Molecular aggregation in aqueous dispersions of phosphatidyl and lysophosphatidyl cholines

Novel bioactive glycerol-based lysophospholipids: new data -- new insight into their function

Based on the results of research conducted over last two decades, lysophospholipids (LPLs) were observed to be not only structural components of cellular membranes but also biologically active molecules influencing a broad variety of processes such as carcinogenesis, neurogenesis, immunity, vascular development or regulation of metabolic diseases. With a growing interest in the involvement of extracellular lysophospholipids in both normal physiology and pathology, it has become evident that those small molecules may have therapeutic potential. While lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have been studied in detail, other LPLs such as lysophosphatidylglycerol (LPG), lysophosphatidylserine (LPS), lysophosphatidylinositol (LPI), lysophosphatidylethanolamine (LPE) or even lysophosphatidylcholine (LPC) have not been elucidated to such a high degree. Although information concerning the latter LPLs is sparse as compared to LPA and S1P, within the last couple of years much progress has been made. Recently published data suggest that these compounds may regulate fundamental cellular activities by modulating multiple molecular targets, e.g. by binding to specific receptors and/or altering the structure and fluidity of lipid rafts. Therefore, the present review is devoted to novel bioactive glycerol-based lysophospholipids and recent findings concerning their functions and possible signaling pathways regulating physiological and pathological processes.

The interaction of penicillins with phospholipids

The interaction of the antibiotics, penicillin G and ampicillin, with sonicated sols of phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine has been examined by Sephadex gel nitration and dynamic dialysis. Nuclear magnetic resonance spectroscopy provided evidence of a predominantly hydrophobic interaction between the antibiotics and the phospholipids, phosphatidylserine and phosphatidylcholine. Confirmation of hydrophobic interaction was provided by a rheological investigation of the effects of urea and guanidine hydrochloride on the antibiotic-phospholipid complex. Penicillin G was found to interact to a greater degree than ampicillin, a result which is of interest in the light of present knowledge of in vivo activity of these antibiotics.

Different mechanisms of lysophosphatidylcholine-induced Ca(2+) mobilization in N2a mouse and SH-SY5Y human neuroblastoma cells

In mice, lysophosphatidylcholine (LPC) was found to be a physiological substrate of neuropathy target esterase, which is also bound by organophosphates that cause a delayed neuropathy in human and some animals. However, the mechanism responsible for causing the different symptoms in mice and humans that are exposed to neuropathic organophosphates still remains unknown. In the present study, we examined and compared the effect of exogenous LPC on intracellular Ca(2+) overload in mouse N2a and human SH-SY5Y neuroblastoma cells. LPC caused an intracellular Ca(2+) level ([Ca(2+)](i)) increase in both N2a and SH-SY5Y cells; moreover, the amplitude was higher in N2a cells than that in SH-SY5Y cells. Preincubation of the cells with verapamil, an L-type Ca(2+) channel blocker, did not affect the LPC-induced Ca(2+) increase in N2a cells, verapamil inhibited the response by 23% in SH-SY5Y cells. In Ca(2+)-free medium, LPC produced a significant [Ca(2+)](i) decrease in N2a cells, while it caused 64% of total [Ca(2+)](i) increase in SH-SY5Y cells. The results of a cell viability test suggest that N2a cells were more sensitive to LPC than were SH-SY5Y cells. These data suggested that the LPC-induced [Ca(2+)](i) increase was produced in each cell line through different mechanisms. In particular, the [Ca(2+)](i) increase occurred via entry through a permeabilized membrane in N2a cells, but through L-type Ca(2+) channels as well as by Ca(2+) release from intracellular Ca(2+) stores in SH-SY5Y cells. Thus, the symptomatic differences of organophosphate-induced neurotoxicity between mice and humans are probably not related to the diverse amplitudes of intracellular Ca(2+) overload produced by LPC. Moreover, the demyelination effect induced by LPC in mice may be a consequence of its detergent effect on membranes.

The focally demyelinated rat fimbria: a new in vitro model for the study of acute demyelination in the central nervous system

We have produced controlled local demyelination in Wistar rat fimbriae by injection of microliter quantities of the detergent lysophosphatidyl choline (LPC) stereotactically. Six to seven days later the hippocampus and fornix was dissected out en bloc and maintained in vitro for electrical evaluation of conduction through the damaged area. The tissue was then fixed for verification of the lesion by light and electron microscopy. All fimbriae showed a normal conducted action potential with a latency of 0.5 to 1.5 ms when the conduction distance was 4-5 mm. LPC-lesioned fimbriae also showed a later wave with a latency of 4-5 ms. This later wave had the same stimulus-response curve as the primary action potential, but was more sensitive to repeated stimulation. We interpret this wave as evidence of conduction into or through a demyelinated region. LPC-lesioned fimbriae also showed histological evidence of demyelination. This preparation provides an in vitro model for the study of acute local demyelination of central nervous system white matter, such as that induced by multiple sclerosis and other focally demyelinating diseases.

Tryptophan sidechain dynamics in hydrophobic oligopeptides determined by use of 13C nuclear magnetic resonance spectroscopy

Two oligopeptides, t-boc-LAWAL-OMe and t-boc-LALALW-OMe, were synthesized for the purpose of examining the sidechain dynamics of the tryptophan residue in hydrophobic environments by 13C nuclear magnetic resonance and fluorescence spectroscopy. In both peptides, the tryptophan sidechain was greater than 95% enriched with 13C at the C delta 1 position. Spin-lattice relaxation time (T1) and steady-state nuclear Overhauser effect (NOE) data were obtained at 50.3 and 75.4 MHz for both peptides in CD3OD, and at 75.4 MHz for t-boc-LALALW-OMe in lysolecithin-D2O micelles. We have adapted the model-free approach of G. Lipari and A. Szabo (1982, J. Am. Chem. Soc. 104:4546) to interpret the 13C-NMR data. Computer-generated curves based on experimental data obtained at a single frequency demonstrate relationships between an effective correlation time for tryptophan sidechain motion (tau e), a generalized order parameter (sigma) describing the extent of motional restriction, and an overall correlation time for the peptide (tau m). Assuming predominantly dipolar relaxation, least-squares fits of the dual frequency relaxation data provide values for these parameters for both peptides. The contribution of chemical shift anisotropy (CSA), however, is also explicitly assessed in the data analysis, and is shown to perturb the predicted sigma, tau e, and tau m values and to decrease chi(2) values observed in nonlinear least-squares analysis of the data. Because of uncertainty in the contribution of CSA to the relaxation of the indole ring 13C delta 1 atom, nonlinear least-squares analysis of the relaxation data were performed with and without inclusion of a CSA term in the appropriate relaxation equations. Neglecting CSA, an overall peptide correlation time of 0.69 ns is predicted for t-boc-LAWAL-OMe in CD3OD at 20 degrees C compared with 1.28 ns for t-boc-LALALW-OMe. Given these tau m values and taking into account the effect of measurement error in the T1 and NOE data, the internal dynamics of the tryptophan residue of t-boc-LAWAL-OMe in this isotropic environment are described by a range of tau e values from 70 to 112 ps and sigma values between 0.22 and 0.36. Similarly, for t-boc-LALALW-OMe, 68 less than or equal to tau e less than or equal to 93 ps and 0.09 less than or equal to sigma less than or equal to 0.17. The Ch-terminal position of the tryptophan residue in the hexapeptide may account for its lower order parameter. In lysolecithin micelles, the model-free approach applied tot-boc-LALALW-OMe predicts a Te between 0.87 and 1.08 ns, and an order parameter range of 0.72-0.80, assuming an average Tm of 14 ns (Saunders, L. 1966. Biochim. Biophys. Acta. 125:70) for a typical peptide-micelle complex. In this case, measurement of only two 13C relaxation parameters at a single frequency yields sufficient information, plotted in the form of a composite T1-NOE solution curve, to constrain the allowed values of the model-free motional parameters within a relatively narrow range. The predicted range of eV and Te values for the peptide-micelle complex demonstrate that both the rate and spatial mobility of the indole moiety are markedly restrained in the anisotropic micelle environment relative to free methanol solution. Steady-state fluorescence anisotropy measurements made on the peptides dissolved in methanol or with synthetic lysolecithins in water were used to calculate apparent order parameters for tryptophan motion; these values agree well with order parameters calculated from 13C NMR data. The reported results are relevant to the issue of protein dynamic events occurring on the picosecond time scale predicted by molecular dynamics simulations.

Gastric mucosal damage induced by combination of ethanol and lysophosphatidylcholine

The purpose of this study was to determine the effect of lysophosphatidylcholine on the guinea pig stomach after dosing the animal with 20% ethanol by orogastric intubation. We studied four groups of animals; one control group received saline orogastrically followed by buffer and one test group received saline followed by buffer plus 1 mM lysophosphatidylcholine. Two other groups were challenged with 20% ethanol (5 ml) orogastrically followed by buffer or buffer plus 1 mM lysophosphatidylcholine. Compared to other groups, the stomachs of animals given ethanol followed by lysophosphatidylcholine displayed statistically significant increases in the number of gross hemorrhagic lesions, in back-diffusion of hydrogen ion, in net secretion of sodium ion, and in morphologic damage. Transmucosal potential differences in this group were also decreased. We conclude that 90 min after dosing with ethanol, the guinea pig stomach is more susceptible to damage by lysophosphatidylcholine. Our data further suggest that these agents cause mucosal damage by different mechanisms and that the combination acts synergistically.

Mechanism of modification of rat brain lysophospholipase A activity by cationic amphiphilic drugs

The three psychotropic cationic amphiphilic drugs, chlorpromazine, desmethylimipramine and propranolol were found to have biphasic effects on rat brain lysophospholipase A, stimulating the enzyme at low, and inhibiting it non-competitively at higher concentrations. Low concentrations (less than or equal to 50 microM) of the drugs prevented the formation of micelles of lysophosphatidylcholine, whereas high concentrations caused a phase transition of the substrate with formation of a highly ordered membranous lattice. A possible mechanism of stimulation and inhibition of the enzyme activity by cationic amphiphilic drugs is proposed. Stimulation is explained by a decrease in the concentration of substrate micelles, which are inhibitory for the activity, whereas inhibition may be caused by adsorption of the enzyme onto the membranous lattice formed by the substrate in the presence of high cationic amphiphilic drug concentrations.

Bilayer asymmetry in lysophosphatidylcholine/cholesterol (1:1) vesicles. A phosphorus-31 NMR study

Prolonged sonication (3 h) of equimolar amounts of lysophosphatidylcholine (lysoPC) and cholesterol (chol) produces small unilamellar vesicles. Phosphorus-31 NMR (32.20 MHz) of the vesicles gave rise to a single peak (40.5 ppm) which was split upon addition of lanthanide ions. An additional, more intense signal appeared downfield near 51.0 ppm due to 2.4 mM Pr3+, upfield near 34.3 ppm due to 5 mM Yb3+. The more intense signals responsive to paramagnetic ions were assigned to lysoPC located in the outer vesicle leaflet; the signal not shifted by the ions was assigned to inside lysoPC. Based on peak intensities, an outside-to-inside lysoPC ratio (Ro/i) of 6.5-6.6 was determined. Essentially the same Ro/i values (6.6-6.8) were obtained when Pr3+ was present only in the vesicle interior or when Pr3+ was on the inside and Pr3+ and Yb3+ were on the outside. Ion leakage did not occur. Our data demonstrate that lysoPC/chol (1:1) vesicles are drastically asymmetric and that lysoPC shows a distinct preference for the outer bilayer leaflet.

Supramolecular organization of lysophosphatidylcholine-packaged Gramicidin A

Heat derived gramicidin A'/L-alpha-lysophosphatidylcholine complexes were separated on a sucrose gradient to form two fractions: Fraction A which had an approximately constant Gramicidin A' to phospholipid ratio of 8 to 10 lipid molecules per Gramicidin A' molecule and Fraction B which had a larger but variable ratio. Fluorescence and circular dichroism studies confirmed Fraction A to be a lipid-incorporated channel state. Electron microscopic studies, using uranyl acetate negative staining, showed fraction A to be a membranous state with the formation of bilayer vesicles, that is, the interaction of peptide and phospholipid micelles causes the lipid to reorganize into a bilayer structure. Freeze-fracture replicas of the channel incorporated state demonstrated the presence of a supramolecular organization of particles exhibiting a tendency to form rows with a 50-60 A periodicity along the row and with 70-80 A distance between rows. An idealized working model for the incorporated state is presented.

Solubilization of phospholipids by detergents. Structural and kinetic aspects

Most amphiphiles in biological membranes including phospholipids, steroids, and membrane proteins are insoluble amphiphiles and would form liquid crystals or insoluble precipitates alone in aqueous media. Detergents are soluble amphiphiles and above a critical concentration and temperature form micelles of various sizes and shapes. Much of the recent progress in studying the insoluble amphiphiles is due to the formation of thermodynamically stable isotropic solutions of these compounds in the presence of detergents. This process, which is commonly denoted as "solubilization,' involves transformation of lamellar structures into mixed micelles. The information available to date on the solubilization of phospholipids, which constitute the lipid skeleton of biomembranes, by the common detergents is discussed in this review, both with respect to the kinetics of this process and the structure of the various phospholipid-detergent mixed micelles formed. It is hoped that this discussion will lead to somewhat more useful, although still necessarily fairly empirical, approaches to the solubilization of phospholipids by detergents.

The effects of unsaturated fatty acids on the synthesis of arachidonic acid in rat kidney cells

Cultured rat kidney cells absorbed exogenous linoleic acid (cic, cis-18:2n-6) and esterified it mostly into glycerophospholipids. As the concentration of 18:2 was increased (5-200 microM) the quantity absorbed increased linearly and the amount esterified in the triacylglycerol increased. The cells possessed active acyl delta 6-desaturase and elongase which facilely converted 18:2n-6 to 20:4n-6. At low intracellular concentrations of 18:2n-6 other unsaturated fatty acids, i.e., gamma-linolenic (18:3n-6), alpha-linolenic (18:3n-3), dihomo-gamma-linolenic (20:3n-6), and especially trans, trans-linoleic acid (trans, trans-18:2n- -6) at concentrations ranging from 25 to 200 microM depressed delta 6-desaturase activity. However, suppression of 20:4 synthesis even by trans, trans-18:2 was readily overcome by increasing the concentration of available cis, cis-18:2n-6.

The apoprotein fraction of the bile lipoprotein complex: isolation, partial characterization and phospholipid binding properties

A bile apoprotein fraction (Apo BLC) was isolated by preparative isoelectric focusing (I.E.F.) from the detergent-free form of the bile lipoprotein complex (BLC). Analytical I.E.F. of Apo BLC yields a characteristic and reproducible pattern of two narrow acidic bands (pI 4,8-5,0). This apoprotein presents a strong tendency to undergo self-aggregation in aqueous buffer. A low molecular weight constituent of Apo BLC has been isolated after gel filtration, its mean Mw is estimated by SDS-PAGE at 7,500 daltons. The binding capacity of Apo BLC for phospholipids was investigated on dimyristoylphosphatidylcholine liposomes by gel filtration and zone electrophoresis. The resulting structures, larger than the original single-shelled vesicles, acquire and anodic electrophoretic mobility. Apo BLC has a weaker affinity for lysophosphatidylcholines: these phospholipids decrease the degree of aggregation of the apoprotein. These studies contribute additional data concerning the high affinity of Apo BLC for phosphatidylcholines, which are the major phospholipid constituents of bile. The discussion deals with the fact that association of Apo BLC with bile phosphatidylcholines may present some implications in the pathogeny of LpX and in the process of intestinal fat absorption.

Structure and thermodynamic properties of the complexes between phospholipase A2 and lipid micelles

The interaction between porcine pancreatic phospholipase A2 and a homogeneous population of micelles of the subtrate analogue n-hexadecylphosphorylcholine containing 155 lipid monomers was studied by light scattering, equilibrium gel filtration, and isothermal calorimetry. From the detergent/protein molar ratio and the equivalent "molecular weight" of the resulting complex it is concluded that insertion of the enzyme into the detergent micelle results in a protein--detergent complex containing two phospholipase A2 molecules and 80 lipid monomers at 25 degrees C. The affinity constants and complex composition have been determined at different temperatures, allowing calculation of the thermodynamic parameters of the binding process. It is concluded that the interaction of phospholipase A2 with micellar lipids is predominantly hydrophobic.

Interaction of the phosphatidylcholine exchange protein with phospholipids. A fluorescence and circular dichroism study

The phosphatidylcholine exchange protein from bovine liver has a fluorescence emission maximum at 327 nm. Fluorescence was enhanced in the presence of vesicles containing phosphatidylcholine and various amounts of either phosphatidic acid or phosphatidylglycerol. From the increase in fluorescence it was derived that the apparent dissociation constant of the exchange protein-vesicle complex decreased with an increased vesicle content of acidic phospholipids. Fluorescence indicated that the exchange protein interacted with lysophosphatidylcholine micelles at pH 3.5, 5.9 and 8.5. The increase in fluorescence was most prominent at the acidic pH. Circular dichroism indicated that the alpha-helix content of the native protein was low between pH 3.6 and 8.0. Interaction with lysophosphatidylcholine micelles had a negligible effect on the secondary structure of the protein, except at pH 3.6 where distinct minima at 208 nm and 220 nm in the circular dichroic spectrum became apparent.

Histidine and lysine residues and the activity of phospholipase A2 from the venom of Bitis gabonica

Chemical modification of phospholipase A2 (phosphatide 2-acyl-hydrolase, EC 3.1.1.4) from the venom of gaboon adder (Bitis gabonica) showed that histidine and lysine residues are essential for enzyme activity. Treatment with p-bromophenacyl bromide or pyridoxal 5'-phosphate resulted in the specific covalent modification of one histidine or a total of one lysine residue per molecule of enzyme, respectively, with a concomitant loss of enzyme activity. Competitive protection against modification and inactivation was afforded by the presence of Ca2+ and/or micellar concentrations of substrate analogue, lysophosphatidylcholine. Neither modification caused any significant conformational change, as judged from circular dichroic properties. Amino acid analyses and the alignment of peptides from cyanogen bromide and proteolytic cleavage of modified enzyme preparations delineated His-45 as the only residue modified by p-bromophenacyl bromide. However, pyridoxal 5'-phosphate was shown to have reacted not with a single lysine but with four different ones (residues 11, 33, 58 and 111) in such a manner that an overall stoichiometry of one modified lysine residue/molecule enzyme resulted. Apparently, the essential function of lysine could be fulfilled by any one out of these four residues.

Selective solubilization of the components of the mitochondrial inner membrane by lysolecithin

1. Of various phospholipids tested, lysolecithin was the most efficient in the solubilization of the components of beef heart submitochondrial particles. Lysolecithin solubilized selectively nicotinamide nucleotide transhydrogenase, succinate dehydrogenase, NADH dehydrogenase and oligomycin-sensitive ATPase. Various cytochromes other than cytochrome c were only slightly solubilized. 2. The effect of various parameters, e.g. ionic strength, pH, time of centrifugation, and concentrations of lysolecithin and protein was investigated. Increasing times of centrifugation led to a partial sedimentation of NADH dehydrogenase, and a complete sedimentation of oligomycin-sensitive ATPase and cytochrome oxidase. 3. Further fractionation of the lysolecithin extract by centrifugation in the presence of low concentrations of cholate gave a complete separation of NADH dehydrogenase and transhydrogenase, indicating that these enzymes are not related functionally. 4. With the lysolecithin fractionation procedure a more than 10-fold purification of transhydrogenase was achieved. Polyacrylamide gel electrophoresis of the partially purified transhydrogenase in the presence of sodium dodecyl sulphate showed major increases in protein-stained bands corresponding to between 70 000 and 54 000 daltons. 5. A possible mechanism for the detergent action of lysolecithin involving a specific exchange of bound phospholipids for lysolecithin is discussed.

An essential tryptophan in the active site of phospholipase A2 from the venom of Bitis gabonica

The role of tryptophan in phospholipase A2 (EC 3.1.1.4) from the venom of the gaboon viper, Bitis gabonica, has been investigated. Modification of the enzyme with N-bromosuccinimide and 2-nitrophenylsulfenylchloride showed that the two tryptophan residues in the enzyme, viz. Trp-28 and Trp-59, differ in reactivity towards the reagents. Only Trp-28 reacted with N-bromosuccinimide while a preferential reaction occurred between Trp-59 and 2-nitrophenyl-sulfenylchloride. In each case it was found that loss of enzyme activity was specifically correlated with modification of TRP-28. CD spectra indicated that neither the local nor the gross conformation of the enzyme was altered by modification of Trp-28 and it was therefore concluded that Trp-28 is crucial for enzyme activity. The active enzyme was protected against N-bromosuccinimide inactivation by micellar concentrations of substrate or substrate analogue, suggesting that Trp-28 is involved in substrate binding.

Molecular aggregation of the slow reacting hemolytic lysolecithin analog 1-octadecyl-2-benzyl-glycero-3-phosphorylcholine in aqueous solution

An attempt has been made to relate the retarded adsoprtion to red cells of the slow reacting hemolytic phosphatide Rac. 1-octadecyl-2-benzyl-glycero-3-phosphorylcholine (benzyl-lysolecithin) to its aggregation status in aqueous solution. Light scattering measurements indicate a critical micelle concentration at 37 degrees of less than 2 X 10(-6) M. The micellar weight as determined by angle dependent light scattering is 6 X 10(7) with about 97 000 molecules per micells. The aggregates, which according to electron-microscopic observations are more similar to lecithin-liposomes than to usual lysolecithin-micelles, undergo a phase transition at 14 degrees from a tightly packed liquid-crystalline state to the more loose structure of a gel phase with increased mobility of the aliphatic chains. The enthalpy of transition is 4.2 kcal/mole. These changes of the micellar structure are reflected in the binding kinetics of benzyl-lysolecithin to red cells in that the binding rate is rather constant below, but strongly increasing above the transition temperature. It is concluded that the unusual micellar structure is responsible for the retarded adsorption of this lysolecithin analog to red cells and that the rate of adsorption is probably determined by the rate of escape of single lysophosphatide molecules from the micelles.

Delipidation of the phosphatidylcholine exchange protein from beef liver by detergents

1. A method is described to introduce [14C] phosphatidylcholine into the phosphatidylcholine exchange protein from beef liver. The effects of various detergents on this 14C-labelled phospholipid - protein complex are considered. 2. As shown by spectrophotometric and radioactivity analysis of polyacrylamide gels, sodium deoxycholate, Triton X-100, lysophosphatidylcholine and sodium dodecyl sulfate delipidate the exchange protein, while mixed phosphatidylcholine-detergents micelles are formed. 3. Protein delipidated by sodium deoxycholate, Triton X-100 and lysophosphatidylcholine retains its ability to catalyze the transfer of phosphatidylcholine between membranes. The immunological properties are similar to those of native protein as shown by double immunodiffusion in agar against an antiserum gamma-globulin fraction. 4. Sodium dodecyl sulfate and cetyltrimethylammonium bromide interact very strongly with the protein conferring their charge to the complex and destroying the antigenic determinants.

A model for the packing of lipids in bilayer membranes

A number of known structural properties of mixed lipid bilayer membranes and monolayers are accounted for by a model in which lipids pack into bilayers and monolayers like building blocks, each characterized by a surface head group area and characteristic solid angle. In phospholipids above the melting transition the head group area (at a given temperature and degree of hydration) is fairly invariant while the hydrocarbon region may be liquid-like so long as the molecule is not compressed beyond its characteristic solid angle. Phosphatidylcholine and phosphatidylserine are tapered lipids, i.e. their surface head group areas are greater than their non-polar end areas; cholesterol is frayed, i.e. its polar end area is less than its non-polar end area; while phosphatidylethanolamine is almost cylindrical. The "condensing" effect of cholesterol in mixed phospholipid-cholesterol films is seen as a taper-fray accommodation. The lipid distribution in erythrocyte membrane is shown to be conducive to a stable strain-free membrane.