Plasma membrane aminophospholipid distribution in transformed murine fibroblasts

Early ethanolamine phospholipid translocation marks stress-induced apoptotic cell death in oligodendroglial cells

The consequences of H(2)O(2)/Fe(2+)-induced oxidative stress on translocation of ethanolamine phosphoglyceride (EPG) and serine phosphoglyceride (SPG) were studied in an oligodendroglia-like cell line (OLN 93) following 3 days of supplementation with 0.1 mM docosahexaenoic acid (DHA) and a series of polar head group precursors, including N-monomethyl- and N,N-dimethylethanolamine at millimolar concentrations. Added DHA was predominantly esterified in EPG species and those cells enriched in DHA showed enhanced sensitivity to oxidative stress and eventually died by apoptosis. Co-supplements with ethanolamine and DHA resulted in a rapid, but transient, EPG translocation with a maximum at 30 min following stress, as characterized by a trinitrobenzenesulfonic acid reagent. There was no significant translocation of SPG as evidenced by annexin V binding. Unlike SPG, which is usually irreversibly translocated to subserve as a tag for phagocytosis, EPG acted as a signaling molecule with biphasic kinetic characteristics. N-Monomethyl- and N,N-dimethylethanolamine supplements reduced EPG synthesis, prevented its externalization and rescued cells from apoptotic death. Following stress, the fatty acid profile of the externalized EPG showed marked losses in polyunsaturated fatty acids and aldehydes compared with the remaining intracellular EPG. Prevention of EPG species selective translocation to the outer membrane leaflet by altering phospholipid asymmetry may be important in the mechanism of rescue from cell death.

Oxidative and drug-induced alterations in brush border membrane hemileaflet fluidity, functional consequences for glucose transport

Oxidation of biological membranes has been suggested as a major pathological process in a variety of disease states including intestinal ischemia and inflammatory bowel disease. Previous studies on the small intestinal brush border membrane have shown that part of the decrease in the activity of the Na(+)-dependent glucose transporter (SGLT1) observed after oxidation could be secondary to the derangement in membrane fluidity that accompanied oxidative damage. The present study examined the relationship between oxidative-induced hemileaflet fluidity alterations and the resultant change in Na(+)-dependent glucose transport activity. To address this issue, in vitro oxidation of guinea pig brush border membrane vesicles was induced by incubation of the vesicles with ferrous sulfate and ascorbate. We found that oxidation decreased the fluidity of both the outer and inner hemileaflets, the decrease being greater in the outer leaflet. Moreover, the preferential alteration in hemileaflet fluidity was accompanied by a decrease in glucose transport. However, when membrane perturbing agents such as hexanol and A(2)C were used to restore membrane fluidity to levels comparable to controls, rates of glucose transport could not be interpreted in terms of variation of bulk membrane fluidity or variation in fluidity of any specific membrane leaflet. On the basis of these experiments, we propose that previous studies that reported coincidental alteration in membrane fluidity and glucose transport cannot be interpreted on the basis of bulk fluidity or hemileaflet fluidity.

Transbilayer movement of fluorescent and spin-labeled phospholipids in the plasma membrane of human fibroblasts: a quantitative approach

All phospholipids in the plasma membrane of eukaryotic cells are subject to a slow passive transbilayer movement. In addition, aminophospholipids are recognized by the so-called aminophospholipid translocase, and are rapidly moved from the exoplasmic to the cytoplasmic leaflet of the plasma membrane at the expense of ATP hydrolysis. Though these principal pathways of transbilayer movement of phospholipids probably apply to all eukaryotic plasma membranes, studies of the actual kinetics of phospholipid redistribution have been largely confined to non-nucleated cells (erythrocytes). Experiments on nucleated cells are complicated by endocytosis and metabolism of the lipid probes inserted into the plasma membrane. Taking these complicating factors into account, we performed a detailed kinetic study of the transbilayer movement of short-chain fluorescent (N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl); NBD) and, for the first time, spin-labeled analogues of phosphatidylcholine (PC), -ethanolamine (PE), -serine (PS), and sphingomyelin (SM) in the plasma membrane of cultured human gingival fibroblasts. At 20 degrees C, the passive transbilayer diffusion of NBD analogues was very slow, and the choline-containing NBD analogues were internalized predominantly by endocytosis. Spin-labeled analogues of PC and SM showed higher passive transbilayer diffusion rates, and probably entered the cell by both passive transbilayer movement and endocytosis. In contrast, the rapid uptake of NBD- and spin-labeled aminophospholipid analogues could be mainly ascribed to the action of the aminophospholipid translocase, since it was inhibited by ATP depletion and N-ethylmaleimide pretreatment. The initial velocity of NBD-aminophospholipid translocation was eight to ten times slower than that of the corresponding spin-labeled lipid, and the half-times of redistribution of NBD-PS and spin-labeled PS were 7.2 and 3.6 minutes, respectively. Our data indicate that in human fibroblasts the initial velocity of aminophospholipid translocation is at least one order of magnitude higher than that in human erythrocytes, which should be sufficient to maintain the phospholipid asymmetry in the plasma membrane.

Expression of liver fatty acid binding protein alters plasma membrane lipid composition and structure in transfected L-cell fibroblasts

Liver fatty acid binding protein, L-FABP, is an abundant protein that binds fatty acids in vitro. The effects of L-FABP on plasma membrane lipid composition, distribution, and physical structure were determined in intact L-cell fibroblasts transfected with cDNA encoding L-FABP. L-FABP expression altered plasma membrane phospholipids by decreasing both phosphatidylethanolamine and esterified oleic acid content, and increasing sphingomyelin. L-FABP also binds sterols and stimulates sterol uptake and esterification. The fluorescent sterol dehydroergosterol was used to examine sterol distribution in the transfected cell plasma membrane. Dehydroergosterol codistributed equally with the cholesterol in both the bulk membrane and the individual bilayer leaflets. The sterol/phospholipid ratio was decreased in the inner leaflet due to sterol depletion. Concomitantly, intermembrane sterol transfer from the rapidly exchangeable lateral sterol domains as measured by exchange of dehydroergosterol, was reduced. The fluidity of the plasma membrane was measured with the fluorescent molecule diphenylhexatriene by multifrequency (1-250 MHz) phase and modulation fluorometry. Both the bulk plasma membrane and the plasma membrane outer leaflet lipids were fluidized in transfected cells. These alterations of plasma membrane structure and composition are consistent with a role for L-FABP in regulating intracellular sterol and fatty acid distribution and thereby membrane lipid domain structure.

Effects of snake venom phospholipase A2 toxins (beta-bungarotoxin, notexin) and enzymes (Naja naja atra, Naja nigricollis) on aminophospholipid asymmetry in rat cerebrocortical synaptosomes

The effects of snake venom phospholipase A2 (PLA2) toxins (beta-bungarotoxin, notexin) and PLA2 enzymes (Naja nigricollis, Naja naja atra) on aminophospholipid asymmetry in rat cerebrocortical synaptic plasma membranes (SPM) were examined. Incubation of intact synaptosomes with 2 mM 2,4,6-trinitrobenzene sulfonic acid (TNBS) for 40 min, under non-penetrating conditions, followed by SPM isolation, allowed us to calculate the percentage of phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the outer leaflet of the SPM, while incubation with disrupted synaptosomes provided total labeling values with the difference representing labeling of the inner leaflet. We found that 30% of the PE and 2% of the PS were in the outer leaflet, with 54% of the PE and 80% of the PS in the inner leaflet; 16% of the PE and 18% of the PS was inaccessible to TNBS. PLA2 toxins and enzymes increased in a concentration-dependent manner the percentage of PS and, to a lesser extent, the percentage of PE in the outer leaflet of the SPM, due to a redistribution from the inner to the outer leaflet. There was no correlation between the PLA2 enzymatic activities and the increased percentage of PS in the outer leaflet of the SPM induced by the PLA2 toxins and enzymes. Alteration of aminophospholipid asymmetry does not explain the greater presynaptic specificity and potencies of the PLA2 toxins as compared to the PLA2 enzymes, but may be associated with the increased acetylcholine release from synaptosomes induced by both the toxins and enzymes.

Phosphatidylcholine asymmetry in electroplax from the electric eel: use of a phosphatidylcholine exchange protein

Phosphatidylcholine asymmetry in the inner and outer leaflets of the plasma membrane bilayer of the innervated and noninnervated surfaces of the electroplax cell was determined, using a phosphatidylcholine exchange protein. The exchange protein from bovine liver catalyzed the exchange of phosphatidylcholine from small unilamellar vesicles to the outer monolayer of the plasma membrane bilayer. The exchange protein did not penetrate to the inner monolayer of the plasma membrane, did not modify the permeability of the electroplax, and did not alter the phospholipid or cholesterol content of the electroplax. In the innervated plasma membrane, 42% of the phosphatidylcholine is in the outer leaflet, 33% is in the inner leaflet, and 25% is inaccessible to the exchange protein. Corresponding values for the noninnervated plasma membrane are 56, 26, and 18%, respectively. These results are similar to phosphatidylcholine asymmetry in other biological membranes. This unique cell can be used as a model to test the effects on phospholipid asymmetry of compounds that act on the membrane.

Transmembrane distribution of sterol in the human erythrocyte

The transbilayer cholesterol distribution of human erythrocytes was examined by two independent techniques, quenching of dehydroergosterol fluorescence and fluorescence photobleaching of NBD-cholesterol. Dehydroergosterol in conjunction with leaflet selective quenching showed that, at equilibrium, 75% of the sterol was localized to the inner leaflet of resealed erythrocyte ghosts. NBD-cholesterol and fluorescence photobleaching displayed two diffusion values in both resealed ghosts and intact erythrocytes. The fractional contribution of the fast and slow diffusion constants of NBD-labelled cholesterol represent its inner and outer leaflet distribution. At room temperature the plasma membrane inner leaflet of erythrocyte ghosts as well as intact erythrocytes cells contained 78% of the plasma membrane sterol. The erythrocyte membrane transbilayer distribution of sterol was independent of temperature. In conclusion, dehydroergosterol and NBD-cholesterol data are consistent with an enrichment of cholesterol in the inner leaflet of the human erythrocyte.

Aminophospholipid molecular species asymmetry in the human erythrocyte plasma membrane

The transbilayer distribution of the molecular species of aminophospholipids in human red blood cell plasma membrane has been investigated using a covalent labelling technique. Separation and quantitative analysis of the molecular species of phosphatidylethanolamine (PE) and phosphatidylserine (PS) was performed using high-performance liquid chromatography with UV detection of the trinitrophenyl derivatives obtained after reaction with trinitrobenzenesulfonic acid (TNBS). When the molecular species distribution obtained with intact cells was compared to that of the whole membrane, a molecular species asymmetry was evident. This phenomenon was most clearly evident when the reaction was performed at low temperatures (0 degrees C) and was obscured by the excessive labelling or probe permeation associated with higher temperatures or longer incubation times. The monoene species were enriched in the outer leaflet, they comprised about 30% of the PE species in this leaflet. The polyunsaturates were preferentially localized in the inner leaflet and this was true of the arachidonyl species in particular as they represented up to 35% of this pool. The w-3 polyunsaturated fatty acids displayed a preferential localization in the plasmalogen subclass in comparison to the diacyl fraction, i.e., they comprised about 58 of the former and 42% of the latter subclass of cellular PE w-3 species. Data concerning the separation, identification and quantification of PS molecular species in human erythrocytes is also presented. The internal localization of the polyunsaturated species as well as the compartmentalization of the w-3 and w-6 pools will have metabolic, structural and physical implications for membrane function.

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.

Membrane properties of metastatic and non-metastatic cells cultured from C3H mice injected with LM fibroblasts

Little is known regarding the membrane properties of metastatic cells as compared to non-metastatic tumor cells. In order to remove variables such as site of growth and nutrition, C3H mice and LM fibroblasts were used as a model system to derive cell lines from local tumors and lung metastases. LM cells were injected subcutaneously into C3H mice and local skin tumors and secondary lung tumors were isolated, cultured in vitro and analyzed. The activities of lipid-sensitive membrane enzymes, membrane lipid composition, and membrane structure were correlated with metastatic ability. Plasma membranes and microsomes of the cultured metastatic cells had 3.8 +/- 0.5- and 5.4 +/- 0.6-fold elevated 5'-nucleotidase activity, respectively, as compared to plasma membranes and microsomes of cultured non-metastatic cells. The mitochondria of cultured metastatic cells had 3.5 +/- 0.5-fold decreased succinate-dependent cytochrome-c reductase activity as compared to mitochondria of the cultured non-metastatic cells. The lipids of plasma membranes from the metastatic cells had 30 +/- 2% and 46 +/- 7% lower phosphatidylinositol and sterol/phospholipid ratio, respectively, and 30 +/- 3% increased unsaturated/saturated fatty acid as compared to cultured non-metastatic cells. The lower sterol/phospholipid ratio correlated with a 30 +/- 1% lower level of cytosolic sterol carrier protein in the cultured metastatic cells as compared to cultured non-metastatic cells. Multifrequency phase and modulation fluorometry in conjunction with the fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, was used to determine the static and dynamic aspects of membrane fluidity. The plasma membranes and microsomes of cultured metastatic cells were more fluid than those of cultured non-metastatic cells as indicated by 24 +/- 3% and 7 +/- 1%, respectively, lower limiting anisotropy of 1,6-diphenyl-1,3,5-hexatriene in the membranes of the metastatic as compared to non-metastatic cells.

Macrophages and cancer

The uncontrolled growth of metastases resistant to conventional therapeutic modalities is a major cause of death from cancer. Data from our laboratory and others indicate that metastases arise from the nonrandom spread of specialized malignant cells that preexist within a primary neoplasm. These metastases can be clonal in their origin, and different metastases can originate from different progenitor cells. In addition, metastatic cells can exhibit an increased rate of spontaneous mutation compared with benign nonmetastatic cells. These data provide an explanation for the clinical observation that multiple metastases can exhibit different sensitivities to the same therapeutic modalities. These findings suggest that the successful therapy of disseminated metastases will have to circumvent the problems of neoplastic heterogeneity and the development of resistance. Appropriately activated macrophages can fulfill these demanding criteria. Macrophages can be activated to become tumoricidal by interaction with phospholipid vesicles (liposomes) containing immunomodulators. Tumoricidal macrophages can recognize and destroy neoplastic cells in vitro and in vivo, leaving nonneoplastic cells uninjured. Although the exact mechanism(s) by which macrophages discriminate between tumorigenic and normal cells is unknown, it is independent of tumor cell characteristics such as immunogenicity, metastatic potential, and sensitivity to cytotoxic drugs. Moreover, macrophage destruction of tumor cells apparently is not associated with the development of tumor cell resistance. Macrophages are found in association with malignant tumors in a definable pattern, suggesting that the most direct way to achieve macrophage-mediated tumor regression is in situ macrophage activation. Intravenously administered liposomes are cleared from the circulation by phagocytic cells, including macrophages, so when liposomes containing immunomodulators are endocytosed, cytotoxic macrophages are generated in situ. The administration of such liposomes in certain protocols has been shown to bring about eradication of cancer metastases. Macrophage destruction of metastases in vivo is significant, provided that the total tumor burden at the start of treatment is minimal. For this reason, we have been investigating various methods to achieve maximal cytoreduction in metastases by modalities such as chemotherapy or radiotherapy prior to macrophage-directed therapy. It is important to note that even the destruction of 99.9% of cells in a metastasis measuring 1 cm2 would leave 10(6) cells to proliferate and kill the host. The ability of tumoricidal macrophages to distinguish neoplastic from bystander nonneoplastic cells presents an attractive possibility for treatment of the few tumor cells which escape destruction by conventional treatments. Macrophage-directed therapy has been studied in several human protocols, yielding important biological information about the use of liposome-encapsulated macrophage activators in cancer patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of polyunsaturated fatty acids and lipid peroxidation in LM fibroblast plasma membrane transbilayer structure

The effects of polyunsaturated fatty acids and lipid peroxidation on LM fibroblast plasma membrane individual leaflet sterol distribution and structural order were examined. The cytofacial (inner) leaflet was more rigid and contained more sterol than the exofacial (outer) leaflet. The static (limiting anisotropy) and dynamic (rotational relaxation time) structural components of diphenylhexatriene (DPH) motion in each leaflet were determined by phase and modulation fluorometry measurements combined with leaflet-specific quenching by trinitrophenyl groups. Polyunsaturated fatty acids, incorporated into the membrane phospholipids by culture medium supplementation, decreased the limiting anisotrophy of DPH in the cytofacial but not the exofacial leaflet thereby abolishing the transbilayer difference in fluidity. Peroxidation by Fe(II) + H2O2 resulted in a rigidification (increase in limiting anisotropy and rotational relaxation time) of the plasma membrane exofacial leaflet, regardless of whether the membranes contained saturated and monounsaturated fatty acids or were enriched in either linoleate or linolenate. The structure of the cytofacial leaflet reported by DPH was unaffected. Plasma membrane transbilayer sterol distribution, measured by leaflet-specific quenching of dehydroergosterol fluorescence, indicated that 20-28% of the sterol was localized in the exofacial leaflet. Polyunsaturated fatty acid supplementation of LM fibroblasts resulted in a complete reversal of plasma membrane transbilayer sterol distribution (72-76% exofacial leaflet). Sterol transbilayer distribution between the membrane leaflets was completely resistant to alteration by exposure to crosslinking agents and peroxidation in control plasma membranes and by peroxidation in linoleate- or linolenate-supplemented membranes.

Polyunsaturated fatty acids alter sterol transbilayer domains in LM fibroblast plasma membrane

Sterols are asymmetrically distributed between the leaflets of animal cell plasma membranes. Although transbilayer migration of sterols is extremely rapid, s to min, previous experimental manipulations have not altered their transmembrane steady-state distribution. However, the effect of polyunsaturated fatty acids has not been reported. When cultured in a lipid-free, chemically defined culture medium, LM fibroblasts do not synthesize polyunsaturated fatty acids but will incorporate polyunsaturated fatty acids into their plasma membranes if supplied in the medium. Sterol transbilayer distribution in LM plasma membranes was determined from quenching of fluorescence of dehydroergosterol by trinitrophenyl groups selectively attached to the exofacial leaflet. When cells are cultured in lipid-free media, 28.1% of the plasma membrane sterol is located in the exofacial (outside) leaflet. In contrast, when cells are cultured with linoleate- or linolenate-supplemented medium, 71.8% and 75.5% of the plasma membrane sterol is exofacial, respectively.

Topological distribution of aminophospholipid fatty acids in trout intestinal brush-border membrane

The transbilayer distribution of aminophospholipids in trout intestinal brush-border membrane has been investigated using trinitrobenzene sulfonic acid (TNBS). In the middle intestine, phosphatidylethanolamine (PE) is symmetrically distributed between the two leaflets while 68% of the phosphatidylserine (PS) are located in the inner membrane leaflet. In the posterior intestine, 64% of the PE and 69% of the PS are located in the inner membrane leaflet. When asymmetrically distributed, the inner species of PE and PS have a higher content of 22:6(n-3) than the outer ones. This asymmetric distribution of docosahexaenoic acid in trout intestinal brush-border membrane might be related to the rod-like shape of the microvillus membrane and to its metabolism to hydroxylated derivatives.

Charged anaesthetics alter LM-fibroblast plasma-membrane enzymes by selective fluidization of inner or outer membrane leaflets

The functional consequences of the differences in lipid composition and structure between the two leaflets of the plasma membrane were investigated. Fluorescence of 1,6-diphenylhexa-1,3,5-triene(DPH), quenching, and differential polarized phase fluorimetry demonstrated selective fluidization by local anaesthetics of individual leaflets in isolated LM-cell plasma membranes. As measured by decreased limiting anisotropy of DPH fluorescence, cationic (prilocaine) and anionic (phenobarbital and pentobarbital) amphipaths preferentially fluidized the cytofacial and exofacial leaflets respectively. Unlike prilocaine, procaine, also a cation, fluidized both leaflets of these membranes equally. Pentobarbital stimulated 5'-nucleotidase between 0.1 and 5 mM and inhibited at higher concentrations, whereas phenobarbital only inhibited, at higher concentrations. Cationic drugs were ineffective. Two maxima of (Na+ + K+)-ATPase activation were obtained with both anionic drugs. Only one activation maximum was obtained with both cationic drugs. The maximum in activity below 1 mM for all four drugs clustered about a single limiting anisotropy value in the cytofacial leaflet, whereas there was no correlation between activity and limiting anisotropy in the exofacial leaflets. Therefore, although phenobarbital and pentobarbital below 1 mM fluidized the exofacial leaflet more than the cytofacial leaflet, the smaller fluidization in the cytofacial leaflet was functionally significant for (Na+ + K+)-ATPase. Mg2+-ATPase was stimulated at 1 mM-phenobarbital, unaffected by pentobarbital and slightly stimulated by both cationic drugs at concentrations fluidizing both leaflets. Thus the activity of (Na+ + K+)-ATPase was highly sensitive to selective fluidization of the leaflet containing its active site, whereas the other enzymes examined were little affected by fluidization of either leaflet.

Inhibition of serum complement haemolytic activity by lipid vesicles containing phosphatidylserine

The effect of artificial model membranes on the complement system was investigated. Incubation of the model membranes with human serum resulted in consumption of complement haemolytic activity when phosphatidylserine-containing vesicles were used. The activation of the complement system appeared to proceed through the alternative pathway. This conclusion was supported by the failure of [125I]Clq to bind to the membranes suggesting that the classical pathway was not involved. Although always obtained when phosphatidylserine was present in the model membranes, the activation of complement was enhanced by the contemporaneous presence of phosphatidylethanolamine. Liposomes prepared from lipid extracts of red blood cells were also able to stimulate a concentration-dependent activation of complement. Fresh, intact erythrocytes, however, could not initiate the same effects unless opsonized by antibodies. When artificially aged in vitro, red blood cells were lysed if incubated with normal human serum or with Clq-depleted serum. However, no lysis was obtained if the 'aged' erythrocytes were incubated with serum pretreated with ammonia to destroy the C3 component of complement. It is suggested that one of the mechanisms of macrophage recognition of senescent erythrocytes might be provided by the activation of the alternative pathway of complement if phosphatidylserine becomes exposed on the surface of the aging cells.

Characteristics of phospholipid methylation in human erythrocyte ghosts: relationship(s) to the psychoses and affective disorders

Recent studies have shown that patients with a schizophrenic-like illness have a significant deficit in erythrocyte ghost membrane (EGM) phosphatidylcholine (PC); patients with the most severe deficiency showed a marked decrease in Na+-Li+ counterflow activity (Hitzemann et al. 1984a and b). The present study was undertaken to see if the decrement in PC is associated with a decrease in phospholipid methylation activity. Phospholipid methylation in human EGMs is distinctly different from that in rat EGMs (Hirata and Axelrod 1980) in that the human activity is not Mg++-dependent, and apparent methyltransferase I activity is located in the external membrane surface. The patient population consisted of 20 DSM-III schizophrenics (SCZ), 13 DSM-III schizophreniform (SF) disorder patients, and 11 DSM-III manics (M). Twelve age- and sex-matched controls were used for the comparison group. Methylation activity was significantly decreased in all three patient groups, although the M group had significantly higher activity than the SF group. Twenty-four of the SCZ and SF patients entered a Li+ trial. The Li+ responder group (n = 8) showed significantly lower activity than the nonresponder group (n = 16). Overall, we conclude that the decrement in phospholipid methylation activity partially contributes to the decrement in PC levels.

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)

Role of membrane lipid asymmetry in aging

Recent advances in our understanding of the asymmetric distribution of lipids across nervous system membranes coupled with the application of biophysical techniques to examine transbilayer structure and function have led to the formulation of a new hypothesis. The author hopes that the insights presented herein will stimulate investigation into this developing new field. The theory provides an approach to correlation the accumulation of nervous tissue membrane peroxidative and cross-linking damage, the loss of transbilayer lipid asymmetry, and loss of transbilayer neuroendocrine, transport, secretory and immunoregulatory functions. Central to this scheme is the role of membrane lipid asymmetry in regulation to and/or coupling of transbilayer functions.

Fluorescence probes in metastatic B16 melanoma membranes

Fluorescence probe molecules, trans-parinaric acid and 1,6-diphenylhexatriene, were utilized to characterize the structure of plasma membranes, microsomes and mitochondria from B16 melanoma cells. High metastastic B16-F10 and low metastatic B16-F1 melanoma cell lines had markedly different membrane structures. The fluorescence polarization, fluorescence lifetime and limiting anisotropy of trans-parinaric acid were significantly lower (P less than 0.05) in all three membrane fractions of the B16-F1 cell line than in the corresponding membranes of the B16-F10 cell line. These data indicated less restriction to rotational motion in the solid lipid domains of B16-F1 cell membranes preferentially sensed by trans-parinaric acid. The limiting anisotropy of both trans-parinaric acid and 1,6-diphenyl-1,3,5-hexatriene was significantly lower in the outer monolayer than the inner monolayer of the plasma membrane of B16-F1 cells but not in B16-F10 cells. A breakpoint in Arrhenius plots of fluorescence near 30-34 degrees C indicated the presence of a phase separation that was assigned to the inner monolayer of the plasma membrane. However, no differences in this breakpoint temperature were noted between the B16-F1 and B16-F10 melanoma membranes. Thus, more fluid solid membrane domains and a distinct transbilayer fluidity difference were characteristic of plasma membranes from low metastatic B16-F1 melanoma cells in contrast to high metastatic B16-F10 melanoma cells.

Asymmetric lipid fluidity in human erythrocyte membrane: new spin-label evidence

We have synthesized spin-labeled analogues of phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine with a short beta chain (C5) bearing a doxyl group at the fourth position. When added to an erythrocyte suspension, the labels immediately incorporate in the membrane. The orientation of the spin-labels was assessed in the bilayer (i) by addition in the medium of a nonpermeant reducer (ascorbate at 5 degrees C) or (ii) by following spontaneous reduction at 37 degrees C due to the endogenous reducing agents present in the cytosol. Both techniques prove that the spin-labels are originally incorporated in the outer leaflet and redistribute differently after incubation. After a 5-h incubation at 5 degrees C, the phosphatidylcholine derivative remained in the outer layer, while the phosphatidylethanolamine and phosphatidylserine derivatives were found principally in the inner leaflet. During the incubation, a small fraction of the spin-labels is hydrolyzed, particularly the phosphatidylserine derivative, presumably by an endogenous phospholipase A2. Because the hydrolyzed spin-labeled fatty acids are rejected in the aqueous phase, the spectra of the intact membrane-bound phospholipids can be obtained by an adequate spectral subtraction. The ESR spectrum corresponding to a probe in the outer leaflet indicates a more restricted motion than that associated with probes in the inner leaflet. Additional experiments have been carried out to prove that the difference in viscosity, which is likely to be due to anisotropic cholesterol distribution, is not attributable to modification of the cell morphology.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane anomalies in Huntington's disease fibroblasts

Plasma membranes, microsomes, and mitochondria were isolated from paired, passage number matched, cultured human fibroblasts. The cells were obtained from skin biopsies of Huntington's disease (HD) subjects and from sex and age matched controls. All fibroblasts were cultured in identical media for three to seven passages. Enrichment of surface marker enzymes such as Na+,K+-ATPase indicated a 10-fold purification of the isolated plasma membrane. The specific activity of Na+,K+-ATPase was 62 and 82% greater in the crude homogenate and isolated plasma membrane, respectively, of HD fibroblasts than in control fibroblasts. The specific activity of plasma membrane Na+,K+-ATPase was correlated with lipid composition and with membrane structure as determined by measurement of the rotational relaxation time and limiting anisotropy of fluorescence probe molecules. Major alterations in the structure of the plasma membranes in HD fibroblasts were not noted. The rotational relaxation time and limiting anisotropy of 1,6-diphenyl-1,3,5-hexatriene and of trans-parinaric acid were not significantly different between the plasma membrane, microsomes, or mitochondria of HD versus those of control fibroblasts. trans-Parinaric acid demonstrated the coexistence of fluid and solid domains in all three subcellular membrane fractions of the normal and HD skin fibroblasts. Lastly, both trans-parinaric acid and 1,6-diphenyl-1,3,5-hexatriene displayed characteristic breakpoints in Arrhenius plots of absorbance corrected fluorescence in plasma membranes, microsomes, and mitochondria. In all cases, similar breakpoint temperatures, indicative of phase alterations, were noted near 20 degrees and 30 degrees C. These breakpoints were unaltered in HD. In summary, the data do not support the concept of major membrane structural defects in HD.

Sex and age alter plasma membranes of cultured fibroblasts

Human skin fibroblasts were taken from age-matched male and female subjects. The cells were then cultured under identical conditions and passage-number matched. Plasma membranes were isolated and membrane enzyme activities, lipid composition, and structure of isolated plasma membranes were measured in order to determine the presence of significant sex differences in human fibroblast membrane properties. The results indicated that plasma membranes from normal female subjects had a 1.6-fold and 3.6-fold higher cholesterol/phospholipid ratio and oleic acid (18:2) content than normal male subjects. The limiting anisotropy and the rotational relaxation time of fluorescence probe molecules such as trans-parinaric acid and 1,6-diphenyl-1,3,5-hexatriene in the plasma membranes was not significantly different from fibroblasts of male versus female normal subjects. The total activity of plasma membrane (Na+, K+)-ATPase was significantly higher in female than male normal subjects. A potential 'membrane structural disorder', Huntington's disease, was confirmed in fibroblast membranes from male but not from female Huntington's disease subjects. The possibility that Huntington's disease was a 'premature membrane aging' phenomenon was considered. A comparison of plasma membrane enzymes, lipids, and structure from old and young Huntington's disease subjects did not show differences consistent with accelerated membrane aging as explaining the molecular basis for the disease. The age-dependent differences noted in aged Huntington's disease subjects: increased phosphatidylcholine/phosphatidylethanolamine ratio and sphingomyelin + lysophosphatidylcholine content of fibroblast plasma membranes were not significantly altered when compared to normal age-matched controls. However, (Na+, K+)-ATPase activity was significantly enhanced in fibroblast plasma membranes of older Huntington's disease subjects unlike those of control subjects. In conclusion, sex and age differences in membrane properties of cultured cells represent important potential variables in the elucidation of human genetic disorders that may be membrane-related.

Age-related alterations in cultured human fibroblast membrane structure and function

Membrane enzyme activities, lipid composition, and fluorescence probe characteristics in isolated plasma membranes, microsomes and mitochondria of cultured human fibroblasts were used to determine if structural alterations occurred as a function of donor age. The cells were sex matched and allowed to undergo approximately 8 population doublings under identical culture conditions. Plasma membrane (Na+, K+)-ATPase, microsomal NADPH cytochrome c reductase, and mitochondrial succinate cytochrome c activities showed variation as a function of increasing donor age but these changes were not statistically significant. At the same time the cholesterol/phospholipid molar ratio was unaltered in plasma membranes, decreased 50% in microsomes, and unchanged in mitochondria with increasing donor age. The phosphatidylcholine/phosphatidylethanolamine ratio increased in all three membrane fractions with increasing age of the fibroblast donor. The ratio of unsaturated/saturated fatty acids decreased in the phospholipids of microsomes but not of plasma membranes or mitochondria. The structural properties of the membranes were determined with two different fluorescence probe molecules, trans-parinaric acid and 1,6-diphenyl-1,3,5-hexatriene. These probe molecules indicated that the fluorescence lifetime and/or fluorescence polarization of the trans-parinaric acid probe decreased in microsomes, mitochondria, and in the plasma membrane, such that the limiting anisotropy, indicative of restrictions to probe motions, was significantly lower (high fluidity) with increasing subject age in plasma membranes, microsomes and mitochondria. The trans-parinaric acid fluorescence lifetime displayed two components in plasma membranes, microsomes, and mitochondria, a finding consistent with the coexistence of fluid and solid membrane lipid areas in the cultured human fibroblast subcellular membranes. The trans-parinaric acid partitioned preferentially into solid membrane areas. The limiting anisotropy of 1,6-diphenyl-1,3,5-hexatriene, a fluorescent probe that partitioned almost equally into different lipid domains, was also decreased in microsomes and mitochondria with increasing donor age. In contrast, 1,6-diphenyl-1,3,5-hexatriene indicated a small increase in limiting anisotropy (0.219 vs 0.195) in plasma membranes. Arrhenius plots of trans-parinaric acid and 1,6-diphenyl-1,3,5-hexatriene absorbance-corrected fluorescence in plasma membranes, microsomes and mitochondria demonstrated characteristic breakpoints near 20 degrees C and 30 degrees C. These breakpoints were not altered as a function of age.(ABSTRACT TRUNCATED AT 400 WORDS)

Compartmentation of membrane phosphatidylethanolamine formed by base-exchange reaction in rat brain microsomes

The compartmentation of membrane phosphatidylethanolamine (PE) formed by base-exchange reaction in rat brain microsomal vesicles has been investigated. After labelling membrane PE by base-exchange in vitro, microsomal vesicles were treated with trinitrobenzenesulfonic acid (TNBS). The amount of membrane PE reacting with TNBS depends on the duration and the temperature of the reaction as well as on the TNBS concentration. It was found that almost all of the labelled PE molecules, but only about 24% of membrane PE, were accessible to TNBS, under very mild reaction conditions. It is concluded that PE labelled by base-exchange is completely localized in the cytoplasmic leaflet of microsomal vesicles.

Asymmetric distribution of phosphatidylethanolamine in the endoplasmic reticulum demonstrated using trinitrobenzenesulphonic acid as a probe

At pH 7.4 approximately one third of the phosphatidylethanolamine (PE) of rat liver microsomes is labelled by trinitrobenzenesulphonic acid (TNBS). The same fraction of the PE was labelled, when a fixed concentration of microsomes were incubated with concentrations of TNBS from 1.5 mM to 12 mM, or when the TNBS concentration was fixed at 3.0 mM and the microsomal protein varied between 1.2 and 12.0 mg. Microsomes incubated with TNBS remain closed indicated by retention of mannose-6-phosphatase latency, retention of labelled vesicular contents and by the appearance of the vesicles in the electron microscope. When the microsomal vesicles were opened by alkaline pH or after passage through the French pressure cell the % of PE labelled increased up to 90% of the total. The small % remaining unlabelled may be due to some vesicles remaining closed or to steric hindrance by the relatively bulky label on both phospholipid and protein. Phospholipase C hydrolyses approximately one third of the PE in closed microsomal vesicles. After treatment of microsomes with phospholipase C the % PE available for labelling by TNBS decreased and was inversely proportional to the % PE hydrolysed. These results suggest that the same pool of PE is available for either hydrolysis by phospholipase C or for labelling by TNBS, and that this pool is that of the outer leaflet of the microsomal membrane bilayer.

Phagosomal membrane lipids of LM fibroblasts

Murine fibroblasts, LM cells, were cultured in suspension or monolayer in a chemically defined medium without serum and exposed to polystyrene beads. The LM cells endocytized the beads in direct proportion to the bead/cell ratio and the bead surface area. However, equal volumes of beads irrespective of size or surface area were internalized. The lipid composition of the phagosome membrane differed significantly from the parent primary membrane in having higher contents of phosphatidylcholine, phosphatidylserine, and sterol but lower contents of sphingomyelin and lysophosphatidylcholine. When phagosomes isolated from suspension-cultured LM fibroblasts were exposed to trinitrobenzene-sulfonic acid at 4 degrees C, 55 +/- 1.6% of the phagosomal membrane phosphatidylethanolamine was trinitrophenylated. The asymmetric distribution of phosphatidylethanolamine across the phagosomal membrane was not affected by the bead/cell ratio, bead diameter, or exposure time of LM fibroblasts to the beads. When cells were reacted with trinitrobenzenesulfonic acid at 4 degrees C prior to phagocytosis, the amount of trinitrophenylphosphatidylethanolamine was greater in the isolated phagosomes than in the parent primary plasma membrane. Culturing LM fibroblasts in suspension or monolayer had no effect on the asymmetric distribution of phosphatidylethanolamine across primary plasma membrane bilayers. The data are consistent with the observation that LM fibroblasts grown either in suspension or monolayer internalize polystyrene beads at selective sites in the surface membrane.

LM fibroblast plasma membrane subfractionation by affinity chromatography on con A-sepharose

Affinity chromatography was used to determine the heterogeneity and orientation of plasma membrane vesicles isolated from LM fibroblasts subjected to Dounce homogenization. Two plasma membrane subfractions were obtained by Con A-Sepharose affinity chromatography of LM fibroblast plasma membranes prepared by Dounce homogenization. The desmosterol-phospholipid molar ratio, the phospholipid composition, and the phospholipid fatty acid composition were almost identical between the two fractions. However, the lipid to protein ratio was almost 2-fold greater in the nonadherent fraction A. The binding of fluorescein-concanavalin A was the same in both fractions indicating a right-sided-out orientation of the vesicles. Similarly and asymmetric distribution of phosphatidylethanolamine in both membrane fractions was the same. In contrast, sialic acid content, 5'-nucleotidase activity, and (Na+ + K+)-ATPase activity were 47%, 3.7-fold, and 2.5-fold greater, respectively, in the nonadherent, lipid-rich fraction A. Structural properties of the two membrane fractions determined by fluorescence polarization and arrhenius plots of trans-parinaric acid fluorescence were similar. These results indicate that concanavalin-A affinity chromatography separates two membrane fractions differing in sialic acid content, lipid content, and enzyme profile but having the same right-side-out orientation.

Resolution of plasma membrane lipid fluidity in intact cells labelled with diphenylhexatriene

The partitioning of fluorescence probes into intracellular organelles poses a major problem when fluorescence methods are applied to evaluate the fluidity properties of cell plasma membranes with intact cells. This work describes a method for resolution of fluidity parameters of the plasma membrane in intact cells labelled with the fluorescence polarization probe 1,6-diphenyl-1,3,5-hexatriene (DPH). The method is based on selective quenching, by nonradiative energy transfer, of the fluorescence emitted from the plasma membrane after tagging the cell with a suitable membrane impermeable electron acceptor. Such selective quenching is obtained by chemical binding of 2,4,6-trinitrobenzene sulfonate (TNBS), or by incorporation of N-bixinoyl glucosamine (BGA) to DPH-labelled cells. The procedures for determination of lipid fluidity in plasma membranes of intact cells by this method are simple and straightforward.

Asymmetric transbilayer distribution of sterol across plasma membranes determined by fluorescence quenching of dehydroergosterol

A new method for measurement of transbilayer distribution of sterol in plasma membranes is reported. The procedure utilized a fluorescent sterol, dehydroergosterol, and a chemical quenching agent, trinitrobenzenesulfonic acid. Dehydroergosterol was useful as a probe molecule for sterols for the following reasons, (a) Dehydroergosterol contained no bulky side chains as reporter groups. (b) Dehydroergosterol structurally resembled cholesterol and desmosterol, the primary sterol synthesized by LM fibroblasts. (c) Dehydroergosterol interacted with digitonin, filipin, and served as a substrate for cholesterol oxidase. (d) The phase transition of dipalmitoylglycerophosphocholine was completely abolished by dehydroergosterol. (e) The native sterol of LM fibroblasts, desmosterol, was completely replaced by dehydroergosterol without effect on LM cell growth, cell doubling time, plasma membrane (Na+, K+)-ATPase and 5'-nucleotidase activity, microsomal NADPH-dependent cytochrome c reductase activity, and mitochondrial succinate-dependent cytochrome c reductase activity. (f) Neither the phospholipid composition nor the sterol/phospholipid ratio of LM fibroblasts were altered by supplementation with dehydroergosterol. The trinitrophenyl group of trinitrophenylglycine or of surface membranes of LM fibroblasts or red blood cells treated with trinitrobenzenesulfonic acid was an excellent quencher of dehydroergosterol fluorescence. Fluorescence in mouse very-low-density lipoproteins, LM fibroblasts plasma membranes, red blood cell surface membranes, and in rat red blood cell membranes was quenched 95 +/- 3%, 20 +/- 2%, 75 +/- 4%, and 69 +/- 4% respectively when the quenching agent was present on only the extracellular site of the membrane. Trinitrophenyl residues effectively quenched the dehydroergosterol fluorescence in the plasma membrane of LM cells by 20% when dehydroergosterol was present from 1-85 mol/100 ml of the membrane sterol. When both sides of the plasma membrane were trinitrophenylated, greater than 95% of the dehydroergosterol fluorescence was quenched. In addition, when LM cells were cultured with dehydroergosterol, exposed latex beads, and the endocytosed particles isolated as phagosomes and treated with trinitrobenzenesulfonic acid under non-penetrating conditions, the fluorescence of the dehydroergosterol was quenched nearly 64%. From these and other results we deduced that the inner monlayer of the LM fibroblasts plasma membrane was enriched with dehydroergosterol. In contrast, the distribution of the sterol in red blood cell membranes indicated an enrichment in the outer monolayer.

Altered phospholipid composition affects endocytosis in cultured LM fibroblasts

The phospholipid polar head group composition of LM fibroblast membranes was altered by growing the cells in a chemically defined, serum-free medium containing choline, N,N'-dimethylethanolamine, N-monomethylethanolamine, or ethanolamine. The cells incorporated these bases into their membrane phospholipid such that 29-40% of the total plasma membrane phospholipids contained these polar head groups. Alteration of the phospholipid composition correlated with a depression of polystyrene bead phagocytosis by 36, 55 and 85% when the cells had been supplemented with N,N'-dimethylethanolamine, N-monoethylethanolamine, or ethanolamine, respectively. Pinocytotic uptake of horseradish peroxidase was depressed 44, 39, and 32%, respectively. The phagosomal membrane phospholipid composition qualitatively resembled that of the primary plasma membrane from which it was derived. However, enrichment of phosphatidylcholine, and other quantitative differences were noted in the phagosomal membranes as compared to the parent primary plasma membrane. Approx. 50% of the phagosomal membrane's phosphatidylethanolamine was accessible to the chemical labelling reagent trinitrobenzenesulfonate at 4 degrees C. The asymmetric distribution of phosphatidylethanolamine across the phagosomal membrane did not appear to be altered by base analogues except in the case of phagosomes from cells supplemented with ethanolamine. The data were consistent with a nonrandom site for endocytosis with regard to phospholipid composition.

Drug-induced surface membrane phospholipid composition in murine fibroblasts

The effects of drug on phospholipid composition of cell surface membranes are not well understood at this time. The effects of membrane-active drugs and membrane depolarization on the phospholipid composition were determined in murine LM fibroblasts. Receptor-aggregating drugs such as concanavalin A and cytoskeleton-disrupting agents such as colchicine, vinblastine, and cytochalasin B decreased phosphatidylserine content of the plasma membrane from 5.4 +/- 1.5% to as low as 1.4 +/- 0.2%. In addition, concanavalin A and colchicine increased the phosphatidylglycerol content from 6.9 +/- 1.6% to 13.1 +/- 0.7% and 10.6 +/-1.7%, respectively, while vinblastine and cytochalasin B had no effect. Pentobarbital decreased the content of phosphatidylinositol+ phosphatidylserine and of phosphatidylglycerol almost 2-fold. Propranolol, ethanol, and depolarization with 120 mM KCl had small or ne effects on plasma membrane phospholipid composition. None of the above drugs or treatments significantly altered the asymmetric distribution of phosphatidylethanolamine across the LM cell plasma membrane under the conditions tested. In addition, energy inhibitors that deplete the proton-motive force of the cell (NaN3 and KCN) and inhibitors of ATP synthesis such as NaAsO4 did not affect the asymmetric distribution of phosphatidylethanolamine. It is concluded that the mechanism of action of membrane-active drugs such as concanavalin A, vinblastine, colchicine and pentobarbital may involve alterations in plasma membrane composition. It also appears that microfilaments, microtubules, beta-adrenergic receptors, membrane fluidity, and membrane potential are not critical for the regulation of the asymmetric distribution of membrane phosphatidylethanolamine.

Penetration of 2,4,5-trinitrobenzenesulfonate into human erythrocytes. Consequences for studies on phospholipid asymmetry

The glutathione content of human erythrocytes rapidly diminishes when cells are exposed to 2,4,6-trinitrobenzenesulfonate (20 mumol/l cells) at 37 degrees C. Even at 0 degrees C a slow decrease in glutathione content is observed. The uptake of trinitrobenzenesulfonate by the cells is retarded by inhibitors of the inorganic anion exchange system, indicating that trinitrobenzenesulfonate enters the cells by this pathway. The disappearance of glutathione most probably results from the reaction: 2 GSH + trinitrobenzenesulfonate leads to GSSG + aminodinitrobenzenesulfonate. The reaction of trinitrobenzenesulfonate with glutathione occurs prior to its covalent binding to amino groups of hemoglobin which makes this reaction a more sensitive method of detection of penetration of trinitrobenzenesulfonate into erythrocytes. Results of studies on the asymmetric distribution of phospholipids using trinitrobenzenesulfonate as the only probe should be reconsidered in the light of these new data.

Membrane asymmetry. A survey and critical appraisal of the methodology. II. Methods for assessing the unequal distribution of lipids

In the companion paper, I have reviewed the techniques employed for assessment of the asymmetric distribution and orientation of membrane proteins. This article deals with methods applicable to the investigation of the unequal distribution of lipids between the two membrane leaflets. Among the techniques I will discuss are the use of immunological techniques and lectins, chemical reagents, enzymatic isotopic labeling and degradation of membrane lipids, exchange proteins and physical techniques. Whenever appropriate, problems of crypticity and non-availability of lipids to interact with the appropriate ligands, reagents, modifying enzymes or exchange proteins have been envisaged. It appears that in many case, highly discordant results, sometimes with the same biological material, have been obtained. Some of the difficulties encountered presumably stem from the reported existence of non-bilayer arrangements and isotropic movement of lipids as evidenced by freeze-fracture and NMR studies. Other problems may be related to the induction of such arrangements, especially the inverted micellar arrangement, by the modifying agents, particularly degradation enzymes or exchange proteins when they cause severe unilateral modification of the lipids of the exposed leaflet. In addition, the situation is complicated by the role of the induced increase in the flip-flop rate under different experimental conditions and by modification of the rearrangement of lipid molecules as a result of the metabolic state of the cell or ghost preparation and of the reactivity of lipids as a consequence of temperature changes. Here, more so than with proteins, one must be cautious in interpreting experimental results. Moreover, it would appear that the use of different techniques in conjunction and the consequent comparison of results should be recommended. It has been emphasized that 'general rules' do not hold and that each new material should be assay again. To give one example, it is not pertinent to state that proteins enhance the flip-flop rate in lipid vesicles (and hence in membranes). This holds true for glycophorin from erythrocyte membrane, but could not be proved when mitochondrial cytochrome oxidase was used. There seems to be no rule for the distribution of lipids between the two leaflets of different membranes. For example, even for different strains of the same bacterial species, highly divergent results have been reported. It is generally (and probably under the influence of different studies with erythrocytes) believed that in mammalian plasma membranes, choline phospholipids are enriched in the outer leaflet and aminophospholipids in the inner leaflet. Though this contention may prove to be correct, different instances of contradictory results have been given in the text. This shows that if rules do exist, they remain to be discovered or established...

Regulation of aminophospholipid asymmetry in murine fibroblast plasma membranes by choline and ethanolamine analogues

The regulation of the asymmetric distribution of aminophospholipids in mammalian cell plasma membranes is not understood at this time. One approach to determine the nature of such regulatory mechanisms is to attempt alteration of the plasma membrane phospholipid composition. Choline analogues such as N,N'-dimethylethanolamine and N-monomethylethanolamine lowered the quantity of phosphatidylethanolamine in the plasma membrane of LM fibroblasts grown in defined medium without serum. Ethanolamine supplementation increased the phosphatidylethanolamine content while ethanolamine analogues such as 2-amino-2-methyl-1-propanol, 2-amino-1-butanol, 1-aminopropanol, and 3-aminopropanol did not alter the aminophospholipid content significantly. The transverse distribution of aminophospholipids in the plasma membrane was determined by use of a chemical labelling reagent trinitrobenzenesulfonic acid. The percent phosphatidylethanolamine trinitrophenylated by trinitrobenzenesulfonate in the outer plasma membrane monolayer of LM cells supplemented with choline analogues was not altered. In contrast, ethanolamine analogue supplementation increased the percentage of aminophospholipid in the outer monolayer 2--3-fold. Ethanolamine analogue-containing phospholipids were distributed asymmetrically across the plasma membrane with 85 to 91% being located in the inner monolayer of the plasma membrane, a distribution similar to that of phosphatidylethanolamine. The fatty acyl composition of aminophospholipids in the outer monolayer was in all cases more saturated than in the corresponding phospholipids of the inner monolayer. However, choline analogues and especially the ethanolamine analogues reduced this difference. Thus, base analogues of choline and ethanolamine may alter the aminophospholipid asymmetry, the surface charge, and the acyl chain asymmetry of LM cell plasma membranes.