressed. Often its concentration is high in the plasma membrane of cancer cells, where it causes multidrug resistance by pumping lipophilic drugs out of the cell. In addition, MDR1 Pgp can transport analogues of membrane lipids with shortened acyl chains across the plasma membrane. We studied a role for MDR1 Pgp in transport to the cell surface of the signal-transduction molecule platelet-activating factor (PAF). PAF is the natural short-chain phospholipid 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. [(14)C]PAF synthesized intracellularly from exogenous alkylacetylglycerol and [(14)C]choline became accessible to albumin in the extracellular medium of pig kidney epithelial LLC-PK1 cells in the absence of vesicular transport. Its translocation across the apical membrane was greatly stimulated by the expression of MDR1 Pgp, and inhibited by the MDR1 inhibitors PSC833 and cyclosporin A. Basolateral translocation was not stimulated by expression of the basolateral drug transporter MRP1 (ABCC1). It was insensitive to the MRP1 inhibitor indomethacin and to depletion of GSH which is required for MRP1 activity. While efficient transport of PAF across the apical plasma membrane may be physiologically relevant in MDR1-expressing epithelia, PAF secretion in multidrug-resistant tumours may stimulate angiogenesis and thereby tumour growth.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

Effect of a glycerol-containing hypotonic medium on erythrocyte phospholipid asymmetry and aminophospholipid transport during storage

Previous studies from our laboratory have shown that under blood bank storage conditions red blood cell (RBC) ATP and lipid content were better maintained in a glycerol-containing hypotonic experimental additive solution (EAS 25) than in the conventional storage medium Adsol. The objective of this study was to determine the mechanism of the protective effect of EAS 25, by measuring transmembrane phospholipid asymmetry and the membrane integrity of stored RBCs. Split units of packed RBCs were stored in either EAS 25 or Adsol. RBCs were analyzed after 0, 42, and 84 days and vesicles shed from stored RBCs were analyzed after 84 days of storage. Phospholipid asymmetry was measured by phospholipase A2 digestion (RBCs) and activation of the prothrombinase complex (RBCs, vesicles). RBC membrane exhibited a significantly greater (P < 0.01) amount of phosphatidylethanolamine externalized after storage in Adsol than in EAS 25 (44.3% +/- 11.7 vs. 25.3% +/- 5.7, respectively). Prothrombin converting activities in RBCs were significantly lower than in shed vesicles (P < 0.001) suggesting the presence of phosphatidylserine in the outer monolayer of vesicle, but not in RBC membranes. The rates of inwardly-directed aminophospholipid transport in RBCs decreased by 50% and glutathione levels decreased by approximately 50% in both media. RBC cholesterol and phospholipid content of stored RBCs remained significantly greater (P < 0.01) in EAS 25 than in Adsol. The results indicate that despite comparable reduction in the rate of aminophospholipid transport and reduced GSH concentrations, RBC phospholipid asymmetry was better maintained during storage in EAS 25 than in Adsol. The data suggest that glycerol in the hypotonic EAS helps preserve RBC lipid organization and membrane integrity during storage.

The influence of a fish oil-enriched diet on the phospholipid fatty acid turnover in the rabbit red cell membrane in vivo

The influence of the polyunsaturated fatty acid (PUFA) composition of the diet on the rate of fatty acid turnover of individual phospholipids in the erythrocyte membrane in vivo was studied. Following modification of the fatty acid composition of the membrane phospholipids by the use of a fish oil or a linoleic acid enriched diet, phospholipids--labelled in the unsaturated fatty acid at the 2-position of the glycerol moiety--were introduced into the membrane of freshly isolated rabbit erythrocytes. Thereafter, the labelled erythrocytes were reinjected into the bloodstream of the animal. It appears that, with the exception of 1-palmitoyl,2-linoleoyl phosphatidylcholine, all other phosphatidylcholines disappear faster from the erythrocytes of fish oil-fed rabbits than from the red cells of linoleic acid-fed rabbits. Another parameter, which possibly influences the turnover rates of PUFA containing phospholipids, can be peroxidation. An attempt was made to measure peroxidative damage of lipids in vivo by the introduction of 1-palmitoyl,2-cis-parinaroyl phosphatidylcholine (PnPC)--a probe to measure oxidative stress--into the membrane of freshly isolated erythrocytes, in the same way as is described for the radioactive phospholipids. The data demonstrate that the fluorescent signal from the PnPC decreases at a fast rate which is independent of the dietary conditions.

Phospholipid composition and organization in model beta-thalassemic erythrocytes

The membrane phospholipid organization in human red blood cells (RBC) is rigidly maintained by a complex system of enzymes. However, several elements of this system are sensitive to oxidative damage. An important component in the destruction of beta-thalassemic RBC is the generation of reactive oxygen species and the release of redox-active iron by the unpaired alpha-hemoglobin chains. Consequently, we hypothesized that the presence of this oxidative stress to the RBC membrane could lead to alterations in membrane lipid organization. Model beta thalassemic RBC, prepared by the introduction of excess alpha-globin in the cell, have previously been shown to exhibit structural and functional changes almost identical to those observed in beta-thalassemic cells. After 24 hr at 37 degrees C, the model beta thalassemic cells exhibited a significant loss of deformability, as measured by ektacytometric analysis, indicative of extensive membrane damage. However, a normal steadystate distribution of endogenous phospholipids was found, as evidenced by the accessibility of membrane phospholipids to hydrolysis by phospholipases. Similarly, the kinetics of transbilayer movement of spin-labeled phosphatidylserine (PS) and phosphatidylethanolamine (PE) in all samples was in the normal range and was not affected by the presence of excess alpha-globin chains. In contrast, a faster rate of spin-labeled phosphatidylcholine (PC) transbilayer movement was observed in these cells. While control RBC exhibited a complete loss of their initial (2 mol%) lysophosphatidylcholine (LPC) levels following 24 hr of incubation at 37 degrees C, 1.5 mol% LPC was still present in model beta-thalassemic cells, suggesting an altered phospholipid molecular species turnover, possibly as a result of an increased repair of oxidatively damaged phospholipids.

In vivo turnover of 1,2-dipalmitoylphosphatidylcholine and sphingomyelin in rabbit erythrocytes

The in vivo turnover of both 1,2-dipalmitoylphosphatidylcholine (DPPC) and sphingomyelin (SM) in rabbit erythrocytes was studied. DPPC, either 14C-labelled in the fatty acyl chain at the 2-position of the glycerol moiety or 3H-labelled in the choline's methyl group, and [N-methyl-14C]SM (bovine) were introduced into the membrane of freshly isolated rabbit erythrocytes by using phospholipid transfer proteins. Thereafter, the labelled erythrocytes were reinjected into the bloodstream of the animal. Analysis of blood samples shows that both labels disappear from the circulating cells with the same rate, resulting in a half-time value of about 6.4-6.6 days. This result demonstrates that the loss of DPPC from the cells is due to transfer of intact molecules to the plasma and that a deacylation process is of no or minor importance as mechanism of renewal of DPPC. Labelled sphingomyelin, introduced into the rabbit erythrocyte membrane in a similar way, disappears from the circulating red cell with a half-time value of 15.5 days. This accounts for a daily replacement of the total SM pool by 3.2%.

Retrograde lipid traffic in yeast: identification of two distinct pathways for internalization of fluorescent-labeled phosphatidylcholine from the plasma membrane

Digital, video-enhanced fluorescence microscopy and spectrofluorometry were used to follow the internalization into the yeast Saccharomyces cerevisiae of phosphatidylcholine molecules labeled on one acyl chain with the fluorescent probe 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD). Two pathways were found: (1) transport by endocytosis to the vacuole and (2) transport by a non-endocytic pathway to the nuclear envelope and mitochondria. The endocytic pathway was inhibited at low temperature (< 2 degrees C) and by ATP depletion. Mutations in secretory (SEC) genes that are necessary for membrane traffic through the secretory pathway (including SEC1, SEC2, SEC4, SEC6, SEC7, SEC12, SEC14, SEC17, SEC18, and SEC21) almost completely blocked endocytic uptake. In contrast, mutations in the SEC63, SEC65, or SEC11 genes, required for translocation of nascent secretory polypeptides into the ER or signal peptide processing in the ER, only slightly reduced endocytic uptake. Phospholipid endocytosis was also independent of the gene encoding the clathrin heavy chain, CHC1. The correlation of biochemical analysis with fluorescence microscopy indicated that the fluorescent phosphatidylcholine was degraded in the vacuole and that degradation was, at least in part, dependent on the vacuolar proteolytic cascade. The non-endocytic route functioned with a lower cellular energy charge (ATP levels 80% reduced) and was largely independent of the SEC genes. Non-endocytic transport of NBD-phosphatidylcholine to the nuclear envelope and mitochondria was inhibited by pretreatment of cells with the sulfhydryl reagents N-ethylmaleimide and p-chloromercuribenzenesulfonic acid, suggesting the existence of protein-mediated transmembrane transfer (flip-flop) of phosphatidylcholine across the yeast plasma membrane. These data establish a link between lipid movement during secretion and endocytosis in yeast and suggest that phospholipids may also gain access to intracellular organelles through non-endocytic, protein-mediated events.

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

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Effect of cholesterol and dipalmitoyl phosphatidylcholine enrichment on the kinetics of Na-Li exchange of human erythrocytes

The effects of cholesterol loading and depletion and of a 10% replacement of native phosphatidylcholine by dipalmitoyl phosphatidylcholine (di 16:0-PC) on kinetic properties of human red cell Na-Li exchange have been studied. Compared to control erythrocytes (cholesterol/phospholipid ratio (C/P = 0.8-0.9], Vmax of phloretin-sensitive Li uptake and of Li efflux stimulated by extracellular Na (Nao) were reduced by 15-30% in cholesterol-loaded red cells (C/P = 1.05-1.33). The apparent Km values for external Li (Lio) and for internal Li (Lii) were decreased by about one-third in these cells. Cholesterol depletion (C/P = 0.7) exerted opposite effects on the kinetics of Nao-dependent Li efflux. On augmenting C/P from 0.66 to 1.0, Vmax of Nao-dependent Li efflux was reduced by about 30%; increasing C/P above 1.0 caused no further lowering of Vmax.Li leakage rates monotonically decreased over the whole range of C/P ratios examined (0.66-1.3). This indicates that Na-Li exchange and Li leak are differentially affected by cholesterol. Incorporation of di 16:0-PC (replacement of 3% of total red cell phospholipids) caused similar kinetic alterations of Na-Li exchange as a rise in membrane cholesterol by 20-50%. Notably, selective incorporation of di 16:0-PC into the outer monolayer increased both intra- and extracellular Li binding affinities of Na-Li exchange and lowered its maximum velocity. Thus, both di 16:0-PC enrichment and cholesterol loading exerted an uncompetitive type of transport inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective internalization of choline-phospholipids in Plasmodium falciparum parasitized human erythrocytes

We have incubated control and Plasmodium falciparum parasitized human erythrocytes with lipid vesicles containing radiolabeled long-chain phosphatidylcholine and sphingomyelin, in the presence of a nonspecific lipid transfer protein. Most of the radiolabeled phospholipids were, immediately thereafter, available for extracellular phospholipases, suggesting that uptake of vesicles as such did not occur. In time, the amount of phosphatidylcholine inserted in the outer leaflet of the host cell membrane of parasitized erythrocytes decreased, indicating that phosphatidylcholine was being internalized in parasitized erythrocytes. The exclusion of sphingomyelin from the internalization process suggests that the removal of phosphatidylcholine from the outer leaflet of the erythrocyte membrane is caused by transbilayer migration, rather than by endocytosis. The extent of phosphatidylcholine internalization indicates that part of it does not remain in the inner leaflet of the host cell membrane, but is taken up by the intraerythrocytic parasite. Individual phosphatidylcholine species, containing 16:0/18:1-, 16:0/18:2- and 16:0/20:4-fatty acids, showed similar extents of internalization, after being incorporated in parasitized erythrocytes by a phosphatidylcholine specific transfer protein.

Magnetic resonance of membranes

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Transmembrane diffusion of fluorescent phospholipids in human erythrocytes

The outside-inside passage and transmembrane equilibrium distribution of several amphiphilic fluorescent phospholipids were examined in human erythrocytes. The results were compared with previous kinetic data obtained with spin-labeled phospholipids and with the equilibrium distribution of endogenous lipids in erythrocytes. When a nitro benzoxadiazole (NBD) was at the terminal position of a 6 carbon beta-chain, the outside-inside diffusion of the fluorescent phosphatidylserine (PS) analogue was slower, and the plateau lower than with long chain radioactive PS or spin-labeled PS. The corresponding phosphatidylethanolamine (PE) did not flip nor did the phosphatidylcholine (PC) analogue. With a NBD at the 12th carbon of a 18C alpha-chain, the amino-derivatives behaved more like endogenous PS and PE, i.e. they accumulated rapidly on the inner monolayer; however, the phosphatidylcholine analogue reached a plateau corresponding to 50% inside within 2 h at 37 degrees C, indicative of an abnormal rapid diffusion. In the latter case, changing the beta-chain from four to eight carbons had no influence on this rapid diffusion. We conclude that when the NBD is close to the glycerol moiety, it diminishes the affinity of the aminophospholipids for the aminophospholipid translocase. When it is close to the methyl terminal of an acyl chain, there is an acceleration of the spontaneous flip-flop. Presumably the polarity of the NBD is responsible for an unconventional orientation of the flexible acyl chain, thereby causing the transmembrane destabilization of the phospholipid. Overall these results illustrate the respective roles of spontaneous diffusion and translocase activity on transmembrane equilibrium distribution of phospholipids. They also show that NBD derivatives should be used cautiously as indicators of endogenous phospholipids.

Transmembrane movements of lipids

Membranes allow the rapid passage of unchanged lipids. Phospholipids on the other hand diffuse very slowly from one monolayer to another with a half-time of several hours. This slow spontaneous movement in a pure lipid bilayer can be selectively modulated in biological membranes by intrinsic proteins. In microsomes, and probably in bacterial membranes, non-specific phospholipid flippases allow the rapid redistribution of newly synthesized phospholipids. In eukaryotic plasma membranes, aminophospholipid translocase selectively pumps phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the outer to the inner leaflet and establishes a permanent lipid asymmetry. The discovery of an aminophospholipid translocase in chromaffin granules proves that eukaryotic organelles may also contain lipid translocators.

Replacement of molecular species of phosphatidylcholine: influence on erythrocyte Na transport

The phosphatidylcholine-specific transfer protein (PC-Tp) from bovine liver was used to replace endogeneous erythrocyte phosphatidylcholine (PC) with various amounts of five different molecular species of PC. Furosemide-sensitive (FS) Rb uptake, Na-Li exchange, and Na-K pump rates were considered in relation to the nature and extent of those replacements. Changes in fatty acid contents of PC after incorporation of different molecular species fell within a variation range (10-30%) similar to that found in large populations of healthy individuals. Di16:0-PC accelerated Na-Li exchange and FS Rb uptake by approximately 40 and 25%, respectively. Some reduction (20%) in FS Rb uptake was seen in 16:0/18:2-PC-enriched erythrocytes. Incorporation of 16:0/22:6-PC accelerated Na-Li exchange and FS Rb uptake by greater than 40 and 20%, respectively. Apart from inhibitory effects of 16:0/18:1-PC and di16:0-PC (24 and 19%, respectively) the Na-K pump rate was virtually unchanged by incorporation of different PC molecular species. Exogeneous PC molecules are exclusively inserted in the outer membrane leaflet and, particularly in the case of di16:0-PC, migrate slowly to the cytoplasmic leaflet. Prolonged incubation of cells (up to 21 h) after replacement with di16:0-PC showed that both Na-Li exchange and FS Rb uptake rates responded differently to redistribution of newly inserted molecules over both bilayer halves. Compared with cells exhibiting a selective incorporation of di16:0-PC in the outer monolayer, additional enrichment with disaturated species in the inner monolayer accelerated FS Rb uptake, whereas Na-Li exchange rate reverted to control values. It is concluded that small changes in fatty acid composition of PC induced by limited replacement of phospholipid molecular species can cause considerable changes in Na-Li exchange rate and FS Rb uptake. Differences in phospholipid molecular species composition could contribute to known interindividual variability of both Na-Li exchange and Na-K cotransport.

In vitro synthesis and transbilayer movement of phosphatidylethanolamine molecules labelled with different fatty acids in chick brain microsomes

The transbilayer fatty acid distribution of diacylglycerophosphoethanolamine and the translocation of newly synthesized phosphatidylethanolamine molecules labelled with different fatty acids has been investigated in chick brain microsomes using trinitrobenzensulfonic acid. The determination of the fatty acid composition of diacylglycerophosphoethanolamine in both the outer and the inner leaflet of the microsomal vesicles revealed a similar distribution indicating that both leaflets share the same molecular species. The in vitro incorporation of radioactive fatty acids (16:0, 18:1 and 20:4(n-6] into ethanolamine phospholipids, known to be catalyzed by the lyosphosphatidylethanolamine acyl transferase, showed that the radioactive diacylglycerophosphoethanolamine molecules appeared first in the outer leaflet and were thereafter transferred to the inner leaflet. The apparent rate of translocation of the newly synthesized ethanolamine phospholipid molecules was the highest for those labelled with 16:0 and the lowest for those labelled with 20:4(n-6). The results indicate that the active site of the acyl-CoA:lysophosphatidylethanolamine acyltransferases is located on the outer leaflet of the microsomal vesicles and that the different newly synthesized molecular species of diacylglycerophosphoethanolamine may be translocated from the outer to the inner leaflet at different rates.

Involvement of ATP-dependent aminophospholipid translocation in maintaining phospholipid asymmetry in diamide-treated human erythrocytes

Crosslinking of membrane skeletal proteins such as spectrin by oxidation of their SH-groups can be provoked by treatment of intact erythrocytes with diamide. Shortly after exposure of human erythrocytes to diamide and despite the transverse destabilization of the lipid bilayer that was observed in these cells (Franck, P.F.H., Op den Kamp, J.A.F., Roelofsen, B. and Van Deenen, L.L.M. (1986) Biochim. Biophys. Acta 857, 127-130), no abnormalities could be detected regarding the asymmetric distribution of the phospholipids when probed by either the prothrombinase assay or brief exposure of the cells to a modified phospholipase A2 with enhanced membrane penetrating capacity. This asymmetry appeared to undergo dramatic changes however, when the ATP content of the cytosol had decreased to less than 10% of its original level during prolonged incubation of the treated cells. These observations indicate that the initial maintenance of phospholipid asymmetry in diamide-treated erythrocytes can be solely ascribed to the action of the ATP-dependent aminophospholipid translocase. This view is supported by experiments involving radiolabeled phospholipids of which trace amounts had been inserted into the outer membrane leaflet of diamide-treated red cells and which still showed a preferential translocation of both aminophospholipids in favour of the inner monolayer, be it that the efficiency of the translocase was found to be impaired when compared to control cells.

Erythrocyte morphology reflects the transbilayer distribution of incorporated phospholipids

The transbilayer distribution of exogenous phospholipids incorporated into human erythrocytes is monitored through cell morphology changes and by the extraction of incorporated 14C-labeled lipids. Dilauroylphosphatidylserine (DLPS) and dilauroylphosphatidylcholine (DLPC) transfer spontaneously from sonicated unilamellar vesicles to erythrocytes, inducing a discocyte-to-echinocyte shape change within 5 min. DLPC-induced echinocytes revert slowly (t1/2 approximately 8 h) to discocytes, but DLPS-treated cells revert rapidly (10-20 min) to discocytes and then become invaginate stomatocytes. The second phase of the phosphatidylserine (PS)-induced shape change, conversion of echinocytes to stomatocytes, can be inhibited by blocking cell protein sulfhydryl groups or by depleting intracellular ATP or magnesium (Daleke, D. L., and W. H. Huestis. 1985. Biochemistry. 24:5406-5416). These cell shape changes are consistent with incorporation of phosphatidylcholine (PC) and PS into the membrane outer monolayer followed by selective and energy-dependent translocation of PS to the membrane inner monolayer. This hypothesis is explored by correlating cell shape with the fraction of the exogenous lipid accessible to extraction into phospholipid vesicles. Upon exposure to recipient vesicles, DLPC-induced echinocytes revert to discoid forms within 5 min, concomitant with the removal of most (88%) of the radiolabeled lipid. On further incubation, 97% of the foreign PC transfers to recipient vesicles. Treatment of DLPS-induced stomatocytes with acceptor vesicles extracts foreign PS only partially (22%) and does not affect cell shape significantly. Cell treated with inhibitors of aminophospholipid translocation (sulfhydryl blockers or intracellular magnesium depletion) and then incubated with either DLPS or DLPC become echinocytic and do not revert to discocytic or stomatocytic shape for many hours. On treatment with recipient vesicles, these echinocytes revert to discocytes in both cases, with concomitant extraction of 88-99% of radiolabeled PC and 86-97% of radiolabeled PS. The accessibility of exogenous lipids to extraction is uniformly consistent with the transbilayer lipid distribution inferred from cell shape changes, indicating that red cell morphology is an accurate and sensitive reporter of the transbilayer partitioning of incorporated exogenous phospholipids.

Phospholipid uptake by Plasmodium knowlesi infected erythrocytes

The uptake of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS) in Plasmodium knowlesi infected erythrocytes has been studied. Whereas uptake of phospholipids, in the absence of phospholipid transfer proteins, is negligible in control cells, the infected cells can incorporate considerable amounts of added phospholipids. The uptake is enhanced by the presence of lipid transfer proteins. Doubly labeled [3H]oleate, [14C]choline) PC does not undergo any appreciable remodelling following uptake, which strongly suggests that plasma PC is used as such for the biogenesis of the parasite membranes. Transport of extracellularly offered PS and PE towards the intraerythrocytic parasite and utilization of these lipids by the parasite are confirmed by the observation that these lipids are converted into respectively PE and PC. The extent and rate of these conversions depend on the way the phospholipids are introduced into the infected cells.

Human erythrocyte protein 4.1 is a phosphatidylserine binding protein

The aminophospholipids phosphatidylethanolamine (PE) and phosphatidylserine (PS) are the major phospholipids contained in the cytoplasmic leaflet of the human erythrocyte (RBC) plasma membrane and are largely confined to that leaflet over the entire RBC lifespan. In particular, PS, which comprises approximately 13% of total RBC membrane phospholipids, is normally restricted entirely to the cytoplasmic leaflet. However, molecular mechanisms that regulate this asymmetric distribution of phospholipids are largely unknown. We examined elliptocytic RBCs that completely lacked protein 4.1 (HE [4.1 degrees]), but contained normal amounts of all other peripheral membrane proteins, and found approximately 10% of total membrane PS was accessible in the exoplasmic leaflet of these membranes. Inside out vesicles (IOVs) derived from HE [4.1 degrees] RBCs bound fewer PS liposomes than did IOVs derived from normal RBCs. Normal IOVs that were depleted of proteins 2.1 (ankyrin), 4.1, and 4.2 bound fewer PS liposomes similar to HE [4.1 degrees] IOVs, and repletion with protein 4.1 restored PS liposome binding to control levels. Addition of purified protein 4.1 to PS liposomes resulted in saturable binding with the extent of binding being proportional to the liposome PS content. Our data suggests that human RBC protein 4.1 is a PS binding protein and may be involved in the molecular mechanisms that stabilize PS in the cytoplasmic leaflet of the human RBC plasma membrane.

An intracellular simian malarial parasite (Plasmodium knowlesi) induces stage-dependent alterations in membrane phospholipid organization of its host erythrocyte

The membrane phospholipid organization in monkey erythrocytes harbouring different developmental stages of the simian malarial parasite Plasmodium knowlesi was studied using phospholipase A2 from two different sources and Merocyanine 540 as the external-membrane probes. Experiments were done to confirm that the phospholipases did not penetrate into the infected cells or hydrolyse phospholipids during membrane isolation. The parasite-free erythrocyte membrane was isolated by differential centrifugation or by using the cationic beads Affi-Gel 731. The purity of the membranes was established by optical and electron microscopy, and by assaying the parasite-specific enzyme glutamate dehydrogenase. About 10% of the phosphatidylethanolamine and none of phosphatidylserine were hydrolysed by the phospholipases in intact normal monkey erythrocytes. However, accessibility of these aminophospholipids to the enzymes was significantly enhanced in the infected cells under identical conditions. The degree of this enhancement depended on the developmental stage of the intracellular parasite, but not on the parasitaemia levels in the infected monkeys, and increased with the parasite growth inside the cells. Analogously, Merocyanine 540 was found to label the trophozoite- or schizont-infected erythrocytes, but not the ring-infected or normal cells. These results demonstrate that the intracellular malarial parasite produces stage-dependent alterations in the membrane phospholipid organization of its host erythrocyte.

Modulation of erythrocyte vesiculation by amphiphilic drugs

Release of acetylcholinesterase-containing vesicles from human erythrocyte membranes induced by dimyristoylphosphatidylcholine (DMPC) was inhibited by exposure of red cells to cationic amphiphilic drugs like tetracaine, chlorpromazine and primaquine which all are known to induce stomatocyte formation. On the other hand, the process was facilitated when red cells were exposed to crenators like the anionic drugs indomethacin and phenylbutazone or when DMPC was added to calcium-loaded red cells. The results suggest that agents which are known to modulate red cell shape do also influence the vesiculation behavior of the cells.

Phospholipid organization in monkey erythrocytes upon Plasmodium knowlesi infection

The phospholipid organization in monkey erythrocytes upon Plasmodium knowlesi infection has been studied. Parasitized and nonparasitized erythrocytes from malaria-infected blood were separated and pure erythrocyte membranes from parasitized cells were isolated using Affi-Gel beads. In this way, the phospholipid content and composition of the membrane of nonparasitized cells, the erythrocyte membrane of parasitized cells and the parasite could be determined. The phospholipid content and composition of the erythrocyte membranes of nonparasitized and parasitized cells and erythrocytes from chloroquine-treated monkeys cured from malaria, were the same as in normal erythrocytes. The phospholipid content of the parasite increased during its development, but its composition remained unchanged. Three independent techniques, i.e., treatment of intact cells with phospholipase A2 and sphingomyelinase C, fluorescamine labeling of aminophospholipids and a phosphatidylcholine-transfer protein-mediated exchange procedure have been applied to assess the disposition of phospholipids in: erythrocytes from healthy monkeys, nonparasitized and parasitized erythrocytes from monkeys infected with Plasmodium knowlesi, and erythrocytes from monkeys that had been cured from malaria by chloroquine treatment. The results obtained by these experiments do not show any abnormality in phospholipid asymmetry in the erythrocyte from malaria-infected (splenectomized) monkeys, neither in the nonparasitized cells, nor in the parasitized cells at any stage of parasite development. Nevertheless, a considerable degree of lipid bilayer destabilization in the membrane of the parasitized cells is apparent from the enhanced exchangeability of the PC from those cells, as well as from their increased permeability towards fluorescamine.

Phospholipid asymmetry during erythropoiesis. A study on Friend erythroleukemic cells and mouse reticulocytes

The distribution of phospholipids over the outer and inner layers of the plasma membranes of differentiated Friend erythroleukemic cells (Friend cells) and mouse reticulocytes has been determined. Phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol were found to be distributed symmetrically over both layers, sphingomyelin was found to be enriched in the outer layer (80-85%) and phosphatidylserine appeared to be present mainly in the inner layer (80-90%) of the plasma membranes of differentiated Friend cells. The outer layer of reticulocyte membranes contains 50-60% of the phosphatidylcholine, 20% of the phosphatidylethanolamine, 82-85% of the sphingomyelin and 40-42% of the phosphatidylinositol. All of the phosphatidylserine is present in the inner layer. The results show, that the asymmetric distribution of phospholipids, typical for erythrocyte membranes, is partially apparent already at an early stage of erythropoiesis, the proerythroblast, while the final organization of phospholipid distribution takes place at some stage during enucleation of the enormoblast and release of the reticulocyte into the blood stream.

Flip-flop rates of individual molecular species of phosphatidylcholine in the human red cell membrane

Trace amounts of four different, well-defined species of phosphatidyl[N-methyl-14C]choline ([14C]PC), differing in their fatty acyl constituents, were introduced exclusively into the outer membrane leaflet of the intact erythrocyte by using a PC-specific phospholipid transfer protein. The rate of transbilayer equilibration of these probe molecules was calculated from the time-dependent decay in specific radioactivity of the PC pool in the outer monolayer, which was discriminated from that in the inner leaflet by treating the intact cells with phospholipase A2 in the presence of sphingomyelinase C. At 37 degrees C, 1,2-dipalmitoyl-, 1,2-dioleoyl-, 1-palmitoyl-2-linoleoyl- and 1-palmitoyl-2-arachidonoyl-PC revealed halftime values for the rate of their transbilayer equilibration of 26.3 +/- 4.4, 14.4 +/- 3.5, 2.9 +/- 1.7 and 9.7 +/- 1.6 h, respectively.

Effect of membrane cholesterol on dimyristoylphosphatidylcholine-induced vesiculation of human red blood cells

During incubation of intact human erythrocytes with sonicated dimyristoylphosphatidylcholine (DMPC) vesicles, the cells change their discoid morphology to form echinocytes and finally give rise to the release of membrane vesicles. In this process, the red cell membrane accumulates DMPC and loses up to 15% of its cholesterol. On the other hand, replacement of 25% of the endogenous phosphatidylcholine species by DMPC without affecting the cholesterol level of the erythrocytes can be achieved by incubation with DMPC/cholesterol (1:1, mol/mol) sonicated vesicles in the presence of the phosphatidylcholine-specific phospholipid-transfer protein from bovine liver. This replacement also gives rise to an echinocytic cell morphology, but no membrane vesiculation can be observed. However, the vesiculation process can as yet be initiated upon a subsequent decrease of the cholesterol level, by incubation of those modified cells in the presence of sonicated vesicles of pure egg phosphatidylcholine. Incubation of native erythrocytes with pure egg phosphatidylcholine vesicles, on the other hand, results in cholesterol depletion, but does neither induce the formation of echinocytes nor the release of membrane vesicles. Cellular ATP levels are not affected during these incubations. From these results, it can be concluded that a decrease in cholesterol content of the erythrocyte membrane is essential for the DMPC-induced vesiculation of those cells.

Lipid molecular shape affects erythrocyte morphology: a study involving replacement of native phosphatidylcholine with different species followed by treatment of cells with sphingomyelinase C or phospholipase A2

In a previous report it was shown that the replacement of native erythrocyte phosphatidylcholine (PC) with different PC species which have defined acyl chain compositions can lead to morphological changes (Kuypers, F.A., W. Berendsen, B. Roelofsen, J. A. F. Op den Kamp, and L.L.M. van Deenen, 1984, J. Cell Biol., 99:2260-2267). It was proposed that differences in molecular shape between the introduced PC species and normal erythrocyte PC caused the membrane to bend outwards or inwards, depending on the shape of the PC exchanged. To support this proposal, two requirements would have to be fulfilled: the exchange reaction would take place only with the outer lipid monolayer of the erythrocyte, and the extent of lipid transbilayer movement would be restricted. If this theory is correct, any treatment causing unilateral changes in lipid molecular shape should lead to predictable morphological changes. Since this hypothesis is a refinement of the coupled bilayer hypothesis, but so far lacks experimental support, we have sought other means to change lipid molecular shape unilaterally. Shape changes of human erythrocytes were induced by the replacement of native PC by various PC species using a phosphatidylcholine-specific transfer protein: by hydrolysis of phospholipids in intact cells using sphingomyelinase C or phospholipase A2, and by the combination of both procedures. The morphological changes were predictable; additive when both treatments were applied, and explicable on the basis of the geometry of the lipid molecules involved. The results strongly support the notion that lipid molecular shape affects erythrocyte morphology.

Incorporation and translocation of aminophospholipids in human erythrocytes

Cell morphology changes are used to examine the interaction of exogenous phosphatidylserine and phosphatidylethanolamine with human erythrocytes. Short-chain saturated lipids transfer from liposomes to cells, inducing shape changes that are indicative of their incorporation into, and in some cases translocation across, the cell membrane bilayer. Dioleoylphosphatidylserine and low concentrations of dilauroyl- and dimyristoylphosphatidylserine induce stomatocytosis. At higher concentrations, dilauroylphosphatidylserine and dimyristoylphosphatidylserine induce a biphasic shape change: the cells crenate initially but rapidly revert to a discocytic and eventually stomatocytic shape. The extent of these shape changes is dose dependent and increases with increasing hydrophilicity of the phospholipid. Cells treated with dilauroylphosphatidylethanolamine and bovine brain lysophosphatidylserine exhibit a similar biphasic shape change but revert to discocytes rather than stomatocytes. These shape changes are not a result of vesicle--cell fusion nor can they be accounted for by cholesterol depletion. The reversion from crenated to stomatocytic forms is dependent on intracellular ATP and Mg2+ concentrations and the state of protein sulfhydryl groups. The present results are consistent with the existence of a Mg2+- and ATP-dependent protein in erythrocytes that selectively translocates aminophospholipids to the membrane inner monolayer engendering aminophospholipid asymmetry.

Abnormal transbilayer mobility of phosphatidylcholine in hereditary pyropoikilocytosis reflects the increased heat sensitivity of the membrane skeleton

We determined whether the membrane defect in hereditary pyropoikilocytosis (HPP) is associated with thermally induced changes in the lipid bilayer, the stability of which was probed by the rate of translocation of phosphatidylcholine (PC) over the two leaflets. [14C]PC was incorporated into the outer leaflet of the lipid bilayer of the intact erythrocytes using a PC-specific phospholipid exchange protein. The transbilayer equilibration of this PC was determined by measuring the time-dependent changes in its accessibility to exogenous phospholipase A2. The rate of transbilayer equilibration of PC was increased in HPP cells at 37 degrees C when compared to normal erythrocytes (rate constants, 0.07 +/- 0.02 and 0.03 +/- 0.01 h-1, respectively). A further dramatic increase in PC transbilayer equilibration was noted in HPP cells incubated at 44 degrees C (rate constant, 0.15 +/- 0.02 h-1). A similar marked acceleration in transbilayer movement of PC was also seen in normal erythrocytes when incubated at 46 degrees C (rate constant, 0.13 +/- 0.03 h-1). Despite the enhanced transbilayer mobility of PC in HPP cells when compared to normal erythrocytes, no major alteration in the asymmetric distribution could be observed when probed with phospholipase A2. Since changes in transbilayer mobility of PC and cell morphology occur in HPP cells at lower temperature than in normal red cells, it may be concluded that the enhanced thermal sensitivity of spectrin is the major factor responsible for these changes. Our results therefore support the view that the structural integrity of the skeletal network is essential for stabilization of the lipid bilayer of the red cell membrane.

The phospholipid organisation in the membranes of McLeod and Leach phenotype erythrocytes

The phospholipid composition, the distribution of phospholipids over the two membrane layers as well as the phosphatidylcholine-specific transfer protein-mediated exchangeability of phosphatidylcholine from the membrane, has been investigated in two types of abnormal erythrocytes--the McLeod phenotype and the Leach phenotype. The acanthocytic McLeod cells appeared to have a normal phospholipid composition and distribution, but the exchangeability of phosphatidylcholine was found to be markedly enhanced. Unlike control erythrocytes, in which 75% of all of the phosphatidylcholine can be exchanged during an 8 h incubation, the McLeod cell showed a complete exchange of this phospholipid within the same time period. This obviously indicates an enhanced transbilayer mobility of phosphatidylcholine in the membrane of McLeod cells. Erythrocytes of the Leach phenotype showed an elliptocytic shape and increased osmotic fragility, but no abnormalities were observed as to the composition and organisation of the phospholipid complement of their membranes.

Phospholipid localization in the plasma membrane of Friend erythroleukemic cells and mouse erythrocytes

The distribution of phospholipids over outer and inner layers of the plasma membranes of Friend erythroleukemic cells (Friend cells) and mature mouse erythrocytes has been determined. The various techniques which have been applied to establish the phospholipid localization include the following: phospholipase A2, phospholipase C, and sphingomyelinase C treatment, fluorescamine labeling of phosphatidylethanolamine, and a phosphatidylcholine transfer protein mediated exchange procedure. The data obtained with these different techniques were found to be in good agreement with each other. Phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol were found to be distributed symmetrically over both layers of the plasma membrane of Friend cells. In contrast, sphingomyelin was found to be enriched in the outer layer of the membrane (80-85%), and phosphatidylserine appeared to be present mainly in the inner layer (80-90%). From these results, it was calculated that the outer and inner layers accounted for 46% and 54%, respectively, of the total phospholipid complement of that membrane. Analogous studies on the plasma membrane of mature mouse erythrocytes showed that the transbilayer distribution of the total phospholipid mass appeared to be the same as in the plasma membrane of the Friend cell, namely, 46% and 54% in outer and inner layers, respectively. The outer layer of this membrane contains 57% of the phosphatidylcholine, 20% of the phosphatidylethanolamine, 85% of the sphingomyelin, and 42% of the phosphatidylinositol, and none of the phosphatidylserine was present.(ABSTRACT TRUNCATED AT 250 WORDS)

Acyl selectivity in the transfer of molecular species of phosphatidylcholines from human erythrocytes

This report describes the molecular species composition of phosphatidylcholines (PC) transferred from human erythrocytes to acceptor vesicles composed of cholesterol and single PC species in the presence of PC-specific transfer protein from bovine liver. The compositions of the PC isolated from the vesicles were determined by capillary GLC as the diacylglycerol trimethylsilyl ethers. The cellular PC species appearing in the acceptor vesicles were enriched in unsaturated species and showed a low content of dipalmitoyl PC compared to untreated erythrocytes. This trend was independent of the composition of the PC used to construct the acceptor vesicles and it was possible to determine that the relative rates of efflux of the palmitoyl-containing phosphatidylcholines decreased in the order: palmitoyl-linoleoyl greater than palmitoyl-oleoyl greater than dipalmitoyl and in the stearoyl series, stearoyl-linoleoyl greater than stearoyl-oleoyl. No clear trend was distinguished for the influence of chain-length on the efflux, thus preventing an unambiguous assignment of the order of removal of all species from the cell membrane. Results derived for arachidonoyl-containing species were compromised by evidence for oxidation occurring during incubations at 37 degrees C. To confirm that acyl selectivity was also possible during transfer in the absence of the transfer protein, the efflux of 14C-labeled soya PC and [14C]dipalmitoyl PC from prelabeled erythrocytes was measured using plasma as the acceptor. As predicted by the chromatographic analyses, 14C-labeled soya PC effused up to 10-times faster than [14C]dipalmitoyl PC from the red cell membrane. Thus, the more rapid transfer of unsaturated PC cannot be explained entirely as a specificity of the transfer protein and is consistent with the hypothesis that intermolecular interactions involving PC molecules within the erythrocyte membrane, become weaker with increasing unsaturation. The results suggest a potential role of PC-specific transfer protein as a probe of the nature of PC interactions within biological membranes.

Uncoupling of the membrane skeleton from the lipid bilayer. The cause of accelerated phospholipid flip-flop leading to an enhanced procoagulant activity of sickled cells

We have previously reported that the normal membrane phospholipid organization is altered in sickled erythrocytes. More recently, we presented evidence of enhanced transbilayer movement of phosphatidylcholine (PC) in deoxygenated reversibly sickled cells (RSC) and put forward the hypothesis that these abnormalities in phospholipid organization are confined to the characteristic protrusions of these cells. To test this hypothesis, we studied the free spicules released from RSC by repeated sickling and unsickling as well as the remnant despiculated cells. The rate of transbilayer movement of PC in the membrane of deoxygenated remnant despiculated cells was determined by following the fate of 14C-labelled PC, previously introduced into the outer monolayer under fully oxygenated conditions using a PC-specific phospholipid exchange protein from beef liver. The rate of transbilayer movement of PC in the remnant despiculated cells was significantly slower than in deoxygenated native RSC and was not very much different from that in oxygenated native RSC or irreversibly sickled cells. The free spicules had the same lipid composition as the native cells, but were deficient in spectrin. These spicules markedly enhanced the rate of thrombin formation in the presence of purified prothrombinase (Factor Xa, Factor Va, and Ca2+) and prothrombin, indicating the exposure of a significant fraction of phosphatidylserine (PS) in the outer monolayer. This effect was not observed when the spicules in this assay were replaced by normal erythrocytes, deoxygenated native RSC, or a deoxygenated sample of RSC after repetitive sickling/unsickling. The results are interpreted to indicate that the destabilization of the lipid bilayer in sickled cells, expressed by the enhanced flip-flop of PC and the exposure of PS in the outer monolayer, occurs predominantly in those parts of the membrane that are in spicular form.

Modification of the erythrocyte membrane by a non-specific lipid transfer protein

The non-specific phospholipid transfer protein purified from bovine liver has been used to modify the phospholipid content and phospholipid composition of the membrane of intact human erythrocytes. Apart from an exchange of phosphatidylcholine between the red cell and PC-containing vesicles, the protein appeared to facilitate net transfer of phosphatidylcholine from the donor vesicles to the erythrocyte and sphingomyelin transfer in the opposite direction. Phosphatidylcholine transfer was accompanied by an equivalent transfer (on a molar basis) of cholesterol. An increase in phosphatidylcholine content in the erythrocyte membrane from 90 to 282 nmol per 100 microliters packed cells was observed. Phospholipase C treatment of modified cells showed that all of the phosphatidylcholine which was transferred to the erythrocyte was incorporated in the lipid bilayer. The nonspecific lipid transfer protein used here appeared to be a suitable tool to modify lipid content and composition of the erythrocyte membrane, and possible applications of this approach are discussed.

Shape changes in human erythrocytes induced by replacement of the native phosphatidylcholine with species containing various fatty acids

Phosphatidylcholine-specific transfer protein from beef liver has been used to replace native phosphatidylcholine (PC) molecules from intact human erythrocytes by a variety of PC species differing in fatty acid composition. These replacements changed neither the total phospholipid content of the membrane, nor the composition of this fraction in terms of the various phospholipid classes. The morphology of the erythrocyte was not modified when native PC was replaced by 1-palmitoyl,2-oleoyl PC, 1-palmitoyl,2-linoleoyl PC, egg PC, or PC isolated from rat liver microsomes. Replacement with the disaturated species 1,2-dimyristoyl PC, 1,2-dipalmitoyl PC, and 1,2-distearoyl PC resulted in the formation of echinocytes and, at higher levels of replacement, in spheroechinocytes. Echinocyte-like erythrocytes were also observed after replacement with 1-palmitoyl,2-arachidonoyl PC, whereas stomatocytes were formed upon replacement with PC species containing two unsaturated fatty acids, e.g., 1,2-dioleoyl PC and 1,2-dilinoleoyl PC. The observations show that the erythrocyte membrane structure and the overall discoid cell shape of the human erythrocyte are optimally stabilized by PC species that contain one saturated and one mono- or diunsaturated fatty acid, and that the cell tolerates only limited variations in the species composition of its PC.