Liposome-cell interaction: transfer and intracellular release of a trapped fluorescent marker

A calmodulin antagonist increases the apparent rate of endocytosis of liposomes bound to MHC molecules via monoclonal antibodies

We have investigated the molecular mechanisms required for endocytosis of MHC-encoded proteins by a cell line, TRH 42, that expresses endogenous murine and introduced human class I molecules. As probes we have used protein A-bearing liposomes which bind to cell surface determinants via monoclonal antibodies. The technique of fluorescence quenching release was used with liposome encapsulated quenched carboxyfluorescein as the marker for endocytosis. We demonstrate that the calmodulin antagonist trifluoperazine (TFP) enhances the apparent rate of endocytosis of liposomes bound to MHC class I molecules. Drugs that interfere with energy metabolism, microfilament organization, or phospholipase A2 activity all block endocytosis both in the presence and absence of TFP. The requirement of extracellular Ca2+ for endocytosis was found to be partial. The implications for the structural and enzymatic requirements of endocytosis of MHC class I molecules are discussed.

Lateral phase separation of phospholipids as a basis for increased permeability of membranes towards fluorescein and other chemical species

Using mouse spleen cells, before and after treatment with glutaraldehyde or mild hyperthermia, we observe a strong correlation between permeability to fluorescein and susceptibility to staining with N epsilon-dansyl-L-lysine (irrespective of the cells' ability to exclude trypan blue). We observe the same correlation using liposomes prepared from phosphatidylcholine and varying amounts of cholesterol. We have recently introduced N epsilon-dansyl-L-lysine as a fluorescent membrane stain, or "probe," whose uptake, we propose, is selective for phospholipid domains in membranes (G.M.K. Humphries & J.P. Lovejoy Biophys. J. 42:307-310, 1983; G.M.K. Humphries & J.P. Lovejoy J. Membrane Biol. 77:115-122, 1984). The results presented here are consistent with the hypothesis that the presence or absence of phospholipid domains in membranes also modifies their permeability toward fluorescein, and suggests that permeability towards other chemical species may be similarly affected. On the basis of work using liposomes, we believe this to be the case for carboxyfluorescein and for glucose.

Origin of a fluorescence increase accompanying the limited proteolysis of fluorescein-labeled human prothrombin by Factor Xa

In a search for a probe which would report its proteolysis to thrombin, the human blood coagulation zymogen prothrombin was covalently labeled with fluorescein. Fluorescein isothiocyanate (FITC) and dichlorotriazinylaminofluorescein (DCTAF) both introduced approximately 1 molecule of dye, but labeling occurred at different locations, as FITC had no effect on clotting activity whereas DCTAF caused 95% inactivation. At pH 9.0 DCTAF, but not FITC, could induce labeling up to 4 mol/mol. All derivatives were activated normally by prothrombinase (the activating complex of Factor Xa, Factor V(a), Ca2+ and phospholipids), as indicated by the pattern of bands on SDS gel electrophoresis and an unaltered yield of activity toward a chromogenic substrate for thrombin. Upon undergoing this limited proteolysis, the most heavily labeled derivative showed a 40% increase in fluorescence of the fluorescein at 520 nm (lambda ex 480 nm). In contrast, the fluorescence of lightly labeled forms was more intense but increased by only 0-5% upon activation. The data suggest that the lower fluorescence of the most labeled form is due to an intramolecular quenching effect between the dye molecules on individual polypeptide chains that is partly relieved when activation occurs.

Hairpin configuration of H-2Kk in liposomes formed by detergent dialysis

H-2Kk is a transmembrane glycoprotein having the N-terminal region of the heavy chain exposed at the cell surface and the C-terminal region exposed at the cytoplasmic face in its native configuration in the plasma membrane. The configuration of H-2Kk in liposomes formed by detergent dialysis was investigated by using fluorescently labeled H-2 and Co2+ ions to quench fluorescence. H-2Kk was incorporated into sealed lipid vesicles when deoxycholate was removed by dialysis from a mixture of protein and lipid. Including 20 mM carboxyfluorescein (CF) in the mixture prior to dialysis resulted in CF trapped inside the vesicles at concentrations where self-quenching occurred. Vesicles with CF trapped inside were shown to be osmotically active and impermeable to Na+ and Co2+ ions. In order to examine the configuration of H-2Kk in these liposomes, the heavy chain was covalently labeled by using the sulfhydryl reactive fluorescent reagents fluorescein-5-ylmaleimide (NFM) or 5-[[2-[(iodoacetyl)amino]-ethyl]amino]naphthalene-1-sulfonic acid (IAEDANS). In both cases, approximately equal amounts of fluorescent label were incorporated into the N- and C-terminal regions of the protein. Incorporation of the labeled H-2 into liposomes and examination of the effect of Co2+ on the fluorescence showed that all of the label was accessible to quenching by Co2+ and thus exposed on the outside of the liposome. The results demonstrate that the H-2Kk is incorporated into these liposomes in a hairpin configuration, not in the transmembrane configuration found in native membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Liposomes as targets for granule cytolysin from cytotoxic large granular lymphocyte tumors

Purified cytoplasmic granules from rat large granular lymphocyte tumors having natural killer activity and/or antibody-dependent cell-mediated cytotoxicity induced a rapid, dose-dependent release of the water-soluble marker carboxyfluorescein from liposomes made of phosphatidylcholine. A solubilized, partially purified cytolytic preparation termed "cytolysin" from these granules showed identical properties. Marker release induced by granules or the cytolysin was strongly dependent on the presence of Ca2+ at a concentration of 0.1 mM or higher in the medium; Ca2+ could be replaced by higher concentration of Sr2+ but not by Ba2+ or by Mg2+. These properties strikingly parallel the lytic effects that granules and granule cytolysin exert on cells. Marker release from liposomes was stopped instantaneously when an excess of EGTA was added to the medium. The remaining carboxyfluorescein inside the liposomes was present at the original internal concentration, indicating that marker release was all-or-none from individual liposomes. Liposomes comprised of lipid in the solid phase released marker more slowly than did comparable liposomes containing fluid-phase lipids. Variation of the lipid headgroup had only minor effects on the cytolysin-induced marker release. Electron microscopy of liposomes exposed to cytolysin in the presence of Ca2+ showed cylindrical structures of 15-nm diameter inserted into the membrane concomitant with the penetration of negative stain into the liposome. These properties of large granular lymphocyte granule cytolysin strongly suggest that it operates through a mechanism similar to the membrane attack of complement.

Transmucosal passage of liposomally-entrapped drugs in rat small intestine

Intestinal absorption of liposomally-entrapped drugs was investigated for egg yolk phosphatidylcholine-cholesterol (2:1 by molar ratio) liposomes (EggPC liposome) and distearoylphosphatidylcholine-cholesterol (2:1) liposomes (DSPC liposome). The release of carboxyfluorescein, an aqueous phase marker, induced by the presence of everted rat intestine was 40 % and 6 % in one hour from DSPC liposomes and EggPC liposomes, respectively, and it is suggested that EggPC liposomes are more stable in the intestinal lumen. The transport of a liposomally-entrapped drug was examined with fluoresceinisothiocyanate-conjugated dextran (FITC-D) as a model drug that has a small mucosal-to-serosal clearance because of its high average molecular weight (64200). The clearance of FITC-D entrapped in DSPC liposomes was largely reduced and could be accounted for by the clearance of the extraliposomal FITC-D concentration in the preparation. On the other hand, the calculated clearance of EggPC liposome-associated FITC-D was similar to or even higher than that of free FITC-D. The serosal appearance of the EggPC liposome-associated drug was inhibited by colchicine, cytochalasin B, and iodoacetate, suggesting that the liposome was incorporated into the epithelial cells by endocytosis. However, the observation that a lipid phase marker, (14)C-dipalmitoylphosphatidylcholine, failed to be transported into the serosal fluid indicates the absence of the penetration by an intact liposomal form.

Lipoprotein lipase- and phospholipase A2-catalyzed hydrolysis of phospholipid vesicles with an encapsulated fluorescent dye effects of apolipoproteins

The self-quenching dye, 6-carboxyfluorescein, has been encapsulated into sonicated vesicles of egg phosphatidylcholine. Porcine pancreatic phospholipase A2 and bovine milk lipoprotein lipase catalyze the hydrolysis of the phosphatidylcholine resulting in the release of the encapsulated dye and a large increase in 6-carboxyfluorescein fluorescence. The fluorescence increase occurs in parallel with the formation of lysophosphatidylcholine and is strongly dependent on Ca2+ for phospholipase A2 catalysis and on apolipoprotein C-II for hydrolysis by lipoprotein lipase. Other apolipoproteins, including apolipoproteins C-III, C-I, and A-I, do not enhance lipoprotein lipase activity towards this substrate. We conclude that the enhancement of lipoprotein lipase activity by apolipoprotein C-II is a specific property of the activator protein due to its interaction with lipoprotein lipase or an enzyme/lipid interface and not a characteristic of lipid-binding proteins in general.

Attachment of immunoglobulin to liposomal membrane via protein carbohydrate

A general method has been developed for the covalent attachment of immunoglobulin molecules to the outer layer of liposomal membranes. Aldehyde groups are generated by the mild oxidation with periodate or galactose oxidase of the carbohydrate groups on the constant region of the heavy chain. The oxidized protein is then reacted with a hydrazide group linked to a membrane component. Detailed studies were carried out with monomers of a monoclonal human IgM and two monoclonal murine IgM antibodies specific for the 1-dimethylaminonaphthalene-5-sulfonyl (Dns) group. Two hydrazide-containing hydrophobic reagents were used: N alpha-lauroyl, N epsilon-Dns-lysine hydrazide and lauric acid hydrazide. The number of protein aldehyde groups formed was assayed by reaction with N-(2,4-dinitrophenyl)-beta-alanylglycylglycine hydrazide. Measurement of the intrinsic affinity for Dns-lysine of the processed anti-Dns IgMs demonstrated no substantial impairment of the specific reactivity of the antibody either from the oxidation step or the subsequent attachment to small unilamellar vesicles. The extent of attachment of antibody to small unilamellar vesicles was evaluated with respect to the mol% of hydrazide in the membrane, the duration of the incubation period for the aldehyde-hydrazide reaction and the ratio of protein to hydrazide. The yield of attached protein was significantly dependent on each of these experimental parameters over the ranges tested. Under the most favorable conditions the extent of covalent attachment of IgMs to small unilamellar vesicles was 535 micrograms of protein per mumol of phospholipid, corresponding to 0.3 mol% of protein. Under these conditions, 61% of the total protein was associated with the small unilamellar vesicle fraction after fractionation by gel filtration. The attachment of the antibody to small unilamellar vesicles did not destroy the integrity of the vesicles, as demonstrated by the retention of carboxyfluorescein following initial encapsulation during the formation of small unilamellar vesicles.

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.

Transfer of herpes simplex virus thymidine kinase synthesized in bacteria by a high-expression plasmid to tissue culture cells by protoplast fusion

The introduction of a protein into living tissue culture cells may permit the in vivo study of functions of the protein. We have previously described a high-efficiency-expression plasmid, pHETK2, containing the herpes simplex virus type 1 thymidine kinase (TK) gene which, upon temperature induction, causes TK to be synthesized as greater than 4% of the bacterial protein. In this report we show that enzymatically active TK was transferred to mouse Ltk- cells by polyethylene glycol-mediated fusion with protoplasts prepared from bacteria containing induced levels of TK. The presence of TK in the Ltk- cells was detected by the incorporation of [3H]thymidine into cell nuclei as measured by autoradiography.

Transport of a fluorescent phosphatidylcholine analog from the plasma membrane to the Golgi apparatus

We have examined the internalization and degradation of a fluorescent analog of phosphatidylcholine after its insertion into the plasma membrane of cultured Chinese hamster fibroblasts. 1-acyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole)-aminocaproyl phosphatidylcholine (C6-NBD-PC) was incorporated into the cell surface by liposome-cell lipid transfer at 2 degrees C. The fluorescent lipid remained localized at the plasma membrane as long as the cells were kept at 2 degrees C; however, when the cells were warmed to 37 degrees C, internalization of some of the fluorescent lipid occurred. Most of the internalized C6-NBD-PC accumulated in the Golgi apparatus although a small amount was found randomly distributed throughout the cytoplasm in punctate fluorescent structures. Internalization of the fluorescent lipid at 37 degrees C was blocked by the presence of inhibitors of endocytosis. Incubation of cells containing C6-NBD-PC at 37 degrees C resulted in a rapid degradation of the fluorescent lipid. This degradation occurred predominantly at the plasma membrane. The degradation of C6-NBD-PC resulted in the release of NBD-fatty acid into the medium. We have compared the internalization of the fluorescent lipid with that of a fluorescent protein bound to the cell surface. Both fluorescent lipid and protein remained at the plasma membrane at 2 degrees C and neither were internalized at 37 degrees C in the presence of inhibitors of endocytosis. However, when incubated at 37 degrees C under conditions that permit endocytosis, the two fluorescent species appeared at different intracellular sites. Our data suggest that there is no transmembrane movement of C6-NBD-PC and that the fluorescent probe reflects the internalization of the outer leaflet of the plasma membrane lipid bilayer. The results are consistent with the Golgi apparatus as being the primary delivery site of phospholipid by bulk membrane movement from the plasma membrane.

Quantitation of the delivery of liposome contents into plant protoplasts

A procedure for the quantitation of the delivery of liposome contents into Catharanthus roseus protoplasts has been developed. The method is based on the uptake of liposome encapsulated methylumbelliferyl beta-D-glucoside and its enzymatic hydrolysis to yield fluorescent methylumbelliferone. Since the free glucoside is not taken up by the protoplasts to a significant extent, the delivery of material in the nanomole range can be measured with ease.

Weak acid-induced release of liposome-encapsulated carboxyfluorescein

Leakage of the entrapped anionic fluorophore carboxyfluorescein was used as a measure of the permeability of liposomes to several different acids. Carboxyfluorescein leakage increased with increasing buffer concentration at a given pH and depended on its chemical nature: apolar weak acids such as acetic or pyruvic acids induced fast leakage at relatively high pH (4 to 5), while glycine, aspartic, citric and hydrochloric acids induced leakage only at lower pH. Fluorescence leakage measurements reflected the acidification of the liposomes' aqueous spaces, which was primarily caused by the diffusion of undissociated acid molecules across the lipid bilayer. A simple mathematical model in accord with this hypothesis and assuming that carboxyfluorescein leakage was directly related to the proportion of its neutral lactone form, described satisfactorily the carboxyfluorescein leakage kinetics and allowed rough estimation of permeability coefficients for carboxyfluorescein (neutral lactone form: 9 X 10(-9) cm X s-1), acetic acid (greater than 1 X 10(-7) cm X s-1) and glycine (cation: 6 X 10(-9) cm X s-1). These results are consistent with low effective proton permeability of liposomes (less than 5 X 10(-12) cm X s-1) and with the permeability coefficient of HCl (3 X 10(-3) cm X s-1) reported by Nozaki and Tanford ( (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 4324-4328). Diffusion of weak acid molecules across lipid membranes has implications for drug encapsulation and delivery, and may be of biological significance.

The interaction of Bacillus protoplasts with sonicated phosphatidylcholine liposomes

When protoplasts from Bacillus subtilis are incubated with sonicated liposomes made from egg-yolk phosphatidylcholine, this phospholipid is incorporated into the protoplast membranes. Biochemical, fluorescence and ultrastructural data suggest that incorporation occurs through membrane fusion.

pH-induced destabilization of phosphatidylethanolamine-containing liposomes: role of bilayer contact

The mechanism of pH-induced destabilization of liposomes composed of phosphatidylethanolamine and a charged cholesteryl ester was studied by following the release of encapsulated aqueous contents. The kinetics of release were measured continuously by using the water-soluble fluorophore 8-aminonaphthalene-1,3,6-trisulfonic acid in combination with the water-soluble quencher p- xylylenebis (pyridinium) bromide. With this fluorescence assay, release of contents from liposomes composed of phosphatidylethanolamine and cholesteryl hemisuccinate was shown to be a function of pH, ratio of phosphatidylethanolamine to cholesteryl hemisuccinate, and acyl chain composition of the phosphatidylethanolamine. Leakage was very slow at pH 5.5 and increased dramatically with decreasing pH down to 4.0. Replacing phosphatidylethanolamine by phosphatidylcholine eliminated the effect of pH on leakage. Analysis of the kinetics of release by a mass action model demonstrated that bilayer destabilization and leakage occur subsequent to aggregation. The requirement of bilayer contact for destabilization has been found previously for acidic phospholipid bilayers in the presence of divalent cation and for saturated phosphatidylcholine bilayers below the isothermal phase transition temperature. The phosphatidylethanolamine-containing bilayers examined here satisfy the same requirement.

Serological specificity of the liposome lysis test for measurement of anti-ganglioside antibodies. A comparison with hemagglutination inhibition

A simple and reliable complement-dependent liposome lysis test for the detection of anti-ganglioside antibodies is described. For sera raised in rabbits against the monosialoganglioside NG-GM3, the sensitivity and specificity of antibody detection was compared with that of the HRBC hemagglutination-inhibition test: the liposome lysis test appears more sensitive. A difference in antigen presentation was also demonstrated.

Liposome-mediated transfer of macromolecules into flagellated cell envelopes from bacteria

We have studied the interaction between flagellated cell envelopes from Escherichia coli and liposomes. Oligolamellar liposomes of ca. 0.45-micron diameter, composed of azolectin, phosphatidylserine, and cholesterol at a molar ratio of 7:1:2, were prepared by freezing and thawing and subsequent extrusion through polycarbonate filters. These liposomes exhibited high entrapment capacity and low leakiness. Liposome-cell envelope interaction was monitored flow cytometrically in a fluorescence-activated cell sorter with a fluorescent aqueous space marker and by a filtration assay with radiolabels for the lipid phase and the liposomal aqueous space. Maximal association of liposomes with the envelopes was observed in both assays after ca. 25 min at 30 degrees C. After such period of time, it seems that up to 200 liposomes (depending on the liposome to envelope ratio) were associated with a single cell envelope, as calculated from the radiotracer studies. Fluorometric measurements of the transfer of liposomal contents and the intermixing of membrane lipids indicated that at least 20% of the envelope-associated liposomes had delivered their content into the envelopes, possibly by fusion. Electron microscopic observations confirmed the transfer of liposome-encapsulated ferritin molecules into the cell envelopes. Our data suggest that liposomal carriers might be employed to deliver cytoplasmic, chemotaxis-related macromolecules into bacterial cell envelopes.

Transbilayer distributions of red cell membrane phospholipids in unilamellar vesicles

The phospholipid organization in unilamellar vesicles comprised of various purified phospholipid components of monkey erythrocyte membrane was ascertained using phospholipase A2 and trinitrobenzenesulfonic acid as external membrane probes. The vesicles were formed by sonication or detergent dialysis and fractionated by centrifugation or gel permeation chromatography. Experiments were done to confirm that the phospholipase A2 treatments did not cause lysis or induce fusion of the vesicles. This enzyme hydrolysed only the glycerophospholipids in the outer surface of the vesicles. The amounts of the external phospholipids determined by this enzymatic method were verified using the chemical probe, trinitrobenzenesulfonic acid. The choline-containing phospholipids and phosphatidylethanolamine localized randomly in the two surfaces of sonicated vesicles (outer diameter, about 30 nm), whereas phosphatidylserine preferentially distributed in the inner monolayer. This phosphatidylserine asymmetry virtually disappeared in detergent dialysed vesicles (outer diameter, about 45 nm). Furthermore, inclusion of cholesterol in both the types of vesicles resulted in more random glycerophospholipid distributions across the plane of vesicles bilayer, presumably due to the cholesterol-induced increases in the size of vesicles. These results demonstrate that the transbilayer distribution of erythrocyte membrane phospholipids in unilamellar vesicles are controlled mainly by the surface curvature rather than by interlipid interactions, and therefore suggest that phospholipid-phospholipid and phospholipid-cholesterol interactions should not play any significant role in determining the membrane phospholipid asymmetry in red cells. It is proposed that this asymmetry primarily originates from differential bindings of phospholipids with membrane proteins in the two leaflets of the membrane bilayer.

The influence of the internal content of negatively charged liposomes on their interaction with high-density lipoprotein

The release of the internal content of negatively charged phosphatidylcholine/phosphatidylserine vesicles under the influence of high density lipoprotein was studied. Under standard conditions (the same composition outside and inside the compartment) the leakage of negative liposomes increased significantly. However, a high internal concentration of calcein provoked a sealing effect, exhibited both in sucrose and in calcein release. This sealing effect is not related to the size of vesicles, the fluidity of the membrane, the distribution of phosphatidylserine molecules, or the membrane potential. Our data indicate that surface potential influences this effect, probably in addition to a lateral pressure effect such as with cholesterol. The surface potential, as measured by the water-lipid partition coefficient of fatty acids, is strongly affected by internal ionic strength when liposomes contain calcein as well as other polyanions (6-carboxyfluorescein, sodium citrate).

Liposomes as a means to introduce fragment A of diphtheria toxin into cells

The incorporation of fragment A of diphtheria toxin into liposomes is described. The intracellular delivery of the entrapped toxin, as evidenced by the inhibition of protein synthesis by a human lymphoblastoid cell line could be demonstrated with liposomes that contained phosphatidylethanolamine or phosphatidylserine in addition to phosphatidylcholine and cholesterol. Free fragment A, either alone or added to empty liposomes of any composition, did not affect protein synthesis, even when present in considerably higher concentrations than the liposome-entrapped form.

Enhanced activity of streptomycin and chloramphenicol against intracellular Escherichia coli in the J774 macrophage cell line mediated by liposome delivery

Streptomycin and chloramphenicol were entrapped within large neutral or anionic unilamellar vesicles of egg phosphatidylcholine prepared by an ether injection method. Both antibiotics in liposomal form were inactive against Escherichia coli in a simple tube dilution assay. A comparison was made of the activities of the free and liposomal forms of the antibiotics against E. coli located within the macrophages of the J774.2 murine cell line. The apparent intracellular antibacterial activity of both antibiotics was increased more than 10-fold by entrapment in neutral liposomes and in the case of chloramphenicol in anionic liposomes containing phosphatidylserine. Anionic liposomes containing phosphatidic acid were much less effective carriers than neutral liposomes for either antibiotic in this in vitro system. Incubation at 4 degrees C of cells with liposomes containing antibiotic or carboxyfluorescein inhibited intracellular antibacterial activity and cell-associated fluorescence. The high intracellular activity of the liposomal antibiotics is consistent with their phagocytic uptake by the macrophages followed by intracellular liposomal degradation and antibiotic release. Liposomal modification of cellular uptake and intracellular distribution of antibiotics may be used to extend the activity of existing and new agents against intracellular infection of the reticuloendothelial system.

Liposomes from polymerizable phospholipids

The synthesis and characterization of a great variety of single and double chain phospholipids containing the diacetylene and butadiene moiety is described. These substances can be dispersed in water by ultrasonication and the resulting vesicles can be photopolymerized with the retention of their original structure. Absorption spectra of the polymerized diacetylenic lipids show significant differences depending on the molecular structure of the monomers. By the polymerization reaction, the gel to liquid crystalline phase transition is suppressed, which does not correspond to the properties of biological membranes. Evidence for enhanced stability of polymerized vesicles is given by treatment with ethanol and detergents showing that trapped markers are released to a much smaller extent than in the case of unpolymerized vesicles. Diacetylenic lipids show a pronounced hysteresis of the phase transition. If the membrane of supercooled vesicles crystallizes, all trapped marker is released within seconds. Possibilities for overcoming this extreme rigidity of the membranes are discussed.

Binding of fluorescein and carboxyfluorescein by human serum proteins: significance of kinetic and equilibrium parameters of association in ocular fluorometric studies

The binding of fluorescein and 5- and 6-carboxyfluorescein by human serum proteins was measured at 37 and 4 degrees C by equilibrium dialysis. The equilibrium association constants (KA) for fluorescein were 3.7 X 10(3) and 7.1 X 10(3)M-1, and for carboxyfluorescein were 3.5 X 10(3) and 7.5 X 10(3)M-1 at 37 and 4 degrees C equilibrium dialysis data, was 3.9 X 10(3) binding sites in human serum, determined from the 37 degrees C equilibrium dialysis data, was 3.9 X 10(-3)M for fluorescein and 3.3 X 10(-3)M for carboxyfluorescein. Utilizing these binding parameters it was calculated that a maximum of 93.5% of the total fluorescein and 92.0% of the carboxyfluorescein would be bound by undiluted human serum proteins at 37 degrees C. Experimental binding data obtained after prolonged equilibrium dialysis (four days) at low total fluorochrome concentrations (1.5 X 10(-4)M or less) indicated that 93.1 +/- 1.0 (S.D.)% of the fluorescein and 90.1 +/- 0.7% of the carboxyfluorescein were bound at 37 degrees C by undiluted human serum proteins. Stopped-flow kinetic spectrophotometric studies of the changes in absorptivity at 487-488 and 510 nm that occurred when the fluorochromes were bound by human serum proteins, indicated that the fluorescein and carboxyfluorescein binding reactions were 99% complete within 0.65 and 1.72 sec. These had second-order association rate constants at 25 degrees C of 3.0 X 10(3) and 1.5 X 10(3)M-1 sec-1, respectively. These findings offer a basis for calculation of bound and free fluorescein and carboxyfluorescein in vivo in human subjects.

Plasma membrane-mediated leakage of liposomes induced by interaction with murine thymocytic leukemia cells

The interaction of liposomes with BW 5147 murine thymocytic leukemia cells was studied using fluorescent probes (entrapped carboxyfluorescein and fluorescent phosphatidylethanolamine) in conjunction with a Ficoll-Paque discontinous gradient system for rapid separation of liposomes from cells. Reversible liposomal binding to discrete sites on the BW cell surface was found to represent the major form of interaction; uptake of intact liposomal contents by a process such as liposome-BW cell membrane fusion was found to apparently represent a minor pathway of interaction (2%). Liposomal lysis was found to be associated with the process of liposomal binding (perhaps as a result of the binding itself). Lysis was followed by release of the entrapped carboxyfluorescein into the media and its subsequent uptake by the cells. This lysis was shown to be dependent upon discrete membrane-associated sites that have some of the properties of proteins. The results of these studies suggest that liposomal binding to the cells, subsequent lysis of the liposomes and cellular uptake of their contents should be seriously considered in all studies of liposome-cell interactions as an alternate mode of interaction to the four modes (fusion, endocytosis, adsorption and lipid exchange) previously emphasized in the literature.

Interactions of clathrin coat with model lipid membranes

The interaction of coat proteins from coated vesicles with model lipid membranes was examined using small unilamellar vesicles of dipalmitoylglycerophosphocholine as model membranes. Changes in membrane permeability were measured by the leakage of entrapped fluorescent dye, carboxyfluorescein. Both clathrin and the 55000-Da protein were found to be active. Density gradient centrifugation showed the formation of an irreversible protein-lipid complex. Dynamic light-scattering measurements showed that this complex is significantly larger than the original vesicles, suggesting that fusion is induced. The effects of pH, urea, Tris and ionic strength were studied and the possible biological relevance of the results is discussed.

Endocytosis of liposomes bound to cell surface proteins measured by flow cytofluorometry

A new technique for the quantification of cellular receptor-mediated endocytosis has been developed based on the analysis by flow cytometry of ligand-bearing liposomes containing the fluorochrome carboxyfluorescein. Carboxyfluorescein encapsulated at high concentrations in protein A-bearing liposomes is self-quenched. Binding and internalization of such liposomes by cells via antibodies directed towards membrane surface determinants results in the release of the liposome-encapsulated carboxyfluorescein into the cytoplasm causing an increase in cell-associated fluorescence. This increase can be quantified on a flow cytofluorometer.

Limiting ischemic liver injury by interfering with lysosomal autophagy

Two regimens designed to ameliorate hepatic injury from complete liver dearterialization (LD) by interfering with lysosome-mediated proteolysis were studied in 200-g Buffalo rats. Five rats received a lysosome membrane-stabilizing flavenoid, catechin, 200 mg/kg/day for 5 days pre-LD. Five others were infused with lysosome protease inhibitors (LPI), leupeptin and pepstatin, delivered in 0.22-micron multilamellar liposomes at 500 micrograms each per hour for 2 hr, beginning 20 min before LD. Control groups (n = 4 or 5) were untreated LD, and treated and untreated sham LD rats. Blood from an arterial catheter pre-LD and 2 hr (peak enzyme release), 2 days, and 4 days post-LD yielded beta-glucuronidase (BG), aspartate transaminase (AST), and alkaline phosphatase values for each rat. Liver histology was not different between groups at 4 days post-LD. Untreated controls and the catechin LD group had similar enzyme levels at all points. LPI treatment values were statistically similar to sham LD values and had peak values significantly lower (P less than 0.05) than untreated LD controls at 2 hr. BG, 75 +/- 10 (SD) units per liter versus 185 +/- 32 units per liter; AST, 134 +/- 47 units per liter versus 459 +/- 175 units per liter. The BG lowering persisted to day 2 (55 +/- 25 units per liter versus 93 +/- 40 units per liter). Other values remained normal or normalized by Day 2. Hepatic damage as measured by enzyme release was not diminished by the membrane stabilizer catechin but was diminished after ischemic injury by specific targeted lysosomal protease inhibitors.

Melittin-induced fusion of acidic liposomes

Melittin-induced fusion of acidic liposomes. Fusion was observed in the electron-microscope and assayed as intermixing of both liposomes' contents and membranes. The melittin concentrations required for fusion induction were in the microM range compared to over 10 mM Ca2+ required for a comparable effect. It is suggested that the high efficiency of melittin is due to its amphipathic nature. Its hydrophobic moiety is probably anchored in one liposome while the positively charged hydrophilic moiety attracts another liposome.

Effect of quinidine on membrane properties. Depression of the lipid phase transition temperature and changes in the permeability of the lipid bilayer

The influence of an antiarrhythmic drug, quinidine, on the physical state of membrane phospholipids was investigated using model membranes, liposomes. Turbidimetric measurements on liposomes prepared from neutral (dipalmitoyl phosphatidylcholine) and acidic (dipalmitoyl phosphatidic acid) phospholipids showed that quinidine reduces the temp of the gel to liquid-crystalline phase transition and broadens the temp range of the transition. The effect of quinidine on the thermal behaviour of model membranes depends on both the pH and the type of phospholipids used. It is markedly stronger for acidic than for neutral phospholipids, suggesting the importance of electrostatic effects in drug-membrane interaction. The ability of quinidine to interact with the lipid bilayer was confirmed by permeability measurements with the use of a self-quenched fluorescent compound, calcein. It is suggested that quinidine-phospholipid interaction may contribute to the mechanisms by which the drug exerts its physiological and pharmacological effects.

Stability of liposomes in circulation is markedly enhanced by structural modification of their phospholipid component

Replacement of the C-2 ester group in phosphatidylcholine by the carbamyloxy function rendered its liposomes completely stable and longer living in the circulation of rats.

Endocytosis of liposomes and intracellular fate of encapsulated molecules: encounter with a low pH compartment after internalization in coated vesicles

We have compared the intracellular fate of several fluorescent probes and colloidal gold entrapped in negatively charged liposomes. Weakly acidic molecules (carboxyfluorescein) appear in the cytoplasm of CV-1 cells in 30 min; agents that raise lysosomal pH block this process. Highly charged molecules (calcein) and large molecules (FITC-dextran: 18 kd) remain confined to extra-or intracellular vesicles. Thin section electron micrographs show gold-containing liposomes bound to coated pits, in intracellular coated and uncoated vesicles, and in secondary lysosomes, including dense bodies. Free gold was not observed in the cytoplasm. We conclude that negatively charged liposomes are endocytosed and processed intracellularly by the coated vesicle pathway, and acidification of the endocytic vesicle, rather than liposome fusion, permits escape of certain molecules to the cytoplasm.