Studies on phosphatidylcholine vesicles. Formation and physical characteristics

Interaction of phosphatidylcholine with bovine serum albumin. Specificity and properties of the complexes

Phosphatidylcholine dispersed on Celite was rapidly solubilized by neutral bovine serum albumin solutions. Stable protein-lipid complexes were isolated by Agrose gel filtration or by ultracentrifugal flotation in high density solvents, and the physicochemical properties of the complexes were investigated in terms of the stoichiometry of binding, effect of fatty acid ligands on phosphatidylcholine binding, effect of high ionic strength on the stability of the complexes, intrinsic fluorescence and circular dichroism spectra, and sedimentation velocity coefficients. Complexes containing from 2 to 30 phosphatidylcholine molecules per protein molecule were observed; however, no saturation of binding sites could be detected in this range of molar ratios. Oleic acid binding by serum albumin prevents interaction of the protein with phosphatidylcholine, indicating possible competition of these ligands at low contents of the phospholipid. For molar ratios of up to 10 phosphatidylcholine molecules per serum albumin, binding is primarily due to hydrophobic interactions that have no effect on the overall shape and secondary structure of the native protein except for local modifications at tryptophan residues, whose fluorescence becomes quenched and blue shifted on phosphatidylcholine binding. Similar phosphatidylcholine uptake experiments performed with a series of globular proteins indicated that the lipid extraction from Celite surfaces is a non-specific process, accelerated by several other proteins (e.g. aldolase, egg albumin, chymotrypsinogen, soybean trypsin inhibitor, and the major apolipoprotein from bovine serum high density lipoprotein). Formation of stable protein-lipid complexes, however, was only observed with bovine serum albumin, which in contrast to the other proteins is known to have affinity binding sites for anions with hydrophobic side chains.

Interaction of the polyene antibiotics with lipid bilayer vesicles containing cholesterol

The interaction of the polyene antibiotics, amphotericin B, nystatin and filipin with cholesterol-containing single bilayer lipid vesicles has been characterized using gel permeation chromatography and proton magnetic resonance. All three antibiotics bind to vesicles at low concentrations without causing a large amount of vesicle destruction. The strength of binding as determined by gel permeation studies is greater for filipin and amphotericin than for nystatin. Nystatin and amphotericin B at these low concentrations induce a rapid loss of internal vesicle contents consistents consistent with pore formation. Filipin induces no leakage beyond that expected from partial vesicle destruction or general detergent action. At antibiotic levels above 1:1 antibiotic: cholesterol ratios the NMR results show all three antibiotics to cause extensive vesicle destruction. The onset of this behavior, which appears to be independent of the total antibiotic concentraion, indicates a well defined antibiotic : cholesterol interaction stoichiometry. Despite the fact that cholesterol is required for antibiotic activity, the NMR spectra prior to vesicle destruction show no changes indicative of an antibiotic-induced reversal of cholesterol restriction of phosphatidylcholine mobility. The contrast with polyene antibiotic behavior in more extended bilayers is discussed.

Ion-binding to phospholipids. Interaction of calcium with phosphatidylserine

The binding of Ca2+ to monolayers and bilayers of phosphatidylserine has been investigated as a function of pH, ionic strength (NaCl concentration) and Ca2+ concentration using surface and colloid chemical techniques. The molar ratio of lipid to bound calcium decreases to 2 as the Ca2+ concentration is increased to about 0.1 mM. At [Ca2+] greater than 0.1 mM a 1:1 complex is formed. The apparent binding constant Ka ranges from about approximately 10(6) - 10(4) l/mol depending on the Ca2+ concentration. After allowing for electrostatic effects and neighbour group interactions, the intrinsic binding constant Ki of the phosphorylserine polar group at pH 7 (I = 0.01 M), where it carries a net negative charge of one, is approximately 10(4) l/mol; consistent values for Ki were obtained using several independent approaches. Ka for Ca2+ binding decreases with increasing NaCl concentration because the monovalent cations compete with Ca2+ for the same binding site. Na+ and K+ are equally effective in displacing 45Ca2+ adsorbed to monolayers of phosphatidylserine, both with respect to the kinetics and the equilibrium of the displacement. Ka for the reaction between phosphatidylserine and monovalent cations is about 10(3)-fold smaller than that of Ca2+. An investigation of the binding of Mn2+ to phosphatidylserine by both surface chemical and nuclear magnetic resonance methods shows that this cation has a similar binding constant to that of Ca2+. The Ca2+-binding capabilities of monolayers containing only carboxyl groups (i.e. arachidic acid) and phosphodiester groups (i.e. dicetyl phosphate) have also been determined; the apparent pK for the - COOH group in monolayers is larger than or equal to 9 and that for the phosphodiester group is less than 4. Since these groups do not retain the same pK values when they are in close proximity in the phosphorylserine group, the relative contributions of the two groups to the binding of Ca2+ to phosphatidylserine is not obvious.

Interaction of apoliprotein C-III with phosphatidylcholine vesicles. Dependence of aproprotein-phospholipid complex formation on vesicle structure

We have studied the interaction of an apolipoprotein from human very low density lipoproteins (apoC-III) with egg yolk phosphatidylcholine in the form of single- and multi-bilayer vesicles. The reactivity of single-bilayer vesicles with apoC-III appears to be greater than that of the multi-bilayer vesicles according to several thermodynamic and spectrosconic criteria. In the complexes formed by the association of apoC-III with single-bilayer vesicles, the alpha-helical content of the peptide backbone and the apolarity of the environment around the tryptophan residues are greater than that observed in the complexes formed with the multibilayer vesicles. A higher yield and more homogeneous density distribution of lipid-apoprotein complexes results from the interaction of apoC-III with the single-bilayer vesicles relative to those obtained with the multi-bilayer vesicles. The enthalpy of association of apoC-III with phospholipid was greater for the single-shelled vesicles (25 kcal/mol apoC-III) than for the multi-shelled ones (18 kcal/mol apoC-III). The difference in reactivity of these two types of liposomes is not due to a difference in their fluidities since their fatty acid compositions are identical, but may be due to a difference in their areas of sterically accessible phospholipid, their permeabilities to the apoprotein, their radii of curvation, or a combination of these factors.

Microviscosity of lipid domains in human serum lipoproteins

Microviscosities for the hydrophobic lipid regions of human serum lipoproteins and for dispersions of lipids extracted from the lipoproteins have been determined using fluorescence polarization measurements with 1,6-diphenyl-1,3,5-hexatriene, a rod-like molecule, as the main fluorescent probe. Additional microviscosity measurements were carried out on LP-X, an abnormal human lipoprotein characteristic of cholestasis. Perylene, a disc-shaped fluorescent probe, was used with intact human lipoproteins in order to confirm relative microviscosity values measured with 1,6-diphenyl-1,3,5-hexatriene and to estimate the anisotropy of the lipid domains. Logarithmic plots of microviscosity against the inverse of absolute temperature, over the range of 0-40 degrees C, gave no indication of phase transitions and yielded activation energy values for all human lipoproteins and for the isolated lipids. The microviscosity results at 25 degrees C range from a high value of 6.1 +/- 0.9 P for low density lipoprotein down to 1.0 +/- 0.2P for chylomicrons, when 1,6-diphenyl-1,3,5-hexatriene is used as the probe. Activation energies vary from 9 kcal per mol to 6 kcal per mol for the intact lipoproteins. In contrast, isolated lipids have microviscosities from 2.4 +/- 0.3P for low density lipoprotein lipids to 1.0 +/- 0.2P for chylomicron lipids, with activation energies around 6 kcal per mol. The absolute microviscosity values indicate fluid yet viscous and anisotropic lipid domains in higher density lipoproteins, and more fluid and disordered states for isolated lipids and chylomicrons. Differences in microviscosities between the intact lipoproteins and isolated lipids can be attributed to the effects of proteins in restricting the mobility of lipids, these effects are strongest for low density lipoproteins, followed by high density lipoproteins, LP-X, very low density lipoproteins, and chylomicrons. Flow activation energies are throught to reflect intermolecular interactions, and are again higher for the higher density lipoproteins than for isolated lipids, or for chylomicrons.

Lectin-receptor interactions in liposomes

The major sialoglycoprotein of mammalian erythrocytes has been incorporated into phosphatidylcholine membranes to generate a model system, glycoprotein-liposomes. Electron microscopic examination revealed these structures to be vesicles, approximately 300 A in diameter. An aqueous compartment inside the glycoprotein-liposomes has been identified by trapped volume studies with [14C]sucrose. These glycoprotein-liposomes were found to interact with the lectins, wheat germ agglutinin, and phytohemagglutinin, to form aggregates of mainly unfused vesicles. The aggregation process has been studied by electron microscopy, 90 degrees light scattering, and differential ultracentrifugation analysis. Hapten inhibitors of the lectins were found to inhibit the lectin-induced aggregation of the glycoprotein-liposomes. Binding of 125I-labeled wheat germ agglutinin to glycoprotein-liposomes was studied by differential ultracentrifugation. Hapten inhibitors of wheat germ agglutinin were also found to inhbit the binding of 125I-labled wheat germ agglutinin to the glycoprotein-liposomes. The characteristics of the lectin interactions with glycoprotein-liposomes appeared to be phenomenologically similar to lectin-cell interactions.

Temperature dependence of N-phenyl-1-naphthylamine binding in egg lecithin vesicles

The temperature dependence of the binding of PhNapNH2 (N-phenyl-1-naphthylamine) to vesicles of egg phosphatidylcholine has been determined. The Arrhenius plot of the association constant exhibits a discontinuity at 20.9 degrees C, some 30 degrees C above the broad phase transition region of the phospholipid. In the temperature range above 20 degrees C, deltaH0 =--6100 cal-mol-1 and deltaS0 = 9.7 e.u.; in the temperature range below 20 degrees C, deltaH0 = 0 cal-mol-1 and deltaS0 = 30.4 e.u. These values are consistent with the view that there are well ordered lipid-lipid bonds below 20 degrees C which are significantly less important above this temperature. The order in the temperature range of 5 to 20 degrees C, though significantly greater than that above 20 degrees C, is still significiantly less than that in the crystalline state.

Interaction of phospholipid vesicles with cultured mammalian cells. II. Studies of mechanism

The mechanism of interaction of artificially generated lipid vesicles (approximately 500 A diameter) with Chinese hamster V79 cells bathed in a simple balanced salt solution was investigated. The major pathways of exogenous lipid incorporation in vesicle-treated cells are vesicle-cell fusion and vesicle-cell lipid exchange. At 37 degrees C, the fusion process is dominant, while at 2 degrees C or with energy depleted cells, exchange of lipids between vesicles and cells is important. The fusion mechanism was demonstrated using vesicles of [14C]lecithin containing trapped [13H]inulin. Consistent with a fusion hypothesis, both components became cell associated at 37 degrees C in nearly the same proportions as they were present in the applied vesicles. Additional arguments in favor of vesicle-cell fusion and against phagocytosis or adsorption of intact vesicles are presented. At 2 degrees C or with inhibitor-treated cells, the [3H]inulin uptake was largely suppressed, while the lipid uptake was reduced to a lesser extent. Evidence for vesicle-cell lipid exchange was obtained using V79 cells grown on 3H precursors for cellular lipids. [14C]lecithin vesicles, incubated with such cells, showed no change in their elution properties when subjected to molecular sieve chromatography on Sepharose 4B. However, radioactivity and thin-layer chromatographic analyses revealed that a variety of cell lipiids had been exchanged into the uniamellar vesicles. Further evidence for the fusion and exchange processes was obtained using vesicles prepared from mixtures of [3H]lecithin and [14C]cholesterol. A two-step fusion mechanism consistent with the present findings is proposed as a working model for other fusion studies.

Differential scanning calorimetry and 31P NMR studies on sonicated and unsonicated phosphatidylcholine liposomes

1. Phase transitions in sonicated (vesicles) and unsonicated liposomes composed of various synthetic phosphatidylcholines are monitored using differential scanning calorimetry and 31P NMR. 2. The temperature (Tc), heat content and width of the phase transition are comparable in both vesicles and liposomes prepared from 1,2-dipalmitoyl phosphatidylcholine and 1,2-dimyristoyl phosphatidylcholine. In vesicles composed of a (1 : 1) mixture of 1,2-dipalmitoyl phosphatidylcholine and 1,2-dioleoyl phosphatidylcholine phase separation occurs as in the bilayers of the unsonicated liposomes. 3. The linewidth of the 31P resonances in vesicles is not greatly dependent upon the fatty acid composition when the lipids are in the disordered liquid crystalline state (above Tc). When the lipids are in the gel state (below Tc), however, there is a marked increase in linewidth, demonstrating a reduction in motion of the phosphate group. 4. The ratio of the amounts of phosphatidylcholine present in the outside and inside monolayter of the vesicle membrane was determined with 31P NMR using Nd3+ as a non-permeating shift reagent. 5. The outside/inside ratio is dependent upon the hydrocarbon chain length. Increasing chain length gives a lower outside/inside ratio and a larger vesicle. Introduction of cis or trans double bonds in the chain influences the outside/inside ratio slightly. 6. The incorporation of cholesterol decreases the outside/inside ratio and increases the size of 1,2-dimyristoyl phosphatidylcholine vesicles. The cholesterol concentration in the outside and inside monolayer is approximately the same. The size of the 1,2-dioleoyl phosphatidylcholine vesicles is also increased by cholesterol incorporation but the outside/inside distribution is also increased, especially between 30 and 50 mol% cholesterol. In these vesicles cholesterol is asymmetrically distributed and strongly prefers the inside monolayer of the vesicle.

Headgroup conformation and lipid--cholesterol association in phosphatidylcholine vesicles: a 31P(1H) nuclear Overhauser effect study

The nuclear Overhauser effect has been observed in the nuclear magnetic resonance spectra of 31P. The information content of the nuclear Overhauser effect has been applied to the structure and dynamic properties of phosphatidylcholine vesicles. In the vesicles only 1/3 of the theoretical maximum nuclear Overhauser effect enhancement is observed. This result is accounted for by dipolar interactions between the N-methyl protons and the phosphate of phosphatidylcholine, and a correlation time for internal motion of 1.4 X 10(-9) sec. Addition of up to 30% cholesterol does not change the nuclear Overhauser effect enhancement or spin-lattice relaxation time of the vesicles. It is argued that the OH group of cholesterol is hydrogen bonded to the ester carbonyl oxygen of the phosphatidylcholine molecules.

Structural studies on phophatidylcholine-cholesterol mixed vesicles

The homogeneous, single-walled phosphatidylcholine-cholesterol mixed vesicles were prepared by ultrasonic irradiation of egg phosphatidylcholine in the presence of various amounts of cholesterol in solution at 4 degrees under a nitrogen atmosphere followed by molecular sieve chromatography on a Sepharose 4B column. Physicochemical studies performed on these systems invluding sedimentation velocity, diffusion, partial specific volume, intrinsic viscosity, and trapped volume measurements allowed estimation of the weight-average vesicle weight, the vesicle shape, and bilayer membrane thickness of the binary mixture of phosphatidylcholine and cholesterol. Vesicle hydration was calculated using two different methods and the agreement between them was excellent up to cholesterol concentration of 0.32 mole fraction. It was observed that the structural parameters change slowly with increasing cholesterol content up to around 0.3 mole fraction and a relatively abrupt structural alteration occurs above this cholesterol content. This abrupt structural change is consistent with the asymmetrical distribution of lipid composition between the inner and outer bilayer face.

Kinetics of a Ca-2+-triggered membrane aggregation reaction of phospholipid membranes

Ca-2+ and other divalent cations can trigger aggregation of phospholipid vesicles containing phosphatidic acid or phosphatidylserine. The reaction, which can be detected by an increase in light scattering, has a critical dependence on the Ca-2+ concentration, with a threshold near 4 mM Ca-2+. This is the concentration for half-saturation of the polar head groups and for full neutralization of the membrane surface charge. The aggregation proceeds as a "polymerization" reaction, eventually forming such large aggregates that the vesicles precipitate. The stopped-flow rapid mixing technique was used to study the vesicle dimerization reaction which is the first step in the overall aggregation process. Vesicle dimerization resulted in a doubling of light scattering and had a vesicle concentration-dependent time constant (t1/2) which varied between 0.4 and 2.0s under the conditions of the study. Analysis of the dependence of the reaction amplitude and l/t 1/2 on the concentrations of vesicles and Ca-2+ showed that the Ca-2+ binding is fast, and that the dimerization proceeds by a mechanism in which the vesicles first collide to form an encounter complex followed by a slower conversion of the encounter complex to a stable complex. For phosphatidic acid vesicles, about 200-700 collisions are necessary to achieve a stable dimer. The rate-limiting step in the overall reaction is thus the transformation of the encounter complex into a stable complex, requiring 0.5 and 1.0 ms. The above-mentioned results are relatively insensitive insensitive to the type of divalent cation or to the choice of negatively charged lipid (phosphatidic acid or phosphatidylserine). Evidence is given that the stable complex is effected by Ca-2+-mediated salt bridges between the two membranes and that the rate constant of the transformation step derives from the statistics of the distribution and the rate of redistribution of Ca-2+-occupied polar head groups on the membrane surfaces. The relevance of these results to the problem of Ca-2+-induced fusion of biological membranes is discussed.

Asymmetric exchange of vesicle phospholipids catalyzed by the phosphatidylcholine exhange protein. Measurement of inside--outside transitions

Purified phosphatidylcholine exchange protein was used to exchange phosphatidylcholine between homogeneous single-walled phosphatidylcholine vesicles and human erythrocyte ghosts. When excess ghosts were present, it was found that only 70% of the vesicle phosphatidylcholine was available for exchange. This fraction corresponds closely to the amount of phosphatidycholine in the outer monolayer of these vesicles, indicating that only the outer surface of the vesicle is accessible to the exchange protein. Also, it was found that all phosphatidylcholine introduced into vesicles by the exchange protein was available for subsequent exchange. Using the exchange protein, asymmetrical vesicles were prepared in which the outer monolayer was either enriched or depleted in radioactive phosphatidylcholine as compared to the inner monolayer. Re-equilibration of the radioactivity between the two surfaces of the vesicle (flip-flop) could not be detected, even after 5 days at 37degrees. It is estimated that the half-time for flip-flop is in excess of 11 days at 37degrees. These results indicate that the properties of the exchange protein can be expolited to measure phosphatidylcholine flip-flop rates and possible phosphatidylcholine asymmetry in biological and model membranes, without altering the structure of the membrane.

13-C nuclear magnetic resonance studies on the lipid organization in enveloped virions (vesicular stomatitis virus)

13-C nuclear magnetic resonance (NMR) studies are described regarding the lipid organization in the envelope of the vesicular stomatitis virion. The fatty acid chains (oleic acid) and the choline moiety of the 3-sn-phosphatidylcholine and spingomyelin have been labeled specifically with 13-C by growing the virions in prelabeled host cells (BHK 21 cells). The results suggest that 130C NMR spectroscopy is a very feasible method for the study of natural membranes provided the isotope is highly enriched in specific positions and incorporated biochemically. Spin-lattice relaxation (T1) measurements of particular C atoms have been carried out with whole virions, with virions deprived of their surface projections by trypsinization but unaltered in their shape and size, and with liposomes prepared from the total lipid mixture of the envelope in order to get insight into the molecular structure of this model membrane. The mobility of the central part of 11-13-C-labeled oleic acid incorporated into the ester and amide lipids and the choline group of 3-sn-phosphatidylcholine and sphingomyelin is very restricted as indicated by their short T1 times. It is concluded from the data presented here that the high cholesterol content (cholesterol/P: 0.7) of the envelope lipid phase is responsible for the rather rigidly packed envelope structure. The mode and extent of the interactions between lipids and glycoprotein surface projections are subjects for further study.

Apparent dependence of interactions between cytochrome b5 and cytochrome b5 reductase upon translational diffusion in dimyristoyl lecithin liposomes

Dimyristoyl lecithin liposomes, containing cytochrome b5 reductase (NADH:ferricytochrome b5 oxidoreductase, EC 1.6.2.2) and varying amounts of cytochrome b5, were used to measure flavoprotein catalysis alone and catalysis requiring electron transfer between the reductase and cytochrome as a function of temperature. Whereas flavoprotein catalysis showed a simple linear temperature dependence in an Arrhenius plot, the reaction involving electron transfer between the two bound enzymes showed a marked, 4-fold, change in rate at the crystalline-liquid crystalline phase transition of the hydrocarbon chains of the lecithin vesicles and a second, minor change involving the minor transition. These data represent strong evidence that protein-protein interactions in this membrane model system are dependent upon translational diffusion of nonpolar segments of the proteins in the hydrocarbon region of the phospholipid bilayer.

A model for the synthesis of glycoproteins

A technique is described for assaying membrane bound enzymes. The results of studies with two sequential glycoprotein:glycosyl transferases are presented. The results are discussed in the light of recent suggestions concerning the structural organisation of these enzymes. A description, of the specificity of these enzymes for high molecular weight substrates, is presented.

The application of nuclear magnetic resonance spectroscopy to the study of natural and model membranes

This review article outlines some potentials and limits of the recent application of high resolution Nuclear Magnetic Resonance technique--coupled to the Fourier transformation methods--to the study of biological membranes. Molecular arrangement and dynamical structure characters can be assessed at the level of individual chemical groups in lipid bilayer regions of natural and model membranes, through the determination of physical parameters like chemical shifts, spin-lattice (T1) and spin-spin (T2) nuclear magnetic relaxation times. The results of some significant experiments carried out on single-wall lecithin vesicles as well as on intact natural membranes, are summarized and discussed. Useful information can be obtained on the lipid fatty-acid chains thermal transition, by comparing two lecithin vesicles of the same size, formed by the same host lecithin, but incorporated with different molecular components. In particular, T1 and T2 measurements, interpreted in terms of a two- (or more-) correlation time theoretical models, are able to demonstrate different degrees of motional anisotropy in bilayers formed by mixed lecithins or by mixtures of lecithin and fatty acids, possessing moderately different chain lengths [13]. Chromophore-containing molecules, like chlorophyll [12] or fluorescent probes [14] can be located, within few Angstroms, in a lipid bilayer through proton chemical shift measurements; in addition the perturbation of the lipid membrane structure, as induced by the incorporated probe, is assessed mainly in terms of the intramolecular dynamical structure of the host lecithin molecules, by means of T1 and linewidth studies. The comparison of the n.m.r. relaxation behaviour in intact membranes and in vesicles formed by their extracted lipids may, finally, provide indirect information on the lipid-protein intermolecular interactions and relative mobility, besides indicating the intramolecular mobility characters of the lipid bilayer regions of the membrane.

Intervesicular phospholipid exchange. An NMR study

Phospholipid exchange between phosphatidylinositol and phosphatidylcholine ?vesicles has been studied by NMR spectroscopy with use of hydrophilic paramagnetic lanthanide probes (Pr-3+ and Eu-3+ ions). The dependence of the lanthanide induced shifts in the1-H and 31-P NMR spectra on the phospholipid composition of the vesicles could be used for its quantitative evaluation. The method has been proved to be applicable for studying phospholipid exchange stimulated by soluble proteins (postmicrosomal supernatant fraction) from rat liver. Furthermore it has been shown that the phospholipid molecules newly introduced by protein-stimulated exchange are predominantly incorporated into the outermonolayer of the vesicular bilayer membrane. This makes it possible to produce liposomes with asymmetric distribution of the phospholipids across the bilayer.

13-C NMR investigation of phospholipid membranes with the aid of shift reagents

13-C NMR spectra of sonicated phospholipid dispersions have been investigated with the aid of paramagnetic ions (Pr-3+) as shift reagents. This method permits discrimination between the outer and the inner surfaces of the vesicular membrane and gives a larger variety of signals than 1-H NMR. The lanthanide induced shifts have been found to depend on other ions present and on the lipid composition of the membrane.

Fusion of fatty acid containing lecithin vesicles

The rate and temperature at which fusion of purified homogeneous lecithin vesicles containing several per cent fatty acid occurs have been determined by analysis of proton nuclear magnetic resonance (nmr) spectra. Dimyristoyllecithin with myristic acid as 2% of the lipid was found to fuse rapidly at temperatures between 17 and 20 degrees, and dimyristoyllecithin with 4% lauric acid was found to fuse rapidly at temperatures between 11 and 15 degrees, while dimyristoyllecithin with 4% palmitic acid did not fuse at an appreciable rate anywhere in the range 17-37 degrees. These results, along with data on dipalmitoyl- and dilauroyllecithin, are discussed in terms of a possible dependence on separation of fatty acid rich phase especially conducive to fusion.

The interpretation of proton magnetic resonance linewidths for lecithin dispersions. Effect of particle size and chain packing

Two previously reported theoretical treatments of the effect of sonication on the PMR spectrum of phospholipid bilayer membranes have led to divergent conclusions regarding the effects of sonication on the structure of the bilayer membrane. In this report these two theoretical treatments will be critically reviewed, and it will be shown that only the theory of Seiter and Chan (Seiter, C.H.A. and Chan, S.I. (1973) J. Am. Chem. Soc. 95, 7541-7553) yields predictions which are in agreement with experiment. Analysis of available and newly acquired NMR results for sonicated bilayer vesicles of different sizes, both above and below the thermal phase transition, indicates that sonication does disrupt the regular molecular packing of the phospholipid molecules in these systems.

The binding of deoxycholate, Triton X-100, sodium dodecyl sulfate, and phosphatidylcholine vesicles to cytochrome b5

Cytochrome b5 is composed of two domains that can be isolated after tryptic cleavage as two polypeptide fragments. One fragment is globular and hydrophilic and contains the heme; the other fragment is rich in hydrophobic amino acids and is essential for recombination of cytochrome b5 with microsomal membranes (Ito, A., and Sato, R. (1968), J. Biol. Chem. 243, 4922; Spatz, L., and Strittmatter, P. (1971), Proc. Nat. Acad. Sci. U.S. 68, 1042). Equilibrium dialysis and sedimentation equilibrium measurements of the binding of deoxycholate, Triton X-100 and dodecyl sulfate show that neither intact cytochrome b5 nor its proteolytic fragments possess high affinity binding sites for any of these amphiphiles. However, each detergent binds to the protein in a highly cooperative manner at concentrations near the critical micelle concentration. Binding measurements using the separated tryptic fragments show that deoxycholate and Triton X-100 (both nondenaturing detergents) bind to the hydrophobic fragment to the same extent as to intact cytochrome b5, and not at all to the polar fragment. Sodium dodecyl sulfate (a denaturing detergent) is bound to both tryptic fragments, but 70% of the detergent is bound to the hydrophobic fragment although it comprises only 30% of the protein mass. Less detailed measurements were made with synthetic and natural phosphatidylcholines, and show that the intact protein is quantitatively incorporated into phosphatidylcholine vesicles, but that no interaction with the polar fragment occurs. These results are interpreted in terms of the hydrophobic domain of cytochrome b5 having a diffuse hydrophobic surface that can act as a nonspecific nucleus for the formation of a micelle with a variety of amphiphilic substances. This domain of the molecule will insert into any available hydrophobic environment, whether it be detergent micelles, synthetic phospholipid vesicles, or the microsomal membrane. The incorporation of cytochrome b5 into the microsomal membrane is only a specialized case of the general property.

Use of phospholipid exchange protein to measure inside-outside transposition in phosphatidylcholine liposomes

The exchange of phosphatidylcholine between (32P)phosphatidylcholine lipososomes and unlabeled mitochondria was catalyzed by a purified phospholipid exchange protein from bovine heart cytosol. The loss of (23P)phosphatidylcholine from the liposomes appeared to proceed in two stages: with 100 units of phospholipid exchange protein per ml the half-time of the initial stage was about 10 min and that of the final stage 4 days or greater. Agarose-gel chromatography of the liposomes showed an elution compatible with a homofwnwoua pool od amLL single walled vesicles. Treatment of phosphatidyl (14C) choline liposomes with phospholipase D (phosphatidylcholine phosphantidohydrolase) showed that labeled phospholipid removable during the rapid exchange phase was subject to hydrolysis by the phospholipase, but that the labeled phospholipid left after the rapid exchange was completed cound not by hydrolyzed by phospholipase D. It is proposed that the rapidly exchanging phosphatidylcholine constitutes the outer layer of the liposome bilayer. The long half-lives of 4 days or more probably represent the transposition of phosphatidylcholine from the inner to the outer layer of the liposome bilayer.

NMR studies on phospholipid bilayers. Some factors affecting lipid distribution

1. 1H-NMR and 31P-NMR are used to measure the outside/inside distribution of phospholipids in mixed vesicles. 2. Ferricyanide is a suitable shift reagent for measuring the outside/inside ratio of lecithin using 1H-NMR even when the phospholipid mixture contains negative lipids. 3. 31P-NMR can be used to measure the distribution of all phospholipids present provided the resonances are separated. 4. At 36.4 MHz the inside and outside phosphorus in lecithin vesicles have different chemical shifts. The separation at room temperature is 4-5 Hz and the individual linewidths are about 4Hz. 5. In a mixture of lecithin with phosphatidylethanolamine the latter has preference for the inside layer of the bilayer. The same holds for mixtures of lecithin with phosphatidylserine, phosphatidylinositol and phosphatidic acid. 6. In mixtures of lecithin and phosphatidylserine the preference of the latter for the inside is increased at lower pH under which conditions the negative charge of the phosphatidylserine is decreased. 7. In mixtures of lecithin with sphingomyelin the lecithin has a higher concentration at the inside. 8. The effect of vesicle size on the 31P-NMR linewidth and the temperature dependence of this linewidth is in agreement with the conclusion of Berden et al. (FEBS Lett. (1974), 46, 55-58) that the chemical shift anisotropy, modulated by the isotropic tumbling of the vesicles, makes a contribution to the linewidth. The chemical shift difference between outside and inside phosphorus can be used as a parameter for the measurement of the packing density at the inside and of the size of the vesicles. 9. It is concluded that both charge and the packing properties of the head group are major factors in determining the distribution of phospholipids in mixed vesicles.

The interaction of atebrin with phospholipid vesicles

The interaction of atebrin with phosphatidylcholine and phosphatidylcholine-phosphatidic acid vesicles has been followed by equilibrium dialysis, and by photometric, fluorimetric and NMR techniques. The presence of negative charges in the phospholipids enhances the binding of atebrin. The absorbance and NMR spectral changes and fluorescence quenching occurring with phosphatidic acid are attributed to dimerization of the dye interacting electrostatically with negative groups. The dissociation constant of the binding of the dye to phosphatidylcholine vesicles was 1.4 mM; those of binding to the negative sites of phosphatidic acid were approx. 150 and 3 muM. The dye is probably located at the interphase with the acridine ring interacting with the anionic groups of phosphatidic acid and the tail freely floating in the aqueous phase. The results are discussed also in view of the use of atebrin as a probe of the energized state in natural membranes and of the suggestion that atebrin may be used as a transmembrane pH indicator in liposomes or natural membranes.

High-molecular-weight DNA polymerases from mouse myeloma. Purification and properties of three enzymes

Cytoplasmic (high-molecular-weight) DNA polymerase was partially purified from mouse myeloma. Upon chromatography on DEAE-Sephadex, following fractionation on phosphocellulose, the enzyme was resolved into three species named CI, CII, and CIII. The species CI and CII have equal sedimentation coefficients (10.5 S) in sucrose gradients without salt. In the presence of 125 mM ammonium sulfate the sedimentation coefficients are reduced to 8.6 S. The species CIII shows sedimentation coefficients of 5.7 S and 5.2 S without salt and in the presence of 125 mM ammonium sulfate, respectively. This species is assumed to be an artifact arising from either CI or to a minor extent from CII. The optima for pH, KCl and Mg2+ concentration, and the extent of inhibition by N-ethylmaleimide are the same. However, the enzymes differ in their responses to Mn2+ (substituting for Mg-2+), and to addition of ethanol, dimethylsulfoxide, and various phospholipids in the assay mixture. The enzymes prefer poly[d(A-T - d(A-T)] or partially degraded (activated) DNA as template rather than double-stranded or single-stranded DNA. The activity on activated DNA relative to that on poly[d(A-T) - D(A-T)] was found to be 93, 66, and 29% for DNA polymerases CI, CII, and CIII, respectively.