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PubMed1987

Depletion of arachidonic acid from GH3 cells. Effects on inositol phospholipid turnover and cellular activation.

We have adapted rat pituitary GH3 cells to grow in delipidated culture medium. In response, esterfied linoleic acid and arachidonic acid become essentially undetectable, whereas eicosa-5,8,11-trienoic acid accumulates and oleic acid increases markedly. These changes occur in all phospholipid classes, but are particularly pronounced in inositol phospholipids, where the usual stearate/arachidonate profile is replaced with oleate/eicosatrienoate (n - 9) and stearate/eicosatrienoate (n - 9). Incubation of arachidonate-depleted cells with 10 microM-arachidonic acid for only 24 h results in extensive remodelling of phospholipid fatty acids, such that close-to-normal compositions and arachidonic acid content are achieved for the inositol phospholipids. In comparison studies with arachidonic acid-depleted or -repleted cells, it was found that the arachidonate content does not affect thyrotropin-releasing-hormone (TRH)-stimulated responses measured at long time points, including [32P]Pi labelling of phosphatidylinositol and phosphatidic acid, stimulation of protein phosphorylation, and basal or TRH-stimulated prolactin release. However, transient events such as stimulated breakdown of inositol phospholipids and an initial rise in diacylglycerol are enhanced by the presence of arachidonate. These results show that arachidonic acid itself is not required for operation of the phosphatidylinositol cycle and is not an obligatory intermediate in TRH-mediated GH3 cell activation. It is possible that any structural or functional role of arachidonic acid in these processes is largely met by replacement with eicosatrienoate (n - 9). However, since arachidonate in inositol phospholipids facilitates their hydrolysis upon stimulation by TRH, arachidonic acid apparently may have a specific role in the recognition of these lipids by phospholipase C.

Animals↗
PubMed1989

Potential phospholipid source(s) of arachidonate used for the synthesis of leukotrienes by the human neutrophil.

The present study has employed two approaches to address the question of whether there are specific phospholipid sources of arachidonate used for leukotriene biosynthesis in the human neutrophil. Firstly, g.c.-m.s. analysis indicated that arachidonate was lost from all major arachidonate-containing phospholipid subclasses during cell activation with ionophore A23187. On a molar basis, the rank order of breakdown among the three major phospholipids was: 1-alk-1-enyl-2-arachidonoyl-sn-glycero-3-phosphoethanolamine greater than 1-alkyl-2-arachidonoyl-sn-3-phosphocholine greater than 1-acyl-2-arachidonyl-sn-3-phosphoinositol. Leukotrienes released into the supernatant fluid accounted for only 10-35% of the total arachidonate depletion. Phospholipid sources were also identified in labelling experiments where the specific radioactivity of arachidonate in phospholipid subclasses, as well as leukotrienes produced during cell activation, was measured. The specific radioactivity of arachidonate within 1-acyl-linked molecular species of phosphatidylcholine and phosphatidylinositol was initially high relative to the leukotrienes and decreased rapidly with stimulation. By contrast, the specific radioactivity of arachidonate in all three subclasses of phosphatidylethanolamine, 1-acyl, 1-alkyl, and 1-alk-1-enyl, was 3-5-fold below that of the leukotrienes throughout cell activation. Of the six major arachidonate-containing subclasses, only in the case of 1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine did the specific radioactivity correlate well with that of leukotriene B4 and 20-hydroxyleukotriene B4. These data strongly suggest that 1-ether-linked phospholipids are an important source of arachidonate used for leukotriene biosynthesis.

Arachidonic Acid↗
PubMed1989

Phospholipid-protein interactions of the plasma-membrane Ca2+-transporting ATPase. Evidence for a tissue-dependent functional difference.

The aim of the present work was to investigate the stimulation of the plasma-membrane Ca2+-transporting ATPase by negatively charged phospholipids. The Ca2+-transporting ATPase was purified from pig stomach smooth muscle and from pig erythrocytes, and was reactivated with phosphatidylcholine (PC) in the presence and absence of negatively charged phospholipids. The substitution of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidic acid (PA) or phosphatidylserine (PS) for PC induced profound changes in the Vmax, the K0.5 and the Hill coefficient of the Ca2+-activation curves for both ATPases. Low concentrations of each of the negatively charged phospholipids increased the Vmax., but high ratios of PIP, PIP2 or PA to PC decreased this parameter. PI, PA and PS increased the Vmax. of the erythrocyte enzyme to a larger extent than that of the smooth-muscle enzyme. This difference was less pronounced for PIP and absent for PIP2. PI (greater than 20% PC substituted), PIP, PIP2, PA and PS all increased the affinity of the two Ca2+-transporting ATPases for Ca2+ in the following order of potency: PIP2 greater than PIP greater than PI approximately PS approximately PA. PI, PA and PS increased the Ca2+ affinity of the smooth-muscle enzyme more than that of the erythrocyte enzyme; this difference was less pronounced for PIP and absent for PIP2. Even in the presence of calmodulin, all of the negatively charged phospholipids were still able to increase the Vmax. of the erythrocyte enzyme, whereas only PIP and PIP2 increased the affinity for Ca2+. The effect of PI at low concentrations (less than 20%) on the erythrocyte enzyme was peculiar in that it caused a decrease in the Ca2+ affinity instead of an increase. This effect was not observed for the smooth-muscle enzyme. All of the negatively charged phospholipids slightly increased the Hill coefficient for Ca2+ of both ATPases, and this effect was additive to that of calmodulin. The stimulation of the erythrocyte enzyme exhibited positive co-operativity towards PI and PIP, whereas that of the smooth-muscle enzyme did not. It is concluded (1) that there is a correlation between the number of negative charges on the phospholipids (PIP2 greater than PIP greater than PA approximately PI approximately PS) and the magnitude of their effect on the Vmax. and the K0.5 for Ca2+, and (2) that the action of the lipids on the smooth-muscle enzyme differs from that on the erythrocyte enzyme, indicating that these two Ca2+-transporting ATPases are not the same.

Animals↗
PubMed1990

Phospholipids as modulators of hepatic recognition of chylomicron remnants. Observations with emulsified lipoprotein lipids.

The lipids extracted from chylomicrons, chylomicron remnants generated in vivo and hepatic-lipase-treated chylomicrons were emulsified by sonication. These emulsified particles retained the capacity of the native lipoproteins to be differentiated by the liver in vivo, i.e. only the particles derived from remnant and hepatic-lipase-treated chylomicron lipids were efficiently taken up by the liver. To investigate the role of phospholipids in this differentiation process, the phospholipids of all three lipoprotein preparations were separated from the remaining lipids by silicic acid chromatography. The phospholipid-free lipid fraction of chylomicrons was then emulsified with the phospholipids derived from each of the three lipoprotein preparations. Only the particles emulsified with phospholipids derived from remnants and hepatic-lipase-treated chylomicrons were efficiently taken up by the liver in vivo. These results support the proposition that phospholipids modulate the hepatic differentiation between chylomicrons and remnants in vivo.

Animals↗
PubMed2002

Hepoxilin B3 and its enzymatically formed derivative trioxilin B3 are incorporated into phospholipids in psoriatic lesions.

In previous studies we observed that normal human epidermis forms 12-oxo-eicosatetraenoic acid (12-oxo-ETE) and hepoxilin B3 (HxB3) as major eicosanoids, both being elevated in psoriasis. We also observed that normal epidermis, in a reaction probably catalyzed by 12-lipoxygenase, only synthesize one of the two possible 10-hydroxy epimers of HxB3. We have now extended these previous studies investigating further transformation of HxB3 into trioxilin B3 (TrXB3) and esterification of both into phospholipids. Phospholipids were extracted from normal epidermis and from psoriatic scales. A combination of high performance liquid chromatography and gas chromatography-mass spectrometry analysis demonstrated the occurrence of HxB3 and TrXB3 in the phospholipids of psoriatic lesions. Alkaline- and phospholipase-A2-mediated hydrolysis of the phospholipids yielded similar quantities of both HxB3 and TrXB3 indicating their preference for the sn-2 position of glycerophospholipids. The thin layer chromatography analysis of the phospholipid classes after incubation of epidermal cells with [14C]-labeled HxB3, TrXB3, 12-hydroxy-eicosatetraenoic acid (12-HETE), 12-oxo-ETE, or 15-HETE showed that 12-HETE was the most esterified (12-HETE >15-HETE > TrXB3 > 12-oxo-ETE > HxB3). HxB3 and TrXB3 were mainly esterified in phosphatidyl-choline and phosphatidyl-ethanolamine. HxB3 was also enzymatically converted into TrXB3 in vitro. HxB3 epoxide hydrolase-like activity was not observed when boiled tissue was incubated with [14C]-HxB3, this activity being located in the cytosol fraction (100,000 x g supernatant) of fresh tissue. These findings suggest that in vivo some part of HxB3 is transformed into TrXB3 and both compounds are partially incorporated into the phospholipids.

8,11,14-Eicosatrienoic Acid↗
PubMed2002

Acidic phospholipids inhibit the DNA-binding activity of DnaA protein, the initiator of chromosomal DNA replication in Escherichia coli.

In order to initiate chromosomal DNA replication in Escherichia coli, the DnaA protein must bind to both ATP and the origin of replication (oriC). Acidic phospholipids are known to inhibit DnaA binding to ATP, and here we examine the effects of various phospholipids on DnaA binding to oriC. Among the phospholipids in E. coli membrane, cardiolipin showed the strongest inhibition of DnaA binding to oriC. Synthetic phosphatidylglycerol containing unsaturated fatty acids inhibited binding more potently than did synthetic phosphatidylglycerol containing saturated fatty acids, suggesting that membrane fluidity is important. Thus, acidic phospholipids seem to inhibit DnaA binding to both oriC and adenine nucleotides in the same manner. Adenine nucleotides bound to DnaA did not affect the inhibitory effect of cardiolipin on DnaA binding to oriC. A mobility-shift assay re-vealed that acidic phospholipids inhibited formation of a DnaA-oriC complex containing several DnaA molecules. DNase I footprinting of DnaA binding to oriC showed that two DnaA binding sites (R2 and R3) were more sensitive to cardiolipin than other DnaA binding sites. Based on these in vitro data, the physiological relevance of this inhibitory effect of acidic phospholipids on DnaA binding to oriC is discussed.

Bacterial Proteins↗
PubMed2000

Biliary excretory function is regulated by canalicular membrane fluidity associated with phospholipid fatty acyl chains in the bilayer: implications for the pathophysiology of cholestasis.

BACKGROUND AND AIMS: Bile canalicular membrane fluidity is modulated by phospholipid molecular species within membrane lipid bilayers. Thus, organellar membrane lipid composition is a determinant of canalicular function. In this study, the effect of phalloidin-induced cholestasis on bile lipid composition and liver subcellular membrane fraction composition in rats was examined to clarify the relationship between cholestasis and hepatic lipid metabolism. METHODS AND RESULTS: Each rat received one phalloidin dose (400 microg/kg, i.v.). After the bile was collected, liver microsomes and canalicular membranes were analysed. The bile flow rate decreased by 50% 3.5 h after phalloidin administration. Although the bile acid output remained almost the same, the phospholipid and cholesterol output were significantly decreased (by 40.3+/-5.97% and 76.9+/-5.56%, respectively). Thus, the cholesterol:phospholipid (C:P) ratio in bile was significantly decreased by 80.4+/-10.1%. Phalloidin administration also increased the saturated: unsaturated fatty acid ratio (S:U) in bile for phosphatidylcholine by 25.5+/-3.2%. In the canalicular membrane, the C:P and S:U ratios for phosphatidylcholine were increased (24.8+/-4.2% and 34.4+/-6.9%, respectively), while the S:U for sphingomyelin was decreased by 61.0+/-6.2%. In microsomes, the C:P was decreased by 41.0+/-6.0%, but the S:U for both phosphatidylcholine and sphingomyelin were unaffected. Canalicular membrane fluidity, assayed by 1,6-diphenyl-1,3,5-hexatriene fluorescence depolarization, decreased significantly. Therefore, increased secretion of hydrophobic phosphatidylcholine into bile was associated with more hydrophobic canalicular membrane phosphatidylcholine, while sphingomyelin in the canalicular membrane was less hydrophobic. CONCLUSIONS: These results indicate that phalloidin uncouples secretion of cholesterol and phospholipids, which causes a redistribution of fatty acyl chain species among canalicular membrane phospholipids that alters membrane fluidity. These changes may be a homeostatic response mediated by the phospholipid translocator in the canalicular membrane, although direct evidence for this is unavailable.

Analysis of Variance↗
PubMed2001

Effect of some fractions of alveolar surfactant (phospholipids and SP-A) on the bactericidal activity of different antimicrobials against some respiratory pathogens.

OBJECTIVES: To investigate the effects of physiologic concentrations, at alveolar level, of some fractions of pulmonary surfactant (phospholipids and SP-A) on the bactericidal activity of different antimicrobials against some respiratory pathogens. METHODS: The antimicrobial agents cefdinir, sparfloxacin, clarithromycin, teicoplanin, cefepime, ciprofloxacin, netilmicin and tobramycin, depending on their specific activity, were investigated against Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Klebsiella pneumoniae and Pseudomonas aeruginosa. Killing curves were carried out with antimicrobials at 0.5 and 2 MIC, SP-A at 1 and 5 mg/L and phospholipids at 50 mg/L. RESULTS: Time-kill experiments showed that while SP-A never modified the activity of antimicrobials, phospholipids exerted, in some cases, a weak antagonistic effect. Among antibacterials and pathogens investigated, phospholipids were able to decrease the rate of killing of cefepime and ciprofloxacin only on P. aeruginosa, both at 0.5 and at 2 MIC, with an increase of about 1 log in CFU. The combination of SP-A and phospholipids never modified the effect observed in the presence of lipids alone. CONCLUSIONS: The paucity of data only allow us to observe that the examined antibiotics do not have substantially reduced activity against respiratory pathogens studied in the presence of physiologic concentrations of some fractions of surfactant. Cefepime alone already exerted a small effect, and ciprofloxacin at 2 MIC, even in the presence of phospholipids, retained its bactericidal activity.

Anti-Bacterial Agents↗
PubMed2001

Circadian regulation of phospholipid metabolism in retinal photoreceptors and ganglion cells.

The neural retina is a key component of the vertebrate circadian system that is responsible for synchronizing the central circadian pacemaker to external light-dark (LD) cycles. The retina is itself rhythmic, showing circadian cycles in melatonin levels and gene expression. We assessed the in vivo incorporation of 32P-phosphate and 3H-glycerol into phospholipids of photoreceptor cells (PRCs) and retina ganglion cells (GCs) from chicks in constant illumination conditions (dark: DD or light: LL) over a 24-h period. Our findings showed that in DD there was a daily oscillation in 32P-labeling of total phospholipids synthesized in GCs and axonally transported to the brain. This metabolic fluctuation peaked during the subjective night (zeitgeber time [ZT] 20), persisted for several hours well into the subjective day and declined at subjective dusk (ZT 10-12). PRCs also exhibited an in vivo rhythm of 32P-phospholipid synthesis in DD. This rhythm peaked around ZT 22, continued a few hours into the day and declined by the end of subjective dusk. The major individual species labeled 1 h after 32P administration was phosphatidylinositol (PI) in both PRCs and GCs. Rhythmic phospholipid biosynthesis was also observed in DD after 3H-glycerol administration, with levels in GCs elevated from midday to early night. PRCs exhibited a similar rhythmic profile with the lowest levels of labeling during midnight. Phosphatidylcholine (PC) accounted for the individual species with the highest ratio of 3H-glycerol incorporation in both cell populations at all phases examined. By contrast, in LL the rhythm of 3H-glycerol labeling of phospholipids damped out in both cell layers. Our findings support the idea that, in constant darkness, the metabolism of retinal phospholipids, including their de novo biosynthesis, is regulated by an endogenous circadian clock.

Animals↗
PubMed2002

Dietary low linolenic acid compared with docosahexaenoic acid alter synaptic plasma membrane phospholipid fatty acid composition and sodium-potassium ATPase kinetics in developing rats.

The objective of this study was to investigate if maternal dietary 20:4n-6 arachidonic acid (AA) and 22:6n-3 compared with adequate or low levels of 18:3n-3 linolenic acid (LNA) increases synaptic plasma membrane (SPM) cholesterol and phospholipid content, phospholipid 20:4n-6 and 22:6n-3 content, and Na,K-ATPase kinetics in rat pups at two and five weeks of age. At parturition, Sprague-Dawley rats were fed semi-purified diets containing either AA + docosahexaenoic acid (DHA), adequate LNA (control; 18:2n-6 : 18:3n-3 ratio of 7.1 : 1) or low LNA (18:2n-6 : 18:3n-39 ratio of 835 : 1). During the first two weeks of life, the rat pups received only their dams' milk. After weaning, pups received the same diet as their respective dams to five weeks of age. No significant difference was observed among rat pups fed the diet treatments for SPM cholesterol or total and individual phospholipid content at two and five weeks of age. Fatty acid analysis revealed that maternal dietary AA + DHA, compared with feeding the dams the control diet or the low LNA diet, increased 20:4n-6 in phosphatidylserine and 22:6n-3 content of SPM phospholipids. Rats fed dietary AA + DHA or the control diet exhibited a significantly increased Vmax for SPM Na,K-ATPase. Diet treatment did not alter the Km (affinity) of SPM Na,K-ATPase in rat pups at two and five weeks of age. It is concluded that dietary AA + DHA does not alter SPM cholesterol and phospholipid content but increases the 22:6n-3 content of SPM phospholipids modulating activity of Na,K-ATPase.

Administration, Oral↗
PubMed1999

Effects of prostaglandin E1 on high density lipoprotein-phospholipid composition.

Serum high-density phospholipids (HDL-phospholipids) composition was determined in rats treated with prostaglandin E1 (PGE1) and control group treated with isotonic saline. Total phospholipids and HDL-phospholipids levels at serum were found lower in rats treated with PGE1, than in controls. Considering the individual phospholipid classes of HDL, we observed that phosphatidylocholine (PC), phosphatidylinositol (PI) and diphosphatidylglycerol (DPG) serum concentrations were significantly higher in treated rats than in controls (P<0.001, P<0.005 and P<0.05 respectively). Furthermore, the serum concentrations of lysophosphatidylocholine (LPC) and phosphatidylserine (PS) were significantly lower in treated rats than in controls (P<0.01 and P<0.001 respectively). These findings suggest that PGE1 influences the composition of HDL-phospholipids and possibly modifies their action on lipid metabolism.

Alprostadil↗
PubMed2001

[Antipsychotics and phospholipid metabolism in schizophrenia].

To date numerous in-vivo (31)P-MRS and in-vitro studies in schizophrenic patients have been able to demonstrate changes in their membrane phospholipid metabolism, which might be relevant for the cause and the therapeutic responsiveness of this disorder. Thus far, however, only limited studies exist regarding the specificity of these findings for schizophrenia and the effect of antipsychotic medication. The present study examined the composition of membrane phospholipids in platelets of 67 neuroleptic-free schizophrenic patients compared to healthy and psychiatric controls. In a subsample of the schizophrenic patients we determined the effect of antipsychotic treatment on the phospholipid metabolism during six-months follow up. While untreated patients showed a decrease in major membrane phospholipid components, i.e. phosphatidylcholine and phosphatidylethanolamine, when compared to control subjects, as well as an increase in their breakdown-product lysophosphatidylcholine (LPC), there was a significant reduction in LPC during three weeks of pharmacotherapy with haloperidol. After six months treatment with different antipsychotics some divergent effects on phospholipid metabolism in schizophrenic patients could be demonstrated. While in the long-term course LPC remained decreased under continuous therapy with typical neuroleptics, patients being treated with the atypical drug zotepine showed an increase in LPC compared to their baseline level before therapy. Thus, specific mechanisms of the different antipsychotic therapies on phospholipid metabolism might serve to explain the divergent findings of (31)P-MRS in medicated patients.

Adult↗
PubMed1994

[Accelerated breakdown of membrane phospholipids in schizophrenia--implications for the hypofrontality hypothesis].

Phospholipase A2 (PLA2) is a key enzyme in the metabolism of membrane phospholipids. We and other authors (Noponen et al., 1993) reported on increased PLA2 activity in serum and plasma from schizophrenic patients as compared to healthy and psychiatric controls. This increment in PLA2 activity could be inhibited by neuroleptic therapy. The breakdown of membrane phospholipids by PLA2 produces cytotoxic products such as lysophosphatidylcholine (LPC). We found in an independent series of studies increased PLA2 activity, decreased membrane phospholipids and increased LPC concentrations in platelets from schizophrenics, suggesting an accelerated breakdown of membrane phospholipids in the disease. To clarify the effects of PLA2 in the brain we investigated the effects of intracerebral PLA2 injections on dopaminergic neurotransmission in rats using Ungerstedt's model of rotational behaviour. Circing behaviour induced by DA agonists after unilateral PLA2 injections into the substantia nigra pars compacta were recorded. One, three and five weeks after intranigral PLA2 injection apomorphine induced an ipsilateral rotation, indicating a long lasting inhibition of ipsilateral nigrostriatal dopaminergic pathway by PLA2 application. Taken together, our findings indicate that a) at least a subgroup of schizophrenic patients shows increased PLA2 activity and consequently an accelerated breakdown of platelet membrane phospholipids, and b) in animal experiments the intranigral application of PLA2 inhibited the dopaminergic activity. How could these findings be related to the biology of schizophrenia? In schizophrenia a reduced dopaminergic activity in the frontal cortex has been hypothesized. Recent spectroscopy studies reported on an accelerated breakdown of membrane phospholipids in the frontal cortex from schizophrenics.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Sequence of prolactin effects on phospholipid synthesis in mouse mammary gland explants.

In cultured mouse mammary gland explants derived from 12-14 day pregnant mice, the effect of prolactin (PRL) on the rate of incorporation of several precursors into neutral lipids and phospholipids was determined. Employing [14C]-acetate as a substrate, PRL stimulates its incorporation into a) neutral lipids by 4-6 hours, b) phosphatidyl choline (PC) and phosphatidyl inositol-phosphatidyl serine (PI-PS) by 1-2 hours, and c) phosphatidyl ethanolamine (PE) by 2-4 hours. Using [3H]-glycerol as a substrate, the temporal response to PRL for its incorporation into the neutral lipids was the same as that for [14C]-acetate, however, PRL did not enhance the rate of [3H]-glycerol incorporation into the phospholipids at any time through 16 hours. PRL similarly had no effect on the rates of [3H]-choline, [3H]-serine, [3H]-ethanolamine, or [32P]O4 incorporation into the phospholipids at hormone exposure periods of 8 hours or more. And finally, PRL had no effect on the rates of [3H]-arachidonate or [14C]-linoleate incorporation into neutral lipids or phospholipids at culture periods up to 18 hours. These data suggest that the early effect of PRL on [14C]-acetate incorporation into the phospholipids is due to either the insertion of newly synthesized fatty acids and/or the extension of fatty acids contained in the phospholipids.

Acetates↗
PubMed1990

Decreased erythrocyte and platelet phospholipids and fatty acids in juvenile neuronal ceroid-lipofuscinosis (Batten disease).

Total cellular and phospholipid fatty acids were analyzed in erythrocytes and platelets from six patients with juvenile neuronal ceroid-lipofuscinosis (Spielmeyer-Vogt, Batten disease, JNCL). The results were compared to those of age-matched controls. The amounts of total fatty acid and the phospholipid classes, phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) were significantly decreased in patients when related to cellular protein. The reductions in total fatty acids were 27% in erythrocytes and 50% in platelets. Erythrocyte PC reduction was 36%, PE was 44% and PS 27%. There were no major qualitative differences in the phospholipid fatty acids, suggesting that the fatty acid composition of the reduced phospholipid was normal, and that there is a generalized phospholipid deficiency in these cells in JNCL. This was not related to nutritional status. The pathogenesis of Batten disease may be related to abnormal membrane function resulting from this marked phospholipid deficiency.

Adolescent↗
PubMed2005

Condensed complexes in vesicles containing cholesterol and phospholipids.

Animal cell membranes pose conceptual problems related to the physical chemistry of liquids. An avenue to the solution of some of these problems has been opened by the discovery of liquid-liquid immiscibility in synthetic membranes composed of cholesterol and phospholipids. This discovery has led to the development of a thermodynamic model involving condensed complexes. In this model, the phospholipids with longer fatty-acid chains react reversibly with cholesterol to form complexes. The complexes themselves can have a repulsive interaction with other phospholipids, leading to immiscibility. A striking example of this effect is revealed in the phase diagrams of ternary mixtures of cholesterol, a saturated phosphatidylcholine (or sphingomyelin), and an unsaturated phosphatidylcholine. As found by a number of investigators, all binary pairs are miscible in bilayers, whereas the ternary mixture can form two liquid phases. The model of condensed complexes accounts for this effect. Condensed complexes also have a major effect on the chemical activity of cholesterol and on the ordering of phospholipid acyl chains both in the presence and absence of phase separations. Model calculations of phospholipid order parameters account for several features of the deuterium NMR spectra of labeled phospholipid molecules in bilayer mixtures with cholesterol.

Cholesterol↗
PubMed1977

Proton spin-lattice relaxation of retinal rod outer segment membranes and liposomes of extracted phospholipids.

A large fraction of the phospholipid protons of bovine retinal rod outer segment (ROS) disc membrane vesicles yield well-resolved nuclear magnetic resonance lines near physiological temperature. The spin-lattice (T1) relaxation rates of the resolved sharp resonance of ROS disc membranes appear biphasic above 10 degrees C. The rate of the more rapidly relaxing component of each resonance matches closely the relaxation rate of the corresponding resonances of liposomes of purified ROS phospholipids. The slowly relaxing component of each disc membrane resonance is most likely due to phospholipids whose motion is affected by rhodopsin. The primary difference in the relaxation behavior of phospholipids in the ROS membrane vesicles and ROS liposomes appears to be in T1, rather than T2, since the corresponding sharp resonances of both preparations have similar linewidths. These observations suggest that the interaction of rhodopsin with the more fluid membrane phospholipids predominantly affects relatively high frequency segmental motions, which determine T1, while having minimal effects on the lower frequency segmental motions, which influence T2. This conclusion can be rationalized by assuming that a substantial fraction of the interacting phospholipids are relatively fluid with respect to less frequent, larger amplitude segmental motions, but that the more frequent segmental motions (such as beta-coupled trans-gauche isomerizations) are significantly restricted by interaction with protein.

Animals↗
PubMed1977

Evidence against phospholipid asymmetry in intracellular membranes from liver.

We have studied the distribution of phospholipids across the membrane of microsomal vesicles and Golgi-derived secretory vesicles from rat liver by the use of phospholipases. Model studies on single-bilayer phospholipid vesicles showed that phospholipase A2 (phosphatide 2-acyl-hydrolase, EC 3.1.1.4) cleaved at least 80% of the lipids on the outer surface of such vesicles without significant attack on the inner surface. In microsomal vesicles approximately 40% of the outer surface phospholipids were cleaved before the enzyme gained access to the interior of the vesicles. The same conclusion was reached for Golgi vesicles. By following the degradation of the three major phospholipids in intact microsomes and in extracted lipids we found that the same fraction of each of these phospholipids was exposed on the outer surface of the microsomal vesicles. Corresponding experiments with Golgi vesicles showed that distinctly different fractions of phosphatidylcholine and phosphatidylethanolamine were present on the surface of these vesicles. However, the difference was accounted for by enrichment of phosphatidylcholine in intravesicular particles rather than by asymmetry across the vesicle membrane. The results from specific hydrolysis of phosphatidylinositol confirmed an essentially symmetric distribution of this phospholipid across the microsomal and the Golgi vesicle membranes.

Animals↗