Incorporation kinetics of lysolecithin into lecithin vesicles. Kinetics of lysolecithin-induced vesicle fusion

"Staying Out" Rather than "Cracking In": Asymmetric Membrane Insertion of 12:0 Lysophosphocholine

Interactions between detergents and model membranes are well described by the three-stage model: saturation and solubilization boundaries divide bilayer-only, bilayer-micelle coexistence, and micelle-only ranges. An underlying assumption of the model is the equilibration of detergent between the two membrane leaflets. However, many detergents partition asymmetrically at room temperature due to slow flip-flop, such as sodium dodecyl sulfate (SDS) and lysolipids. In this work, we use isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) to investigate the solubilization of unilamellar POPC vesicles by 12:0 lysophosphocholine (12:0 LPC). Flip-flop of 12:0 LPC occurs beyond the time scale of our experiments, which establish a characteristic nonequilibrated state with asymmetric distribution: 12:0 LPC partitions primarily into the outer leaflet. Increasing asymmetry stress in the membrane does not lead to membrane failure, i.e., "cracking in" as seen for alkyl maltosides and other surfactants; instead, it reduces further membrane insertion which leads to the "staying out" of 12:0 LPC in solution. At above the critical micellar concentration of 12:0 LPC in the presence of the membrane, micelles persist and accommodate further LPC but take up lipid from vesicles only very slowly. Ultimately, solubilization proceeds via the micellar mechanism (Kragh-Hansen et al., 1995). With a combination of demicellization and solubilization experiments, we quantify the molar ratio partition coefficient (0.6 ± 0.1 mM^-1) and enthalpy of partitioning (6.1 ± 0.3 kJ·mol^-1) and estimate the maximum detergent/lipid ratio reached in the outer leaflet (<0.13). Despite the inapplicability of the three-stage model to 12:0 LPC at room temperature, we are able to extract quantitative information from ITC solubilization experiments and DLS that are important for the understanding of asymmetry-dependent processes such as endocytosis and the gating of mechanosensitive channels in vitro.

Lysophospholipid-containing membranes modulate the fibril formation of the repeat domain of a human functional amyloid, pmel17

Pmel17 is an important protein for pigmentation in human skin and eyes. Proteolytic fragments from Pmel17 form fibrils upon which melanin is deposited in melanosomes. The repeat domain (RPT) derived from Pmel17 only forms fibrils under acidic melanosomal conditions. Here, we examined the effects of lipids on RPT aggregation to explore whether intramelanosomal vesicles can facilitate fibrillogenesis. Using transmission electron microscopy, circular dichroism, and fluorescence spectroscopy, we monitored fibril formation at the ultrastructural, secondary conformational, and local levels, respectively. Phospholipid vesicles and lysophospholipid (lysolipid) micelles were employed as membrane mimics. The surfactant-like lysolipids are particularly pertinent due to their high content in melanosomal membranes. Interestingly, RPT aggregation kinetics were influenced only by lysolipid-containing phospholipid vesicles. While both vesicles containing either anionic lysophosphatidylglycerol (LPG) or zwitterionic lysophosphatidylcholine (LPC) stimulate aggregation, LPG exerted a greater effect on reducing the apparent nucleation time. A detailed comparison showed distinct behaviors of LPG versus LPC monomers and micelles plausibly originating from their headgroup hydrogen bonding capabilities. Acceleration and retardation of aggregation were observed for LPG monomers and micelles, respectively. Because a specific interaction between LPG and RPT was identified by intrinsic W423 fluorescence and induced α-helical structure, it is inferred that binding of LPG near the C-terminal amyloid core initiates intermolecular association, whereas stabilization of α-helical conformation inhibits β-sheet formation. Contrastingly, LPC promotes RPT aggregation at both submicellar and micellar concentrations via non-specific binding with undetectable secondary structural change. Our findings suggest that protein-lysolipid interactions within melanosomes may regulate amyloid formation in vivo.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Fatty acid contamination and dielectric relaxation in phospholipid vesicle suspensions

Aqueous vesicle or micelle suspensions from various synthetic lecithins or surfactants - most of them purified by a simple ion-exchange procedure in methanol - were investigated, some with ionic admixtures. The dielectric permittivity '(nu) between 5 kHz and 100 MHz was determined by different time-and frequency-domain methods, with attention given to electrode polarization below 1 MHz. Pure ether lecithins (used to reduce hydrolysis during preparation) as well as ester lecithins showed no dielectric dispersion below 10 MHz (Delta' 3). In contrast, even dilute colloidal solutions containing about 1 mol% (with respect to solute) ionic amphiphiles normally exhibited large dielectric dispersion (10 < Delta' < 700), especially with electrolyte present. This low-frequency dispersion is sensitive to vesicle coagulation or fusion. Underlying relaxation mechanisms are discussed, and the main relaxation is shown to be the same as for other charged colloids. This conclusion suggest a new interpretation of measurements, previously reported by other authors, who gave an interpretation in terms of correlated zwitterionic head group orientation in multilamellar lecithin liposomes. Possible effects from traces of impurities in lipids are discussed.

Spectroscopic techniques in the study of membrane solubilization, reconstitution and permeabilization by detergents

This review focuses on the use of spectroscopic techniques for the study of membrane solubilization, reconstitution, and permeabilization by detergents. Turbidity and light scattering, visible and infrared spectroscopic methods, fluorescence, nuclear magnetic resonance, electron spin resonance and X-ray diffraction are examined from the point of view of their applicability to the above detergent-mediated phenomena. A short introduction is provided about each of the techniques, and references are given for further study.

Phospholipase A2-like catalytic antibody

The phospholipase A2-like catalytic antibody 13C2-1F6 was elicited against the hapten 1 as the transition state analog for the hydrolysis of the C2 ester in the phospholipid. The Michaelis-Menten kinetics for the hydrolysis of the phospholipid 2 by 13C2-1F6 afforded a kcat of 1.0 x 10(-2) min(-1) and aKm of 71 microM. This antibody hydrolyzes the C2 ester in (R)-2, regio- and enantioselectively.

Anomalous phase transition in dipalmitoylphosphatidylethanolamine/palmitoylphosphatidylcho line/water system

An anomalous phase transition with a marked rise in specific heat, the isobaric thermal expansion coefficient, and the compressibility coefficient at 62.5 degrees C for an equimolar mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) and 1-palmitoyl-sn-glycero-3-phosphocholine (PLPC), in water (34 wt.%) has been shown by differential scanning calorimetry, scanning dilatometry and isothermal compressibility measurements. This transition occurs 15 degrees C above a first-order transition observed in the same system. (31)P and (2)H nuclear magnetic resonance results are consistent with the occurrence of 'defects' in the bilayer in the temperature range between the first and the anomalous phase transitions. It is proposed that conically, PLPC molecules prefer regions with high curvature in the defective bilayer, while DPPE molecules are mostly confined to the flat regions of the bilayers.

Exchange of monooleoylphosphatidylcholine with single egg phosphatidylcholine vesicle membranes

In a previous paper we described the experiments and the framework of a model for the exchange of monooleoylphosphatidylcholine with a single egg phosphatidylcholine membrane. In the present paper a model is presented that relates the experimentally measured apparent characteristics of the overall kinetics of lysolipid exchange to the true rates of lysolipid exchange and interbilayer transfer. It is shown that the adsorption of the lysolipid follows two pathways: one through the adsorption of lipid monomers and other through the fusion of micelles. The desorption of lysolipid follows a single pathway, namely, the desorption of monomers. The overall rate of fast desorption under convective flow conditions gives the true rate of monomer desorption from the outer membrane monolayer. The overall rate of both slow lysolipid uptake and slow desorption gives the rate of interbilayer transfer. Because of the uneven distribution of lysolipid between the two monolayers during its uptake, one of the membrane monolayers is apparently extended relative to the other. This relative extension of one of the monolayers induces a monolayer tension. The induced monolayer tension can increase up to 7 mN.m-1, when most of the intercalated lysolipid only partitions into the monolayer facing the lysolipid solution. This value is similar to the measured value for the critical monolayer tension of membrane failure, which is on the order of 5 mN.m-1. The similarity of the magnitudes of the induced monolayer tension during monooleoylphosphatidylcholine exchange and the monolayer tension of membrane failure suggests that the interbilayer lipid transfer may be affected by the formation of short living membrane defects. Furthermore, the pH-induced interbilayer exchange of phosphatidylglycerol is considered. In this case, it is shown that the rate of interbilayer transfer is a function of the phosphatidylglycerol concentration in the membrane.

The membrane-perturbing properties of palmitoyl-coenzyme A and palmitoylcarnitine. A comparative study

Fatty acyl-coenzyme A's are temporarily converted into fatty acylcarnitines while transferred across the inner mitochondrial membrane, in their catabolic pathway. In search of an explanation for the need of this coenzyme exchange, the present work describes comparatively the abilities of both kinds of fatty acyl derivatives (represented by palmitoyl-coenzyme A and palmitoylcarnitine) in binding to and perturbing the structure of phosphatidylcholine bilayers in the form of large unilamellar vesicles. Both palmitoyl-coenzyme A and palmitoylcarnitine partition preferentially into the bilayer lipids, so that their free concentration in water is in practice negligible. However, palmitoylcarnitine is able to disrupt the membrane barrier to solutes, leading to vesicle leakage, and, at higher concentrations, it produces complete membrane solubilization, while palmitoyl-coenzyme A produces neither leakage nor solubilization. Palmitoylcarnitine has the properties of many commonly used biochemical detergents. The different behavior of both fatty acyl derivatives helps to explain the need for the transitory coenzyme A/carnitine exchange, and provides a pathogenic mechanism for some genetic defects of mitochondrial fatty acid transport. Other pathophysiological processes in which palmitoylcarnitine has been putatively involved are examined in light of the above results.

Lysolipid exchange with lipid vesicle membranes

While the aqueous solubility for bilayer phospholipids is less than 10(-10) M--keeping lipid membranes at essentially constant mass, single chain surfactants can have a significant aqueous solubility. Thus, in surfactant solutions, both monomer and micelles can interact with a lipid bilayer, and the mass and composition of the bilayer can be changed in seconds. These changes in composition are expected to have direct consequences on bilayer structure and material properties. We have found that the exchange of surfactants like lysolecithin can be described in terms of a kinetic model in which monomer and micelles are transported to the membrane from bulk solution. Molecular transport is considered at the membrane interfaces and across the midplane between the two monolayers of the bilayer. Using micropipet manipulation, single vesicles were transferred into lysolecithin solutions, and the measurement of vesicle area change gave a direct measure of lysolecithin uptake. Transfer back to lysolecithin-free media resulted in desorption. The rates of uptake and desorption could therefore be measured at controlled levels of membrane stress. With increasing lysolecithin concentration in the bulk phase, the amount of lysolecithin in the membrane reached saturation at approximately 3 mol% for concentrations below the critical micelle concentration (CMC) and at > 30 mol% for concentrations above the CMC. When convective transport was used to deliver lysolecithin, uptake occurred via a double exponential: initial uptake into the outer monolayer was fast (approximately 0.2 sec-1); transfer across the bilayer midplane was much slower (0.0019 sec-1).

Early and delayed stages in the solubilization of purple membrane by a polyoxyethylenic surfactant

The purpose of this paper is to explore the reasons by which purple membrane solubilization by detergents takes hours, or even days, to reach equilibrium, while most biomembranes are solubilized in a matter of seconds, or minutes. With that aim, changes in the purple membrane absorption spectrum produced by hydrogenated Triton X-100 under equilibrium conditions (24 h) have been compared to those caused by the same surfactant in the minute, second and sub-second time scale. It is found that the various processes that accompany, or lead to, solubilization are already detected, and even reach an apparent equilibrium, in the 10 s that follow detergent addition. No new phenomena are detected in the following minutes, or hours, that are relevant to the process under study. This leads to the conclusion that the long solubilization process consists of the repeated operation of simple phenomena that are relatively fast in themselves. A hypothesis is proposed according to which the tight crystalline organization of the purple membrane prevents the insertion of detergent monomers in the lipid bilayer; instead, the surfactant would bind the periphery of the patches, i.e., the hydrocarbon-water contact region, and solubilization would take place gradually, from the periphery towards the core of the membrane patches, at a progressively lower rate as the amounts of free detergent and detergent-binding sites are decreased by the previous solubilization steps.

Lipid hydroperoxide-induced peroxidation and turnover of endothelial cell phospholipids

The effects of lipid peroxidation on rabbit aortic endothelial cell phospholipid turnover was studied using linoleic acid hydroperoxide (LOOH). Following treatments with 20-40 microM LOOH, cells prelabeled with either arachidonic acid (20:4) or oleic acid (18:1) showed a movement of these fatty acids out of the phospholipids and into neutral lipid and free fatty acid pools. There was also a release of radioactive free fatty acids and phospholipids into the media, which was significantly increased as compared to cells maintained under standard culture conditions. Fatty acid uptake and distribution among phospholipid pools was also affected by LOOH treatment where incorporation of 20:4 and 18:1 into phosphatidylcholine (PC) decreased, while uptake into phosphatidylinositol (PI) increased after 1 h of incubation with 40 microM LOOH. These effects were also inhibited by vitamin E. In cells prelabeled with 20:4 or 18:1 under conditions where approximately 99% of the fatty acids were incorporated into neutral and phospholipid pools, LOOH treatment produced a decrease in radioactivity associated with PC, while the specific activity of PI increased. The extent of these changes was greater for 20:4 than 18:1, but in each case the effects were inhibited by vitamin E. The temporal pattern of uptake for labeled choline and inositol after LOOH treatments paralleled those found for fatty acid incorporation. These cell responses indicate that induction of lipid peroxidation produces rapid fatty acid release and phospholipid turnover involving repair as well as de novo synthesis. The implications of these effects on turnover of specific phospholipids and cell responses to oxidative stress are discussed.

Quantification of the interaction of lysolecithin with phosphatidylcholine vesicles using bovine serum albumin: relevance to the activation of phospholipase A2

The activity of soluble phospholipase A2 to hydrolyze phosphatidylcholine vesicles increases abruptly after a lag time of several minutes. The onset of this apparent activation event probably results from the accumulation of a threshold mole fraction of the hydrolysis products (lysolecithin and fatty acid) in the bilayer. One important observation relevant to the mechanism of this activation process is the biphasic dependence of the lag time on vesicle concentration. To test whether this dependence can be attributed entirely to the strength of partitioning of the lysolecithin into the phosphatidylcholine bilayer, we estimated the apparent partition coefficient of lysophospholipid in the membrane of phosphatidylcholine vesicles. Based on competition between bovine serum albumin and the vesicles for the lysophospholipid, we estimated the partition coefficient to be about 5.10(-7) for palmitoyl lipids at 39 degrees C and about 9.10(-7) for myristoyl lipids at 22 degrees C. These values were able to rationalize the behavior of the lag time with dipalmitoylphosphatidylcholine vesicles, but they were unable to predict the behavior with dimyristoylphosphatidylcholine. Therefore, it appears that the complete dependence of the lag phase on vesicle concentration must be explained by additional means such as the possible contribution of nascent fatty acid or previously proposed kinetic activation mechanisms.

Reversibility of the activation of soluble phospholipase A2 on lipid bilayers: implications for the activation mechanism

The time-courses of hydrolysis of large vesicles of dipalmitoylphosphatidylcholine were compared using four species of phospholipase A2 (Agkistrodon piscivorus piscivorus, Crotalus adamanteus and Naja naja venoms and porcine pancreatic). In all four cases, the hydrolysis rate suddenly increases 10 to 100-fold at the time (tau) when a specific mole fraction of reaction products has accumulated. The intrinsic fluorescence emission of the three venom enzymes also increases suddenly at time tau. Both the activation and the fluorescence change are reversible with a half-time of about 50 s for the activity and 2 to 6 s for the fluorescence. These reversal rates and the vesicle concentration dependence of tau are considered for monomer and dimer enzyme activation models. Apparently, at least three states of the enzyme exist beyond the initial unbound state: (1) inactive and bound, (2) inactive with high fluorescence and (3) active. The dimer model already contains the necessary number of states but requires that the activation rate be much lower than the reversal rate to account for the vesicle concentration dependence of tau. Success of the monomer model requires an enzyme state additional to those proposed previously. Although these results do not exclude either the monomer or dimer models conclusively, they do impose important constraints on each model.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

Complementary molecular shapes and additivity of the packing parameter of lipids

Physical dimensions of a membrane component influence its phase preference upon hydration. A dimensionless packing parameter, S, given by S = V/al, where V is the hydrocarbon volume, a is the area of the head group, and l is the critical length of the hydrocarbon chain, is useful in determining the phase preference of a lipid, and the value of S usually lies between 0.5 and 1 for bilayers. Here, the value of S is calculated for phosphatidylcholine (PC) and lysophosphatidylcholine (lysoPC) as a function of chain length, and it is shown that diacylPC having an S value of less than 0.74 does not form bilayers. For example, diacylPC, up to a chain length of eight carbon atoms, forms only micelles, whereas higher homologs with S greater than 0.74 form bilayers. It is also shown that when lipid molecules having complementary shapes associate, the value of S becomes additive. Using the additivity of S, a number of experimental results for lipid mixtures can be explained. For example, lysoPC and cholesterol form lamellar structures between 45 and approximately 80 mol% cholesterol, and the additive value of S for this region is between 0.74 and 1. Similarly, the additivity of S shows that the maximum amount of cholesterol that can be incorporated into PC bilayers is 50 mol%, in agreement with experimental studies.

Fluorescence polarization changes with gestational age in amniotic fluid of rabbit and guinea pig

The variation of amniotic fluid microviscosity with gestational age was measured in rat, rabbit, and guinea pig. In rat, the changes followed the same pattern as known for women, microviscosity being high during early and mid-gestation and markedly lowering 12 h before delivery. Surprisingly, an opposite trend was observed in rabbit and guinea pig amniotic fluid. Moreover, the lecithin to sphingomyelin ratio markedly rose in late gestation in all species considered. Qualitative and quantitative analysis of lipids and phospholipids were performed in woman and rabbit amniotic fluids at early and late gestational ages. Among all the parameters measured, the most important differences that can influence the amniotic fluid microviscosity are the presence of very high levels of lysophosphatidylcholine both in early and late gestation in rabbit (much higher than in woman) and the cholesterol to total phospholipid ratio which decreased with gestational age in woman but remained stable in rabbit. The Arrhenius plot of the logarithm of microviscosity against the reciprocal of absolute temperature of mature and immature amniotic fluids from woman and rabbit was also determined. The temperature profiles confirmed the differences in lipid profile between woman and rabbit in early and late gestation which could be quantified on a physicochemical basis by determining the activation energy (delta E) at 25 degrees and 37 degrees C for each curve. This confirmed the opposite patterns in woman and rabbit and showed that amniotic fluid from the immature rabbit was the most fluid.

Kinetic studies on the interaction of phosphatidylcholine liposomes with Triton X-100

Sonicated unilamellar and large multilamellar liposome suspensions have been treated with the non-ionic detergent Triton X-100, and the subsequent changes in turbidity have been studied as a function of time. Sonicated liposome suspensions exhibit an increase in turbidity that takes place in two stages, a fast, low-amplitude one is completed in less than 100 ms, and a slow large-amplitude one occurs in 20-40 s. The first increase in turbidity is associated to detergent incorporation into the bilayer, and the second one, to vesicle fusion. The fast stage may be detected at all detergent concentrations, while the slow one is only seen above the critical micellar concentration of Triton X-100. Both processes may be interpreted in terms of first-order kinetics. Studies of the variation of kexp with lipid and detergent concentration suggest a complex multi-step mechanism. In the case of multilamellar liposomes, a fast increase in turbidity is also seen after detergent addition, which is followed by a slow (20-60 s) decrease in turbidity and a very slow (up to 12 h) large scale decrease in turbidity. These processes do not conform to single-exponential patterns. The fast stage is also thought to reflect surfactant incorporation, while the decrease in turbidity is interpreted as bilayer solubilization starting with the outer bilayer (slow stage) and proceeding through the remaining ones (very slow stage).

Lipid miscibility and size increase of vesicles composed of two phosphatidylcholines

The size increase of small unilamellar vesicles composed of binary mixtures either of saturated fatty acid phosphatidylcholines with different chain lengths or of saturated and unsaturated phosphatidylcholines was found to depend on the miscibility properties of the lipid components. No size increase was detected in vesicles formed by two miscible phosphatidylcholines. In vesicles composed of two lipids which are partially immiscible in the gel state, a size increase was observed at temperatures which mainly overlapped the range of temperatures of the lipid phase transition. The rate of size increase of vesicles composed of two lipids which are immiscible in the gel state was faster than that of vesicles composed of two partially immiscible phosphatidylcholines, and the process occurred not only at the temperature ranges of the lipid phase transition, but also when both lipids were in the gel state. The vesicle size increase process occurred without the mixing of the internal content of the vesicles. A model is proposed in which the presence of 'fractures' between membrane regions of different fluidity and/or lipid composition controls the rate of this process.

Purification and properties of acyl-CoA:1-acyl-sn-glycero-3-phosphocholine-O-acyltransferase from bovine brain microsomes

Acyl-CoA:1-acyl-sn-glycero-3-phosphocholine-O-acyltransferase has been purified approximately 3000-fold from bovine brain microsomes by detergent solubilization followed by ion-exchange and affinity chromatography. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed a single protein of molecular weight 43,000. The specificity of the purified enzyme was studied by measuring the catalytic activity with various lysophospholipids and acyl-CoA derivatives. Of the lysophospholipids tested, only lysophosphatidylcholine was a substrate. Less specificity was exhibited toward the acyl-CoA derivatives, although the enzyme showed a clear preference for arachidonoyl-CoA and little or no activity with palmitoyl-CoA or stearoyl-CoA. High concentrations of arachidonoyl-CoA inhibited the enzyme. The velocity was a sigmoidal function of the concentration of lysophosphatidylcholine (LPC) with little activity obtained below 20 microM LPC. The specificity and kinetic properties of the enzyme were altered, however, by incorporation of the enzyme into liposomes composed of a mixture of phospholipids. Decanoyl-CoA and myristoyl-CoA, which were effective substrates for the soluble enzyme, did not serve as acyl donors for the liposome-bound acyltransferase. Furthermore, the liposome-bound enzyme, in contrast to the soluble form of the enzyme, was active at concentrations of LPC below the critical micelle concentration. The liposome-bound enzyme was also substantially less susceptible to thermal denaturation and proteolytic digestion. This modulation of the acyltransferase activity by interaction with phospholipids may relate to the kinetic properties and the regulation of the enzyme in vivo.

Comparison of the membrane-related effects of cytarabine and other agents on model membranes

The membrane-associated effects of a series of chemotherapeutic and other drugs were examined via differential scanning calorimetry and by their modulation of the action of porcine phospholipase A2 (PLA2) on bilayer substrates. The drugs examined included: cytarabine, amino-glycoside antibiotics, adriamycin, dibucaine, butacaine, and VP-16. The bilayers employed were phase-separated ternary lipid mixtures containing dimyristoylphosphatidylcholine: palmitoyllysolecithin: and either hexadecanoic acid (fatty acid ternary mixture) or hexadecanol (alcohol ternary mixture). Effects of the more hydrophilic drugs (cytarabine and aminoglycoside antibiotics) on the calorimetric profiles of the negatively charged (fatty acid-containing) and the neutral (hexadecanol-containing) ternary lipid mixtures indicate that the interaction of these drugs with biomembranes is likely to be dominated by electrostatic interactions. All of the drugs investigated, including the more hydrophobic adriamycin, dibucaine, butacaine, and VP-16, affected the phase equilibrium in the membrane and exhibited apparent noncompetitive inhibition of the action of PLA2 on bilayers composed of ternary lipid substrates. In addition, cytarabine inhibited fusion of fatty acid-containing ternary mixtures.

CONCLUSIONS

These drug:membrane interactions leading to a shift in the phase equilibria were apparently regiospecific. Hydrophilic drug:membrane interactions included an important electrostatic component. The effects of all of the drugs employed in this study on the action of PLA2 on a bilayer substrate (fatty acid-containing ternary lipid mixture) are hypothesized to be a result of the drug-mediated shift in phase equilibria away from the optimally active phase distribution. As a result, PLA2 binds with normal affinity to the membrane, but its membrane substrate is not catalytically turned over. It is evident that these drugs can directly affect cellular homeostasis in a manner that can show a dependence on the nature of the membrane surface.

Effect of antituberculous calixarenes on phospholipase A2 susceptibility and on fusion of phospholipid bilayers

Some homologous calixarenes or polyoxyethylene ethers that are known to suppress or enhance experimental tuberculous infection (depending on their polyoxyethylene chain lengths) were examined for their effects on phospholipid bilayers. The effect of these solutes is seen at 0.5-50p.p.m., and their effect depends upon their structure as well as that of the phospholipid substrate. The antituberculous compound HOC-12.5 (Macrocyclon) inhibits susceptibility to pig pancreatic phospholipase A2 action and to aggregation/fusion of the ternary co-dispersions of dimyristoyl phosphatidylcholine + 1-palmitoyl lysophosphatidylcholine + palmitic acid (50:11:11 molar proportions). In contrast, the protuberculous compound HOC-60 stimulates these effects. Differential scanning calorimetry suggests that these effects are probably due to modulation of the phase equilibrium in substrate bilayers by these polyethers.

Action of phospholipase A2 on bilayers containing lysophosphatidylcholine analogs and the effect of inhibitors

The effects of several lysophospholipid analogs on the phase properties of codispersions with diacylphosphatidylcholine with or without fatty acids were examined. These ternary codispersions were readily hydrolyzed by phospholipase A2, and they underwent a rapid change in turbidity. Nonideal mixing or phase separation in the ternary codispersions is postulated to be responsible for their enhanced susceptibility to pig pancreatic phospholipase A2, as well as for their tendency to undergo spontaneous change in turbidity, presumably due to spontaneous fusion of the vesicles. Both of these processes were inhibited by a variety of structurally unrelated solutes like n-hexanol and mepacrine. These and other observations are interpreted to suggest that structural defects in bilayers of ternary codispersions are a common locus for the binding of phospholipase A2 and are responsible for the process underlying the change in turbidity. The experiments described here suggest that many of the common inhibitors of phospholipase A2 owe their effects to their ability to modify the quality of the substrate interface, rather than to a direct interaction with the enzyme.

Activation of phospholipase A2 by freshly added lysophospholipids

Reaction progress curves for the hydrolysis of dimyristoylphosphatidylcholine by pig pancreatic phospholipase A2 exhibits a latency phase. Addition of 1-palmitoyllysophosphatidylcholine to the preformed vesicles reduces the latency phase and enhances the binding of phospholipase A2 to the vesicles. In contrast, the binary codispersions prepared from diacylphospholipids premixed with lysophosphatidylcholine do not exhibit such enhanced susceptibility to the phospholipase. This effect appears to be due to organizational defects created by asymmetrical incorporation of lysophospholipid molecules into the outer monolayer of the vesicles, and the action of phospholipase is not observed when the additive is equilibrated in both the monolayers of the vesicles.

Spontaneous and protein-catalyzed transfer of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor) between phospholipid bilayers

1-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor or alkylacetyl-GPC), a bioactive phospholipid that possesses hypotensive, platelet-aggregating and inflammatory properties, is known to be secreted by a variety of cell types. The biological activity of alkylacetyl-GPC is related to a precise chemical structure that implicates interaction with proteins. We have studied the spontaneous and protein-catalyzed transfer of alkylacetyl-GPC between phospholipid vesicles and have demonstrated the following: 1. There are at least two transferable pools of alkylacetyl-GPC in sonicated phospholipid vesicles. 2. These two pools differ in the rate at which they dissociate from the vesicles; one pool equilibrates between donor and acceptor vesicles instantaneously while the other pool is transferred much more slowly. 3. Dialysis of alkylacetyl-GPC between phospholipid vesicles through the aqueous phase is slow. 4. A protein fraction derived from rat lung cytosol catalyzes the transfer of the nonequilibrating pool of alkylacetyl-GPC between phospholipid vesicles; this transfer is superimposed on the spontaneous transfer and is unchanged in experiments using vesicles from which the rapidly equilibrating pool has been removed.

Effect of the lipid phase transition on the kinetics of H+/OH- diffusion across phosphatidic acid bilayers

The kinetics of H+/OH- diffusion across dimyristoyl phosphatidic acid bilayer membranes was measured by following the absorbance of the pH-sensitive indicator Cresol red (o-cresolsulfonphthalein) entrapped in single lamellar vesicles after rapidly changing the external pH in a stopped-flow apparatus. The H+/OH- permeability coefficient was found to be in the 10(-5) to 10(-3) cm . s-1 range. The lipid phase transition has a strong influence on the permeation kinetics as the permeability coefficients in the liquid-crystalline phase are drastically higher. The permeability shows no maximum at the phase transition temperature as is the case for other ions, but displays a similar temperature dependence as water permeation. This is also reflected in the high activation energy of approx. 20 kcal/mol and supports the hypothesis (Nichols, J.W. and Deamer, D.W. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 2038-2042) of H+/OH- permeation via hydrogen bonded water molecules. A second slower kinetic phase is also observed, where the permeation is obviously controlled by counterion diffusion. The temperature dependence of this slow process displays the for ion diffusion characteristic maximum in the permeability at the phase-transition temperature.

Myelin basic protein-enhanced fusion of membranes

Myelin basic protein caused rapid aggregation of vesicles containing acidic phospholipids. Aggregation could be reversed by trypsin digestion of the myelin basic protein. Aggregated vesicles containing gel phase phospholipids or vesicles containing greater than 15 mol% lysolecithin underwent fusion. The extent of fusion was measured by irreversible changes in the light-scattering intensities or diffusion coefficients of the vesicles. Fusion was also measured by the fluorescence quenching which occurred when vesicles containing a covalently bound fluorophore. N-4-nitrobenzo-2-oxa-1,3-diazole, were fused with vesicles containing the covalently bound spin label, 4,4-dimethyl-oxazolidine-N-oxyl. The kinetics of fusion were first order in phospholipid and had half-times of 0.5-5 min depending on lysolecithin composition. This protein-enhanced membrane fusion may provide a valuable model system for studying some types of biological membrane fusions.