Surface area per lipid molecule in the intact membrane of the human red cell

Quantitative Estimation of Cyclotide-Induced Bilayer Membrane Disruption by Lipid Extraction with Mesoscopic Simulation

Cyclotide-induced membrane disruption is studied at the microsecond timescale by dissipative particle dynamics to quantitatively estimate a kinetic rate constant for membrane lipid extraction with a ″sandwich″ interaction model where two bilayer membranes enclose a cyclotide/water compartment. The obtained bioactivity trends for cyclotides Kalata B1, Cycloviolacin O2, and selected mutants with different membrane types are in agreement with experimental findings: For all membranes investigated, Cycloviolacin O2 shows a higher lipid extraction activity than Kalata B1. The presence of cholesterol leads to a decreased cyclotide activity compared to cholesterol-free membranes. Phosphoethanolamine-rich membranes exhibit an increased membrane disruption. A cyclotide's ″hydrophobic patch″ surface area is important for its bioactivity. A replacement of or with charged amino acid residues may lead to super-mutants with above-native activity but without simple charge-activity patterns. Cyclotide mixtures show linearly additive bioactivities without significant sub- or over-additive effects. The proposed method can be applied as a fast and easy-to-use tool for exploring structure-activity relationships of cyclotide/membrane systems: With the open software provided, the rate constant of a single cyclotide/membrane system can be determined in about 1 day by a scientific end-user without programming skills.

Influence of the dimensionality of the periodic boundary conditions on the transport of a drug-peptide complex across model cell membranes

Many research efforts are devoted to improving the efficiency of chemotherapy. One of the aspects is to facilitate the transport of drugs across the cell membranes by attaching the therapeutics to a carrier molecule. The current study focuses on computational investigation of such a system with doxorubicin as the model drug, which is covalently bound to a cell-penetrating peptide. The correct description of its membrane translocation at the molecular level requires proper choice of the model membrane and of the simulation parameters. For the purpose, two phospholipid bilayers are built, one containing solely DPPC and another with mixed lipid content mimicking the composition of a human erythrocyte membrane. Atomistic molecular dynamics simulations are carried out in two types of periodic boundary conditions (2D and 3D PBC), in order to assess the effect of the periodicity dimensionality on the intermolecular interactions. The evolution of some basic characteristics of the bilayers and of the drug-peptide complex is tracked: mass density profiles, electrostatic potentials, lateral diffusion coefficients and areas per lipid, lipid-complex radial distribution functions, secondary structure of the peptide and orientation of the drug relative to the membrane. Thus, the influence of the periodic boundary conditions is quantified and it shows that the mixed system in 3D PBC is the most suitable for analysis of the translocation of the transporting moiety across cell membranes.

DOPC versus DOPE as a helper lipid for gene-therapies: molecular dynamics simulations with DLin-MC3-DMA

Ionizable lipids are important compounds of modern therapeutic lipid nano-particles (LNPs). One of the most promising ionizable lipids (or amine lipids) is DLin-MC3-DMA. Depending on their pharmaceutical application these LNPs can also contain various helper lipids, such as phospho- and pegylated lipids, cholesterol and nucleic acids as a cargo. Due to their complex compositions the structures of these therapeutics have not been refined properly. Therefore, the role of each lipid in the pharmacological properties of LNPs has not been determined. In this work an atomistic model for the neutral form of DLin-MC3-DMA was derived and all-atom molecular dynamics (MD) simulations were carried out in order to investigate the effect of the phospholipid headgroup on the possible properties of the shell-membranes of LNPs. Bilayers containing either DOPC or DOPE lipids at two different ratios of DLin-MC3-DMA (5 mol% and 15 mol%) were constructed and simulated at neutral pH 7.4. The results from the analysis of MD trajectories revealed that DOPE lipid headgroups associated strongly with lipid tails and carbonyl oxygens of DLin-MC3-DMA, while for DOPC lipid headgroups no significant associations were observed. Furthermore, the strong associations between DOPE and DLin-MC3-DMA result in the positioning of DLin-MC3-DMA at the surface of the membrane. Such an interplay between the lipids slows down the lateral diffusion of all simulated bilayers, where a more dramatic decrease of the diffusion rate is observed in membranes with DOPE. This can explain the low water penetration of lipid bilayers with phosphatidylethanolamines and, probably, can relate to the bad transfection properties of LNPs with DOPE and DLin-MC3-DMA.

Understanding the Dynamic Behavior of an Anticancer Drug, Doxorubicin, on a Lipid Membrane Using Multiple Spectroscopic Techniques

The interaction, including the adsorption and embedding, of a widely applied anticancer drug, doxorubicin, with a lipid membrane was investigated. Second harmonic generation and two photon fluorescence were used as a powerful combination capable in revealing this dynamic process at the interface. The adsorption, association, deassociation and embedding of doxorubicin on the lipid membrane were clearly identified based on the consistency in the dynamic parameters revealed by the time dependent second harmonic generation and two-photon fluorescence measurements. This work also presents a new approach for in situ measurement of the adsorption density of doxorubicin on lipid membrane, benefiting from the two-photon fluorescence signal of doxorubicin being significantly altered by its chemical environment. The analysis of the location and molecular density based on the fluorescent efficiency of the chromophores makes the fluorescence measurement a "surface sensitive" technique as well. The analytical procedures used in this work are expected to aid in understanding the interaction between fluorescent molecules and lipid membranes in general.

Liposomal encapsulation of a near-infrared fluorophore enhances fluorescence quenching and reliable whole body optical imaging upon activation in vivo

In the past decade, there has been significant progress in the development of water soluble near-infrared fluorochromes for use in a wide range of imaging applications. Fluorochromes with high photo and thermal stability, sensitivity, adequate pharmacological properties and absorption/emission maxima within the near infrared window (650-900 nm) are highly desired for in vivo imaging, since biological tissues show very low absorption and auto-fluorescence at this spectrum window. Taking these properties into consideration, a myriad of promising near infrared fluorescent probes has been developed recently. However, a hallmark of most of these probes is a rapid clearance in vivo, which hampers their application. It is hypothesized that encapsulation of the near infrared fluorescent dye DY-676-COOH, which undergoes fluorescence quenching at high concentrations, in the aqueous interior of liposomes will result in protection and fluorescence quenching, which upon degradation by phagocytes in vivo will lead to fluorescence activation and enable imaging of inflammation. Liposomes prepared with high concentrations of DY-676-COOH reveal strong fluorescence quenching. It is demonstrated that the non-targeted PEGylated fluorescence-activatable liposomes are taken up predominantly by phagocytosis and degraded in lysosomes. Furthermore, in zymosan-induced edema models in mice, the liposomes are taken up by monocytes and macrophages which migrate to the sites of inflammation. Opposed to free DY-676-COOH, prolonged stability and retention of liposomal-DY-676-COOH is reflected in a significant increase in fluorescence intensity of edema. Thus, protected delivery and fluorescence quenching make the DY-676-COOH-loaded liposomes a highly promising contrast agent for in vivo optical imaging of inflammatory diseases.

Fluid phase lipid areas and bilayer thicknesses of commonly used phosphatidylcholines as a function of temperature

The structural parameters of fluid phase bilayers composed of phosphatidylcholines with fully saturated, mixed, and branched fatty acid chains, at several temperatures, have been determined by simultaneously analyzing small-angle neutron and X-ray scattering data. Bilayer parameters, such as area per lipid and overall bilayer thickness have been obtained in conjunction with intrabilayer structural parameters (e.g. hydrocarbon region thickness). The results have allowed us to assess the effect of temperature and hydrocarbon chain composition on bilayer structure. For example, we found that for all lipids there is, not surprisingly, an increase in fatty acid chain trans-gauche isomerization with increasing temperature. Moreover, this increase in trans-gauche isomerization scales with fatty acid chain length in mixed chain lipids. However, in the case of lipids with saturated fatty acid chains, trans-gauche isomerization is increasingly tempered by attractive chain-chain van der Waals interactions with increasing chain length. Finally, our results confirm a strong dependence of lipid chain dynamics as a function of double bond position along fatty acid chains.

Energetics of peptide (pHLIP) binding to and folding across a lipid bilayer membrane

The pH low-insertion peptide (pHLIP) serves as a model system for peptide insertion and folding across a lipid bilayer. It has three general states: (I) soluble in water or (II) bound to the surface of a lipid bilayer as an unstructured monomer, and (III) inserted across the bilayer as a monomeric alpha-helix. We used fluorescence spectroscopy and isothermal titration calorimetry to study the interactions of pHLIP with a palmitoyloleoylphosphatidylcholine (POPC) lipid bilayer and to calculate the transition energies between states. We found that the Gibbs free energy of binding to a POPC surface at low pHLIP concentration (state I-state II transition) at 37 degrees C is approximately -7 kcal/mol near neutral pH and that the free energy of insertion and folding across a lipid bilayer at low pH (state II-state III transition) is nearly -2 kcal/mol. We discuss a number of related thermodynamic parameters from our measurements. Besides its fundamental interest as a model system for the study of membrane protein folding, pHLIP has utility as an agent to target diseased tissues and translocate molecules through the membrane into the cytoplasm of cells in environments with elevated levels of extracellular acidity, as in cancer and inflammation. The results give the amount of energy that might be used to move cargo molecules across a membrane.

Protein area occupancy at the center of the red blood cell membrane

In the Fluid Mosaic Model for biological membrane structure, proposed by Singer and Nicolson in 1972, the lipid bilayer is represented as a neutral two-dimensional solvent in which the proteins of the membrane are dispersed and distributed randomly. The model portrays the membrane as dominated by a membrane lipid bilayer, directly exposed to the aqueous environment, and only occasionally interrupted by transmembrane proteins. This view is reproduced in virtually every textbook in biochemistry and cell biology, yet some critical features have yet to be closely examined, including the key parameter of the relative occupancy of protein and lipid at the center of a natural membrane. Here we show that the area occupied by protein and lipid at the center of the human red blood cell (RBC) plasma membrane is at least approximately 23% protein and less than approximately 77% lipid. This measurement is in close agreement with previous estimates for the RBC plasma membrane and the recently published measurements for the synaptic vesicle. Given that transmembrane proteins are surrounded by phospholipids that are perturbed by their presence, the occupancy by protein of more than approximately 20% of the RBC plasma membrane and the synaptic vesicle plasma membrane implies that natural membrane bilayers may be more rigid and less fluid than has been thought for the past several decades, and that studies of pure lipid bilayers do not fully reveal the properties of lipids in membranes. Thus, it appears to be the case that membranes may be more mosaic than fluid, with little unperturbed phospholipid bilayer.

Membrane association of glutathione S-transferase mGSTA4-4, an enzyme that metabolizes lipid peroxidation products

Lipid peroxidation products have signaling functions and at higher concentrations are toxic and may trigger cell death. The compounds are metabolized predominantly by glutathione S-transferases exemplified by mGSTA4-4, an enzyme highly efficient in glutathione conjugation of 4-hydroxyalkenals, and possessing glutathione peroxidase activity toward phospholipid hydroperoxides. mGSTA4-4 belongs to the predominant group of "canonical" glutathione S-transferases that are soluble and generally localized in the cytoplasm. The intracellular localization of mGSTA4-4 was examined in hepatocytes of normal mouse liver and in transfected HepG2 cells by fluorescence microscopy and digital deconvolution. mGSTA4-4 was found to be predominantly localized at or near the plasma membrane in transfected HepG2 cells, as well as in hepatocytes endogenously expressing the protein. In vitro, mGSTA4-4 associated with liposomes, and this interaction was potentiated when the liposomes contained negatively charged phospholipids. Mutating lysine 115 to glutamic acid resulted in a loss of the plasma membrane targeting of mGSTA4-4 as well as in a significant reduction of its binding to liposomes in vitro. These data suggest preferential targeting of mGSTA4-4 to the plasma membrane that may contain the major substrate(s) for this enzyme. Lysine 115 is critically important for the membrane association of mGSTA4-4, most likely by entering into an electrostatic interaction with negatively charged phospholipid headgroups.

Helical membrane protein folding, stability, and evolution

Helical membrane protein folding and oligomerization can be usefully conceptualized as involving two energetically distinct stages-the formation and subsequent side-to-side association of independently stable transbilayer helices. The interactions of helices with the bilayer, with prosthetic groups, and with each other are examined in the context of recent evidence. We conclude that the two-stage concept remains useful as an approach to simplifying discussions of stability, as a framework for folding concepts, and as a basis for understanding membrane protein evolution.

Reconstitution of beef heart mitochondrial F0F1 in reverse phase evaporation vesicles

Beef heart mitochondrial F0F1 was reconstituted in proteoliposomes by a new procedure. MF0F1 was inserted in preformed reverse phase evaporation vesicles of large diameters prepared from asolectin (MF0F1-REV). Reconstitution was mediated by Triton X-100, which was subsequently removed by treatment with Bio-Beads. Parameters which resulted in optimal reconstitution were described. The MF0F1-REV proteoliposomes catalyzed an exchange between Pi and ATP and were capable of proton pumping. Both reactions were inhibited by oligomycin and uncoupler of oxidative phosphorylation. The range of Pi-ATP exchange activity of the proteoliposomes (70-110 nmol min[-1] mg[-1]) compared favorably with activities obtained in vesicles reconstituted by cholate dialysis or cholate dilution. The most important aspect of this method is that, unlike other reconstitution methods, exogenous F1 and other coupling factors are not required to obtain high Pi-ATP exchange activity by MF0F1-REV. This simple and rapid reconstitution procedure should be useful for future studies dealing with functional analysis of MF0F1.

Interaction between proteins localized in membranes

We present a conceptual framework for evaluating the effect on the self-association of proteins in membranes due to the presence of other proteins at high concentrations (excluded volume effect) and the high concentration and preoriented state of the reactive species. We have calculated the magnitude of such effects using plausible values for the concentrations of proteins in membranes, for the degree to which proteins may tilt and move vertically, and for their dimensions. Compared to the association of monomers tumbling freely in an experimentally realistic volume, we calculate that these factors alone can increase the likelihood of forming dimers 10(6)-fold and of forming trimers and higher oligomers many orders of magnitude greater. We discuss the implications of our calculations for experimental manipulations of membrane proteins, for biosynthetic assembly of multisubunit membrane proteins and formation of membrane lesions by assemblies of exogenous proteins, and for the activation of cellular events induced by the interaction of membrane receptors with themselves or with other membrane proteins.

Density of newly synthesized plasma membrane proteins in intracellular membranes II. Biochemical studies

Using two independent methods, incorporation of radioactive amino-acid and quantitative immunoblotting, we have determined that the rate of synthesis of each of the Semliki Forest virus (SFV) proteins in infected baby hamster kidney (BHK) cells is 1.2 X 10(5) copies/cell/min. Given the absolute surface areas of the endoplasmic reticulum and Golgi complex presented in the companion paper (Griffiths, G., G. Warren, P. Quinn , O. Mathieu - Costello , and A. Hoppeler , 1984, J. Cell Biol. 98:2133-2141), and the approximate time spent in these organelles during their passage to the plasma membrane (Green J., G. Griffiths, D. Louvard , P. Quinn , and G. Warren 1981, J. Mol. Biol. 152:663-698), the mean density of each viral protein in these organelles can be calculated to be 90 and 750 molecules/micron 2 membrane, respectively. In contrast, we have determined that the density of total endogenous integral membrane proteins in these organelles is approximately 30,000 molecules/micron 2 so that the spike proteins constitute only 0.28 and 2.3% of total membrane protein in the endoplasmic reticulum and Golgi, respectively. Quantitative immunoblotting was used to give direct estimates of the concentrations of one of the viral membrane protein precursors (E1) in subcellular fractions; these agreed closely with the calculated values. The data are discussed with respect to the sorting of transported proteins from those endogenous to the intracellular membranes.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

New models of cellular control: membrane cytoskeletons, membrane curvature potential, and possible interactions

The concepts of "membrane cytoskeletons" (proteins attached to the cytoplasmic face of the membrane to give rigidity and control of lateral protein diffusion) and of membrane curvature potential are briefly reviewed. Possible modes of attachment of the membrane cytoskeleton to the bilayer are discussed, and a detailed calculation of possible sources of membrane curvature potential in the red cell is made. The 2 control systems are then used to illustrate possible mechanisms for some cellular processes, such as vesicle formation and release, pseudopod formation, and red cell aging. It is concluded that combination of these concepts allows control mechanisms which appear to act at a distance, or have other unusual systems properties.

The plasma membrane consists of two polar-nonpolar-polar leaflets

The implications of a double polar-nonpolar-polar leaflet construction of the plasma membrane are investigated. Experimental data from transmission electron microscopic and enzymologic characterization of plasma membranes are advantageously interpreted in these terms compared to interpretation in terms of lipid bilayer. X-ray diffraction and electron spin resonance studies do not differentiate between the present and previous models for the structure of plasma membranes but electron spin resonance data that fail to indicate a statistical distribution of spin labels also fail to support the fluid mosaic model for cell membranes. Results from experiments involving vectorial digestion and labelling of plasma membranes as well as freeze fracture electron microscopic data are compatible with the present model. The molecular composition of the human erythrocyte membrane is investigated whereby the band III protein and glycophorin are suggested to be the structural proteins of the outer leaflet and the spectrins those of the inner leaflet.

Axoplasm architecture and physical properties as seen in the Myxicola giant axon

A technique is described for extracting axoplasm from the giant axon of a marine worm, Myxicola infundibulum. The operation can be completed in 10 sec. 2. Axoplasm is pulled from the axon of a living worm as a long, clear cylinder, up to 35 cm long and 70 mg wet weight. The worm regenerates a new giant axon in about 4 months. 3. Myxicola axoplasm is a gel, 87% water, held together by protein neurofilaments. It contains small amounts of mitochondria and vesicles, but no detectable microtubules. 4. The internal structure of the gel is superficially similar to that of yarn. Closer inspection with light and electron microscopy, and X-ray diffraction, show it to be organized in a hierarchy of helical forms. Squid giant axons have a similar structure. 5. Initial estimates of the bulk physical properties of extracted Myxicola axoplasm give: breaking strength, 1400 g/cm2; specific gravity, 1-05 g/cm3; birefringence, 1-6 X 10(-4); index of refraction, 1-351; resistivity, 57 omega cm. These average values are shown to be compatible with the observed structure and composition. 6. Despite its mechanical strength, the axoplasm gel is so hydrated that Na+, K+ and homarine diffuse through it at rates approaching those in free solution. Fewer than about 5% of each of these ions are tightly bound to the gel. 7. It is argued that (a) the structure and physical properties of Myxicola axoplasm are representative of those in other axons, (b) the compound helix architecture results from twist of parallel, cross-linked fibrous proteins, and (c) this sturcture serves as a flexible internal skeleton for nerve cell processes.

The isolation and characterization of nuclear ghosts from cultured HeLa cells

Macromolecular complexes, which appear as ghosts when viewed by phase contrast microscopy, have been isolated from the nuclei of HeLa cells grown in culture. The preparation of these ghosts involves a detergent wash which removes the unit membranes of the nuclear envelop structure but leaves intact both the nuclear pores and the dense structure conferring nuclear margins (possibly the dense lamella). Detergent-washed nuclei are subsequently treated with 0.5 M MgCl2 and fractionated on continuous sucrose gradients containing 0.5 M MgCl2. The ghosts are recovered as a sharp band at an apparent sucrose density of 47-52% and consist of 72% protein, 10% phospholipid, 14% DNA, And 4% RNA. The release of the majority of intranuclear components is indicated by the large loss of nuclear DNA (95%), RNA (71%), and protein (87%) contrasted to the small loss of phospholipid (27%) druing the conversion of detergent washed nuclei to isolated ghosts. Sodium dodecyl sulfate-polyacrylamide gel patterns of the ghost proteins consist of two major bands with approximate molecular weights of 20,000 and 35,000. The isolation of ghosts with a similar density and protein composition from nondetergent-washed nuclei indicates that the ghost is not an artifact induced by the detergent treatment. The absence of cytoplasmic contamination in the preparations of detergent washed nuclei and nuclear ghosts was demonstrated by chemical, enzymatic, and electron microscope studies. We suggest that the isolated ghosts represent a structural macromolecular complex which underlies and is probably attached to the inner nuclear membrane of intact nuclei. The possible additional presence of intranuclear network proteins has not been excluded.

Lipids in Viruses

This chapter discusses lipids in viruses. Lipid forms an integral part of many viruses and exists either in the form of a continuous envelope or in lipoprotein complexes that surround a nucleoprotein core or helix. In general, the envelope can be described as a molecular container for the genetic material of the virus. Viruses are obligate intracellular parasites and are not known to carry genetic coding for enzymes involved in lipid synthesis. Hence, they generally contain the same classes of lipid as are found in the host cell or their membrane of assembly. Lipids make up 20–35% by weight of most viruses; however, there are exceptions such as vaccinia virus, which has only 5% lipid despite having a complex multimembrane envelope structure. Naked herpesvirus capsids closely resemble non-lipid-containing viruses such as adenovirus or polyoma virus, which are also assembled in the nucleus but show full infectivity without any envelope. Both naked and enveloped herpesvirus particles are found in infected cells; however, only enveloped particles are found in extracellular fluids.

Formation of bimolecular membranes from lipid monolayers and a study of their electrical properties

Bimolecular membranes are formed from two lipid monolayers at an air-water interface by the apposition of their hydrocarbon chains when an aperture in a Teflon partition separating two aqueous phases is lowered through the interface. Formation of the membrane is monitored by an increase of the electrical capacity, as measured with a voltage clamp. Electrical resistance of the unmodified membrane is analogous to that of conventional planar bilayers (black lipid membranes) prepared in the presence of a hydrocarbon solvent, i.e., 10(6)-10(8) ohm cm(2); the resistance can be lowered to values of 10(3) ohm cm(2) by gramicidin, an antibiotic that modifies the conductance only when the membranes are of biomolecular thickness. In contrast to the resistance, there is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayers made by phase transition; the average values are 0.9 and 0.45 muF cm(-2), respectively. The value of 0.9 muF cm(-2) approximates that of biological membranes. Assuming a dielectric constant of 2.1 for the hydrocarbon region, the dielectric thickness, as calculated from a capacity of 0.9 muF cm(-2), is 22 A. This value is 6-10 A smaller than the actual thickness of the hydrocarbon region of bilayers and cell membranes, as determined by x-ray diffraction. The difference may be due to a limited penetration of water into the hydrocarbon region near the ester groups that would lower the electrical resistance of this region and reduce the dielectric thickness. Asymmetric membranes have been formed by adjoining two lipid monolayers of different chemical composition.

Loss of membrane components in the pathogenesis of antibody-induced spherocytosis

The infusion of hyperimmune agglutinating antibodies into man or animals causes spherocytosis and hemolysis. The mechanism of spherocytosis was studied in rats given rabbit anti-rat red cell antiserum intravenously. During the 18 hr after antibody infusion, a time before the onset of reticulocytosis, hematocrits fell from 40.6 to 27.6%. However, no change occurred in mean cell volume, mean cell hemoglobin content, or the red cell concentrations of potassium or adenosine triphosphate (ATP). There was a progressive loss of membrane constituents and membrane surface area which followed first order kinetics. At 18 hr membrane cholesterol had decreased 23.5%, phospholipid 26.3%, protein 4.7%, and surface area (calculated from a measure of osmotic fragility) 14.2%. There was no change in the per cent composition of the various phospholipids. Similar changes occurred in animals splenectomized before receiving antibody.These studies demonstrate that spherocytosis induced by heterologous agglutinating antibodies in vivo results from a loss of surface area with no accompanying change in cell volume or in the concentration of the major intracellular constituents. It is caused by a process acting at the cell surface leading to the loss of lipid-rich, protein-poor components of the red cell membrane.

Calcium binding by human erythrocyte membranes

1. The characteristics of Ca(2+) binding to haemoglobin-free human erythrocyte membranes were investigated by using (45)Ca and centrifugation partition of ;ghosts' from their external incubation medium. Equilibrium of ;ghosts' with external Ca(2+) required less than 15min. 2. The binding did not vary with temperature in the range 0-37 degrees C. 3. At pH7.4 ;ghosts' bound a maximum of 283mumol of Ca(2+)/g of ;ghost' protein, equivalent to 6.85x10(7) Ca(2+) ions per cell. 4. Increasing the ionic strength from 0.01 to 0.46 diminished Ca(2+) binding, as did ATP in concentrations ranging from 0 to 15mm in the incubation medium. 5. An increase of the pH from 3.0 to 9.3 caused a marked increase in the amount of Ca(2+) bound. 6. Extraction of (45)Ca-labelled ;ghosts' with chloroform-methanol showed that the distribution of Ca(2+) was: 79% protein-bound, 16% lipid-bound, 5% in the aqueous phase, presumably non-bound. Most of the lipid-bound Ca(2+) (about 80%) was associated with a phospholipid fraction containing phosphatidylserine, phosphoinositides and phosphatidylethanolamine, giving a molar Ca(2+): phosphorus ratio of about 1:2.