No abstract

The surface vapor pressure-composition diagrams for a variety of binary lipid mixtures have been determined and shown, from the thermodynamics of the mixing process, to be analogous to those for bulk systems. It was observed that (a) two liquid-condensed or two liquid-expanded lipids gave ideal mixing, and (b) the mixing of a liquid-condensed with a liquid-expanded lipid gave either positive deviations from ideal behavior or phase separation (immiscibility). These results are shown to be consistent with regular solution theory, with the mixing process dominated by the hydrocarbon region of the film. Polar group interactions do not contribute significantly to the mixing process. Regular solution theory predicts that mixtures of cholesterol with either a liquid-expanded or a liquid-condensed lipid will show large positive deviations from "ideal" behavior, or phase separation. This is in agreement with the vapor pressure-composition studies. There is no evidence for specific interactions between cholesterol and neutral lipids in surface films.

The optical reflectivity of several well-characterized lipid bilayer systems has been correlated with calorimetric studies of the membrane components. There is a large increase in mean membrane thickness when a bilayer is cooled below the transition temperature of the membrane lipid. Similar studies on membranes generated from a mixture of two lipids possessing different degrees of unsaturation suggest that between the characteristic transition temperatures of the two lipids, the bilayer contains clusters of gel and liquid crystalline lipid which coexist within the plane of the membrane.

The adhesion of artificially generated lipid membrane vesicles to Chinese hamster V79 fibroblasts in suspension was used as a model system for studying membrane interactions. Below their gel-liquid crystalline phase transition temperature, vesicles comprised of dipalmitoyl lecithin (DPL) or dimyristoyl lecithin (DML) adsorbed to the surfaces of EDTA-dissociated cells. These adherent vesicles could not be removed by repeated washings of the treated cells but could be released into the medium by treatment with trypsin. EM autoradiographic studies of cells treated with [3H]DML or [3H]DPL vesicles showed that most of the radioactive lipids were confined to the cell periphery. Scanning electron microscopy and fluorescence microscopy further confirmed the presence of adherent vesicles at the cell surface.Adhesion of DML or DPL vesicles to EDTA-dissociated cells modified the lactoperoxidase-catalyzed iodination pattern of the cell surface proteins; the inhibition of labeling of two proteins with an -60,000-dalton mol wt was particularly evident. Incubation of cells with 3H-lipid vesicles followed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis showed that some of the 3H-lipid migrated preferentially with these 60,000-mol wt proteins.Studies of the temperature dependence of vesicle uptake and subsequent release by trypsin showed that DML or DPL vesicle adhesion to EDTA-dissociated cells increased with decreasing temperatures. In contrast, cells trypsinized before incubation with vesicles showed practically no temperature dependence of vesicle uptake.These results suggest two pathways for adhesion of lipid vesicles to the cell surface-a temperature-sensitive one involving cell surface proteins, and a temperature-independent one. These findings are discussed in terms of current models for cell-cell interactions.The biochemical and physical properties of cell surface membranes are of fundamental importance to our understanding of the complex phenomena underlying cell-cell interactions. One approach toward elucidating these properties is to substitute a synthetic membrane surface of known

Summary. An analytical technique is described for direct determination of the molecular composition of lipid bilayer membranes in aqueous solution. Membranes formed from chemically pure, radioactively labeled components, were sampled by pipetting a mercury droplet through the bilayer-water interface. During this procedure, the membrane remains intact but decreases in area with a concomitant increase in the area of the surrounding bulk phase. It is shown that each mercury droplet is covered with a fragment of the bilayer membrane in the form of a closed vesicle. The chemical composition of the bilayer is determined from an analysis of the readioactivity on the mercury droplet.Bilayers generated from glyceryl monooleate in n-decane or n-hexadecane contain (4.7 -1-0.4) x 1014 molecules of monoglyceride per cm 2 and a minimum of (2.8; ___ 0.7) x 1014 molecules of solvent (n-hexadecane) per cm 2. It is estimated from these numbers that 37 vol % of the hydrocarbon core of the bilayer is occupied by solvent.The composition relationships between the bilayer and bulk membrane-forming solution were determined for mixtures of glyceryl monooleate (GMO) with cholesterol (Chol) or glyceryl monostearate (GMS). It was found that [GMS/GMO]bilayer-----[GMS/GMOlbulk, and [Chol/GMO]bilayer=0.5[Chol/GMO]bul k. While the molecular areas of glyceryl monooleate and glyceryl monostearate are unchanged in the mixed system, the average area for mixtures of cholesterol and glyceryl monooleate is decreased, suggesting a condensing effect of the sterol in the bilayer analogous to that observed in lipid monolayers.Extensive studies on phospholipid model systems in aqueous solution [16,19] and the recent use of physical techniques to probe the organization of natural membranes [11,18] strongly support the existence of a bimolecular lipid lamella as a mosaic element in many biological membranes [8]. Indeed, a number of important membrane-associated functions including facilitated diffusion [15], cation discrimination [21], and electrical excitability [12] have been observed in synthetic lipid bilayer membranes separating two aqueous phases either in unmodified systems or in membranes