Structural and dynamic aspects of phosphatidylcholine in the human erythrocyte membrane

Selective cytotoxicity of dermaseptin S3 toward intraerythrocytic Plasmodium falciparum and the underlying molecular basis

The antimicrobial activity of various naturally occurring microbicidal peptides was reported to result from their interaction with microbial membrane. In this study, we investigated the cytotoxicity of the hemolytic peptide dermaseptin S4 (DS4) and the nonhemolytic peptide dermaseptin S3 (DS3) toward human erythrocytes infected by the malaria parasite Plasmodium falciparum. Both DS4 and DS3 inhibited the parasite's ability to incorporate [3H]hypoxanthine. However, while DS4 was toxic toward both the parasite and the host erythrocyte, DS3 was toxic only toward the intraerythrocytic parasite. To gain insight into the mechanism of this selective cytotoxicity, we labeled the peptides with fluorescent probes and investigated their organization in solution and in membranes. In Plasmodium-infected cells, rhodamine-labeled peptides interacted directly with the intracellular parasite, in contrast to noninfected cells, where the peptides remained bound to the erythrocyte plasma membrane. Binding experiments to phospholipid membranes revealed that DS3 and DS4 had similar binding characteristics. Membrane permeation studies indicated that the peptides were equally potent in permeating phosphatidylserine/phosphatidylcholine vesicles, whereas DS4 was more permeative with phosphatidylcholine vesicles. In aqueous solutions, DS4 was found to be in a higher aggregation state. Nevertheless, both DS3 and DS4 spontaneously dissociated to monomers upon interaction with vesicles, albeit with different kinetics. In light of these results, we propose a mechanism by which dermaseptins permeate cells and affect intraerythrocytic parasites.

Selective association of phospholipids as a clue for the passive flip-flop diffusion through bilayer lipid membranes

We showed that the investigation of the selective association of phospholipids might contribute to the insight of the flip-flop diffusion processes. The process of selective association was studied quantitatively by testing the association probabilities for both parallel and anti-parallel orientations of the polar headgroups. The model of double chain binary mixture confirms a high capacity of phospholipids for self-association in parallel configuration of the electric dipole moments whether the cross-sectional area of the polar headgroups are in an usual range of 25-55 A2. It is demonstrated that the aggregation of a class of phospholipids from a binary mixture is strongly dependent on the dipole-dipole interaction between the same phospholipids and is modulated by the magnitude of the electric dipole moment of the other phospholipids from that binary mixture. There are a great number of mechanisms involved in the transbilayer movement of phospholipids. We referred here only to the passive transport of lipids from one monolayer to the other. The flip-flop mechanisms raised in this paper are the breakdown of bilayer due to the increase of the packing density and the inversion of the coupled phospholipids from the opposite monolayers of the same bilayer. Thus, the pair formation promoting a drop in occupied volume decreases the packing pressure in the respective monolayer and consequently triggers a flip-flop into the other direction since the packing pressure in the other monolayer has not dropped. According to the present model for the binary mixtures of double-chain lipids, the rate of the flip-flop diffusion decreased by increasing the number of the methylene groups added to the acyl chain. This dependence may be perturbed whether the phospholipids possesses a very high cross-section area of the polar headgroups (a > 55 A2). We think that the selective association of phospholipids is neither exclusively, nor only involved in promoting the transbilayer diffusion of phospholipids. Most probably, the selective association determines some phospholipid domains that attract certain particular proteins so that it can modulate the protein activity.

Membrane lipid composition and cell size of Acholeplasma laidlawii strain A are strongly influenced by lipid acyl chain length

The small, cell-wall-less prokaryote Acholeplasma laidlawii strain A-EF22 could grow with membrane lipids having an average acyl chain length Cn varying over 14.5- almost 20 carbons by exogenous supplementation with selected fatty acids. For 16 < Cn < 18, the cells grew with lipids containing 100% (mol/100 mol) monounsaturated acyl chains, whereas for Cn < 16 and Cn > 18, cell growth only occurred with gradually lower fractions of unsaturated chains. Cn was actively increased and decreased by chain elongation or de novo fatty acid synthesis upon incorporation of short-chain and long-chain fatty acids, respectively. The membrane lipid composition was strongly affected by the acyl chain length and unsaturation, and the metabolic responses are readily explained as a regulation mechanism based on the established phase equilibria of the individual lipids in the A. laidlawii membrane. Monoglucosyldiacylglycerol (Glc-acyl2-Gro) was the dominating lipid with short chains but the fraction of this lipid decreased with increasing Cn, correlating with the decreasing lamellar to nonlamellar phase transition temperatures for this lipid. The fractions of diglucosyldiacylglycerol (Glc2-acyl2Gro) and phosphatidylglycerol (PtdGro), forming lamellar phases only, increased with increasing Cn over the entire chain-length interval. A weaker correlation was usually observed between the relative amount of a lipid and the extent of chain unsaturation; however, the fractions of Glc2-acyl2Gro and PtdGro increased clearly with an increasing degree of unsaturation. Moreover, the synthesis of the nonbilayer-forming lipids acyl2Gro and monoacyl-Glc-acyl2Gro was strongly stimulated by a high degree of chain saturation. Concomitantly, the phase equilibria of Glc-acyl2Gro are shifted towards lamellar phases at the growth temperature. The fraction of the three potentially nonbilayer-forming lipids varied over 10-80% (mol/100 mol) total lipids as a function of the acyl chain composition. The combined molar fractions of the three phospholipids increased strongly with chain unsaturation. However, the fraction of phosphate moieties in the different lipids was constant over the entire chain-length interval. It is concluded that the regulation of the membrane lipid composition aims at maintaining similar phase equilibria and surface charge densities of the lipid bilayer. The size of A. laidlawii cells was changed in a systematic manner and correlated qualitatively with the packing properties of the lipids. Cell diameters were increased by an increase in acyl chain length and saturation, and was affected by additives such an n-dodecane and acyl2Gro.

Indicators of long chain polyunsaturated fatty acid status of exclusively breastfed infants at delivery and after 20-22 days

The fatty acid composition of plasma cholesterol esters (CE), erythrocytes (RBC) and mature milk from seven lactating/women and their exclusively breastfed newborns, living on Dominica, were studied. Blood samples were taken from umbilical cord and mother at birth. A sample of breastmilk was collected on day 20-22 postpartum, together with a blood sample from the baby. At birth, cord blood plasma CE and RBC total long chain polyunsaturated fatty acid (LC-PUFA) contents were higher, and linoleic (18:2c, omega 6) and alpha-linolenic (18:3c, omega 3) acid contents lower, than in corresponding maternal compartments. Cord blood RBC LC-PUFA omega 3 content was lower and LC-PUFA omega 6 content higher than in maternal RBC. After birth, feeding with human milk led to a drop in LC-PUFA content in the plasma CE fraction, whereas RBC LC-PUFA content remained virtually constant. Current understanding of the origin and relative affinity of fatty acids incorporated in plasma CE and RBC suggests that RBC LC-PUFA content is a more reliable parameter for LC-PUFA status than plasma CE LC-PUFA content. The RBC LC-PUFA data suggest therefore that at birth the newborn has a lower LC-PUFA omega 3 status than the mother, and that this does not change during three weeks of exclusive breastfeeding.

Fatty acid composition of subcellular particles from sheep platelets and topological distribution of phosphatidylethanolamine fatty acids in the plasma membrane

The fatty acid composition of individual phospholipids in subcellular fractions of sheep platelets and the asymmetrical distribution of phosphatidylethanolamine (PE) fatty acyl chains across the plasma membrane were examined. The main fatty acids of total lipid extracts were oleic (18:1; 32-41%), linoleic (18:2, 10-17%), stearic (18:0; 13-15%), palmitic (16:0; 11-15%) and arachidonic (20:4; 8-12%) acids, with a saturated/unsaturated ratio of about 0.4. Each phospholipid class had a distinct fatty acid pattern. Sphingomyelin (SM) showed the highest degree of saturation (50%), with large proportions of behenic (22:0), 18:0 and 16:0 acids. The main fatty acid in PE, phosphatidylserine (PS) and phosphatidylcholine (PC) was 18:1n-9. Our findings suggest that fatty acids are asymmetrically distributed between the choline versus the non-choline phospholipids, and also between plasma membranes and intracellular membranes. The transbilayer distribution of PE fatty acids in plasma membranes from non-stimulated sheep platelets was investigated using trinitrobenzene-sulfonic acid (TNBS). A significant degree of asymmetry was found, which is a new observation in a non-polar cell. The PE molecules from the inner monolayer contained higher amounts of 18:2 and significantly less 18:1 and 20:5 than those found in the outer monolayer, although no major differences were detected in the transbilayer distribution of total unsaturated versus saturated PE acyl chains.

Effect of ethanol on phospholipid turnover and calcium mobilization in chick embryos

Effects of ethanol on [3H]inositol and [14C]choline incorporation into phosphatidylinositol (PI) and phosphatidylcholine (PC), free intrasynaptosomal Ca2+ ([Ca2+]i) and synaptosomal 45Ca2+ uptake, were investigated in the brain and heart of 17-day-old chick embryos to which a 10% ethanol solution had been injected on the 3rd day of embryogenesis. In brain synaptosomes, ethanol increased the incorporation of [3H]inositol and [14C]choline into PI and PC, increased [Ca2+]i, and decreased 45Ca2+ uptake. On the other hand, in heart synaptosomal membrane, ethanol decreased the incorporation of [3H]inositol and [14C]choline into PI and PC, decreased [Ca2+]i, and increased 45Ca2+ uptake. Ethanol stimulated in vitro [3H]inositol and [14C]choline incorporation into PI and PC in the brain and heart in both the control and ethanol-treated groups. However, addition of ethanol did not affect the release of 45Ca2+ from the synaptosomal membrane of either organ in either group. Addition of ethanol inhibited 45Ca2+ uptake in a dose-dependent manner in the brain but not in the heart. In both organs, there was a relationship between phospholipid turnover and [Ca2+]i after ethanol.

Properties of the mitochondrial membrane and carnitine palmitoyltransferase in the periportal and the perivenous zone of the liver. Effects of chronic ethanol feeding

Rats were fed for 35 days a high-fat diet containing either 36% of total calories as ethanol (ethanol group) or an isocaloric amount of carbohydrate (control group). Then, mitochondria were isolated from the periportal and the perivenous zone of the liver in order to study the acinar heterogeneity of the effects of prolonged ethanol administration upon the properties of carnitine palmitoyltransferase I (CPT-I) and its membrane environment. Chronic ethanol ingestion selectively decreased CPT-I activity in periportal hepatocytes but equally increased enzyme sensitivity to malonyl-CoA and enzyme energy of activation in the two zones of the liver. In control animals, mitochondrial membrane showed higher fluidity and lower degree of saturation of phospholipid fatty acyl moieties in periportal than in perivenous hepatocytes. Prolonged ethanol feeding (i) decreased mitochondrial membrane fluidity; (ii) increased the proportion of palmitic acid and decreased that of arachidonic acid in mitochondrial phosphatidylcholine and phosphatidylethanolamine, whereas it drastically reduced the content of linoleic acid and concomitantly increased that of saturated and monoenoic fatty acids in cardiolipin; (iii) suppressed the disordering effects of the addition of ethanol to mitochondrial suspensions. All these ethanol-induced alterations of membrane fluidity and fatty acyl composition were not significantly different between periportal and perivenous mitochondria. In conclusion, chronic ethanol feeding changes the activity of CPT-I in a zone-selective manner but modifies both the regulatory properties of the enzyme and the properties of its lipid environment in a non-zone-selective manner. Hence factors in addition to the properties of the mitochondrial membrane seem to be involved in the ethanol-induced alterations of the CPT-I enzyme.

Heat-induced alterations in monkey erythrocyte membrane phospholipid organization and skeletal protein structure and interactions

Rhesus monkey erythrocytes were subjected to heating at 50 degrees C for 5-15 min, and the heat-induced effects on the membrane structure were ascertained by analysing the membrane phospholipid organization and membrane skeleton dynamics and interactions in the heated cells. Membrane skeleton dynamics and interactions were determined by measuring the Tris-induced dissociation of the Triton-insoluble membrane skeleton (Triton shells), the spectrin-actin extractability at low ionic strength, spectrin self-association and spectrin binding to normal monkey erythrocyte membrane inside-out vesicles (IOVs). The Tris-induced Triton shell dissociation and spectrin-actin extractability were markedly decreased by the erythrocyte heating. Also, the binding of the heated erythrocyte membrane spectrin-actin with the IOVs was much smaller than that observed with the normal erythrocyte spectrin-actin. Further, the spectrin structure was extensively modified in the heated cells, as compared to the normal erythrocytes. Transbilayer phospholipid organization was ascertained by employing bee venom and pancreatic phospholipases A2, fluorescamine, and Merocyanine 540 as the external membrane probes. The amounts of aminophospholipids hydrolysed by phospholipases A2 or labeled by fluorescamine in intact erythrocytes considerably increased after subjecting them to heating at 50 degrees C for 15 min. Also, the fluorescent dye Merocyanine 540 readily stained the 15-min-heated cells but not the fresh erythrocytes. Unlike these findings, the extent of aminophospholipid hydrolysis in 5-min-heated cells by phospholipases A2 depended on the incubation time. While no change in the membrane phospholipid organization could be detected in 10 min, prolonged incubations led to the increased aminophospholipid hydrolysis. Similarly, fluorescamine failed to detect any change in the transbilayer phospholipid distribution soon after the 5 min heating, but it labeled greater amounts of aminophospholipids in the 5-min-heated cells, as compared to normal cells, after incubating them for 4 h at 37 degrees C. These results have been discussed to analyse the role of membrane skeleton in maintaining the erythrocyte membrane phospholipid asymmetry. It has been concluded that both the ATP-dependent aminophospholipid pump and membrane bilayer-skeleton interactions are required to maintain the transbilayer phospholipid asymmetry in native erythrocyte membrane.

Molecular species of glycerophospholipids and sphingomyelins of human erythrocytes: improved method of analysis

This study reports the application of modern methods of molecular species analysis in determination of the structure of both major and minor glycerophospholipids and sphingomyelins of human erythrocytes. Individual phospholipid classes were resolved from total lipid extracts by thin-layer chromatography. Diradylglycerols were released by phospholipase C and converted into trimethylsilyl ethers, which were resolved into the alkenylacyl, alkylacyl and diacylglycerol subclasses by normal phase high performance liquid chromatography. Molecular species of diradylglycerols and ceramides were quantitated according to carbon and double bond number by gas liquid chromatography using a fused silica capillary column wall-coated with bonded RTx-2330. The molecular species of ceramides were determined by GC/MS. The diradyl glycerophosphocholines contained 93.0% diacyl, 4.6% alkylacyl and 2.5% alkenylacyl, while the diradyl glycerophosphoethanolamines were made up of 48.8% diacyl, 47.8% alkenylacyl and 3.4% alkylacyl subclasses. Analysis of the molecular species showed that the long chain polyunsaturated acids were mainly combined with C16 in all diradyl GPC subclasses and in diacyl GPE, while in the alkylacyl and alkenylacyl GPE and in diacyl glycerophosphoinositol and diacyl glycerophosphoserine they were combined mainly with C18 saturated fatty chains. In addition to the C16 and C18 alkyl and alkenyl, the ether fractions also contained significant proportions of C20, C22 and C24 chains. The molecular species of the ceramide moieties of the SPH were made up largely of mono- and diunsaturated species. Over 200 molecular species were identified and quantitated in a representative sample of human red blood cells.

Molecular species of glycerophospholipids and sphingomyelins of human plasma: comparison to red blood cells

In addition to diacyl glycerophosphocholine and sphingomyelin, human plasma also contains small amounts of other glycerophospholipids, which may have special metabolic function. The structure and origin of these minor plasma lipids has not been determined. Knowledge of the detailed composition of the phospholipids of red blood cells (Myher et al., Lipids 24, 1989) permits evaluation of one of the possible sources. This study reports the detailed analyses of plasma glycerophospholipids made in parallel to those of the erythrocyte lipids obtained from the same blood using HPLC and GLC methods. The proportions of the major phospholipid classes in the plasma and erythrocytes were similar to published values, including the essential absence of diradyl glycerophosphoserine from plasma. Plasma diradyl glycerophosphocholine contained 93.0% diacyl, 3.4% alkylkacyl and 3.6% alkenylacyl, whereas the diradyl glycerophosphoethanolamine consisted of 71.8% alkenylacyl, 19.9% diacyl and 8.3% alkylacyl subclasses. The diradyl glycerophosphoinositol was 100% diacyl. The content of the minor subclasses of plasma diradyl glycerophosphocholine is similar to that of the red cells, but the ether content of the diradyl glycerophosphoethanolamine is higher in plasma than in cells. The lipid ether subclasses of plasma glycerophospholipids also contained a higher proportion of the C20, C22 and C24 alkyl and alkenyl chains than those of the cells. Furthermore, the C16 and C18-containing species in diradyl glycerophosphoethanolamine subclasses varied with the nature of the polyunsaturated acid, whereas in diradyl glycerophosphocholine subclasses the polyunsaturated acids were combined with the C16 and C18 acids in equal proportions. The significant differences in the molecular species of glycerophospholipids and sphingomyelin between plasma and red cells would appear to limit any direct transfer or equilibration of their lipid components.

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

The transbilayer distribution of choline phospholipids in trout intestinal brush-border membrane has been investigated using phospholipase C (from Clostridium welchii). In the middle intestine, 84% of phosphatidylcholine (PC) and 60% of sphingomyelin (SP) are located in the outer membrane leaflet. In the posterior intestine, 89% of PC and 52% of SP are located in the outer membrane leaflet. The externally located PC molecular species are (n - 3) fatty acid-rich in both parts of the intestine. While the sphingomyelin molecular species containing 24:1(n - 9) are exclusively located in the outer leaflet in the middle intestine, those containing 14:0 are more abundant in the same leaflet but in the posterior intestine. This strongly asymmetric distribution of both choline phospholipids may have numerous consequences on the brush-border membrane characteristics.

Abnormal membrane phospholipid organization in Plasmodium falciparum-infected human erythrocytes

The membrane phospholipid organization in Plasmodium falciparum-infected human erythrocytes was analysed by employing phospholipase A2 and Merocyanine 540 as external membrane probes. Both bee venom and pancreatic phospholipases A2 failed to hydrolyse phosphatidylserine in uninfected human red cells isolated from in vitro P. falciparum cultures. However, these enzymes under identical conditions readily degraded this aminophospholipid in P. falciparum-infected erythrocytes. Phosphatidylethanolamine hydrolysis also increased in parasitized cells. The degree to which these aminophospholipids were cleaved by the enzymes in intact infected cells depended on the developmental stage of the intracellular parasite, and was maximum at the schizont stage. This was consistent with the finding that the 'fluid-sensing' fluorescent dye, Merocyanine 540, readily labelled both the schizont and trophozoite-infected cells but not the fresh, uninfected or ring-infected erythrocytes. These results demonstrate that P. falciparum produces stage-dependent changes in the membrane phospholipid organization of its host erythrocyte.

Studies on sickled erythrocytes provide evidence that the asymmetric distribution of phosphatidylserine in the red cell membrane is maintained by both ATP-dependent translocation and interaction with membrane skeletal proteins

In order to study factors which are involved in maintenance of phosphatidylserine (PS) asymmetry within the human red cell membrane, we measured the effect of ATP-depletion and of membrane skeleton/lipid bilayer uncoupling induced by sickling on the distribution of PS within the membrane bilayer of sickle cells. Trace amounts of radiolabeled PS were introduced into the outer membrane leaflet of both fresh and ATP-depleted reversibly sickled cells (RSCs), using a non-specific lipid transfer protein purified from bovine liver. The equilibration of the newly introduced PS over the two halves of the bilayer was monitored by treatment of the cells with phospholipase A2 which selectively hydrolyzes only those molecules present in the outer membrane leaflet. Within 1 h after insertion into fresh RSCs, only 10% of the labeled PS was accessible to the action of phospholipase A2. This fraction was markedly increased when the cells were subsequently deoxygenated. Prolonged deoxygenation of RSCs, deprived of their ATP after incorporation of radiolabeled PS, caused enhanced phospholipase A2-induced hydrolysis of radiolabeled PS. Similarly, phospholipase A2-induced hydrolysis of endogenous PS in intact RSCs was markedly enhanced when ATP-depleted, but not when fresh cells, were incubated under nitrogen for 3.5 h. Deoxygenated ATP-depleted RSCs markedly enhanced the rate of thrombin formation in the presence of purified coagulation factors Xa, Va, prothrombin and Ca2+. This enhancement appeared to be dependent on the duration of incubation under nitrogen. This phenomenon, indicating the presence of increasing amounts of endogenous PS in the outer membrane leaflet, was not observed when either fresh RSCs or ATP-depleted normal erythrocytes were incubated under nitrogen. Our present observations provide evidence that, in addition to the interaction of PS with the skeletal proteins, an ATP-dependent translocation of PS is required to maintain its absolute asymmetric distribution in the human erythrocyte membrane.

Storage of human red blood cells and platelets. Some aspects concerning the factors leading to storage lesion characterized as morphological changes and vesiculation. Minireview based on a doctoral thesis

1. Storage renders erythrocytes more responsive to thermally induced morphological changes, especially the shedding of microvesicles. 4-8 week old cells can be morphologically "rejuvenated" by heating. 2. If pH increases during storage of platelets an extensive loss of small particles occurs. The platelet disintegration is associated with a loss in the metabolic activity, discharge of LDH, increased susceptibility to phospholipid hydrolysis by phospholipase C and is found to be initiated during the actual preparation of platelet concentrates. 3. Activation of platelets during preparation can be decreased by shortening the first centrifugation time or by using adenine in the anticoagulant. 4. A 4 hour prestorage of the whole blood unit prior to centrifugation strongly decreases the activation of platelets upon stimuli and results in platelet concentrates much more stable to storage.

Membrane phospholipid organization in calcium-loaded human erythrocytes

Intracellular Ca2+ levels in human erythrocytes were increased by incubating them with variable concentrations of Ca2+ in the presence of ionophore A23187. Experiments were done to confirm that the Ca2+ loading did induce changes in the cell shape and membrane protein composition. The effect of the increased cytoplasmic Ca2+ levels on the membrane phospholipid organization was analysed using bee venom and pancreatic phospholipases A2, Merocyanine 540 and fluorescamine as the external membrane probes. About 20% phosphatidylethanolamine (PE) and 0% phosphatidylserine (PS) were hydrolysed by the phospholipases in intact control cells, whereas in identical conditions these enzymes readily degraded, 20-30% PE and 7-30% PS, in Ca2+-loaded erythrocytes, depending on the cytoplasmic Ca2+ concentration. Also, Merocyanine 540 failed to stain the fresh or control erythrocytes, but it labeled the cells loaded with Ca2+. Furthermore, fluorescamine labeled approx. 20% PE in fresh or control erythrocytes while in identical conditions, significantly higher amounts of PE were modified in intact Ca2+-loaded cells. These results demonstrate that Ca2+ loading in human erythrocytes leads to loss of the transbilayer phospholipid asymmetry, and suggest that, together with spectrin, polypeptides 2.1 and 4.1 may also play an important role in maintaining the asymmetric distribution of various phospholipids across the erythrocyte membrane bilayer.

Changes in the fatty acid composition of plasma and red blood cell membrane during the first hours of life in human neonates

The objective of this work was to examine the changes in the fatty acid profiles of plasma lipid fractions and red blood cell membrane phospholipids in newborn infants during the first 6-8 h of life. Methyl esters of fatty acids from plasma free fatty acids and phospholipids and from membrane phosphatidylethanolamine, phosphatidylcholine and sphingomyelin for cord blood (n = 20) and venous blood (n = 19) were analyzed by GLC. Important changes were observed in plasma fatty acids. Palmitic and palmitoleic acid increased from birth to 6-8 h of age for both free fatty acids and phospholipids. Palmitic acid also increased in membrane phosphatidylcholine and phosphatidylethanolamine. In the former, stearic acid declined whereas oleic and docosatetraenoic acids increased. Phosphatidylethanolamine and sphingomyelin were less affected than phosphatidylcholine probably because the internal location of the two first fractions in erythrocyte membrane. Polyunsaturated fatty acids dropped slightly during the first hours of life in most lipid fractions. This may be a consequence of the interruption of placental fatty acid supply and the limited capacity of the newborn to desaturate their essential fatty acid tissue stores.

Differential effects of phosphatidylcholine and cardiolipin on carnitine palmitoyltransferase activity

Rates of carnitine palmitoyltransferase-catalyzed conversion of palmitoylcarnitine to palmitoyl-CoA are markedly decreased with the progress of this reaction presumably owing to the build up of inhibitory palmitoyl-CoA in the enzyme vicinity. High, above micellar, concentrations of palmitoylcarnitine, phosphatidylcholine liposomes and high KCl concentrations increased the activity, apparently by facilitating the removal of palmitoyl-CoA from the enzyme surface. The presence of cardiolipin was found to be inhibitory. The enzyme activity followed in the direction of palmitoylcarnitine formation with low palmitoyl-CoA concentration as substrate, was inhibited by phosphatidylcholine, but stimulated by cardiolipin. Both of these lipids markedly stimulated the enzyme activity followed by the isotope exchange procedure which requires progression of both the forward and the backward reactions. The results indicate that one of the effects of phospholipids on carnitine palmitoyltransferase activity is exerted from the ability of these substances to bind the amphipathic reactants of this enzyme, particularly long-chain acyl-CoA. The possibility that the activity of the membrane-bound carnitine palmitoyltransferase may at times be affected by changes in the concentrations and composition of the various phospholipids in the enzyme's vicinity is raised by these findings.