Transient permeabilization induced osmotically in membrane vesicles from Torpedo electroplax: a mild procedure for trapping small molecules

During hypoosmotic stress, membrane vesicles enriched in acetylcholine receptors become more permeable to external tracer molecules. When vesicles are immersed in 3 volumes of water containing 22Na+, 50-70% of 22Na+ equilibration is attained within 90 s. On the other hand, the uptake of 22 Na+ is greatly diminished only 6-10 s after an osmotic shock, and vesicle resealing is completed within 15 s. Furthermore, 90 s after osmotic shock, efflux rates are comparable to those of native vesicles, which also indicates that the vesicles have resealed. During osmotic shock, the entry of molecules into the vesicles increases with the strength of the osmotic shock and also depends on the size of the permeant. With a given strength of osmotic shock, the large molecule [3H]inulin (Mr 5000) is taken up less than the smaller molecules 22Na+ and [3H]sucrose. In addition, alpha-bungarotoxin binding latency of the vesicles is not affected by osmotic shock, indicating that the sideness of the vesicles remains unchanged. The acetylcholine receptors in the vesicles remain functional after osmotic shock. For example, 90 s after 22Na+ and [3H]sucrose are loaded into vesicles by osmotic shock, only 22Na+ is released by dilution in a buffer containing carbamoylcholine (carbamylcholine). Also, the influx of 22 Na+ into previously shocked vesicles can be specifically stimulated by carbamoylcholine. Such stimulations in the shocked vesicles can be blocked by d-tubocurarine or alpha-bungarotoxin, and they can be desensitized by preincubation with carbamoylcholine. These results suggest the possibility of using osmotic shock to load molecular probes into these membrane vesicles, which could provide a powerful tool for studying inner surfaces of the intact vesicles.

Monoclonal antibody covalently coupled with fatty acid. A reagent for in vitro liposome targeting

Monoclonal antibody to the mouse histocompatibility antigen, H-2k, was derivatized with palmitic acid using an activated ester of N-hydroxysuccinimide. About 70% of the resulting amphipathic antibody could be incorporated into liposomes by a detergent-dialysis method. The antibody-bound liposomes were about 900 A in diameter and were heterogeneous in terms of the number of antibody molecules per liposome. These liposomes showed specific binding affinity to mouse L-929 cells (H-2k), but not to A-31 cells (H-2d), whereas native liposomes showed no detectable binding to either cell type. The specific binding of anti-H-2k-bound liposomes to L-929 cells could be blocked by a preincubation of cells with an excess of free, underivatized anti-H-2k antibody but not by normal mouse IgG. Using a fluorescent phospholipid, liposomes containing anti-H-2k specifically labeled L-929 cells but not A-31 cells in a mixed culture. Liposomes containing normal mouse IgG did not significantly label either cell type. These results clearly demonstrated the effectiveness of the monoclonal antibody for liposome targeting.

Studies on the insulin mediator. II. Separation of two antagonistic biologically active materials from fraction II

Insulin treatment significantly altered the elution profile of deproteinized muscle extracts chromatographed on Sephadex G-25 columns, particularly in fraction II, which contains the insulin mediator. Further purification of fraction II by high-voltage paper electrophoresis at pH 1.9 and 3.5 resulted in two active fractions. Fraction 1 leads to 4 stimulated the cyclic AMP-dependent protein kinase and inhibited glycogen synthase phosphoprotein phosphatase, and may be a novel substance. Fractions 1 leads to 6 and 3 leads to 6 inhibited the cyclic AMP-dependent protein kinase and stimulated glycogen synthase phosphatase. It is proposed that the insulin mediator is present in fractions 1 leads to 6 and 3 leads to 6.

Generation by insulin of a chemical mediator that controls protein phosphorylation and dephosphorylation

Deproteinized skeletal muscle extracts free of major nucleotides from control and insulin-treated rats were fractionated and assayed for inhibition of protein phosphorylation by cyclic adenosine monophosphate (AMP)-dependent and -independent protein kinases. A differential effect of insulin on a particular fraction was observed on cyclic AMP-dependent protein kinase but not on cyclic AMP-independent protein kinases. This fraction that inhibited cyclic AMP-dependent protein kinase also stimulated glycogen synthase phosphoprotein phosphatase. It is proposed that this fraction may contain a mediator substance generateed in the presence of insulin.

Entrapment of proteins in phosphatidylcholine vesicles

The trapping efficiency of globular proteins in four different types of phosphatidylcholine vesicles was systematically studied. Vesicles were generated in a mixture of 125I-labeled proteins of various molecular weights. The trapped proteins were separated from untrapped proteins by gel filtration and ultrafiltration and subsequently analyzed by gel electrophoresis and autoradiography. Entrapment of proteins was demonstrated by their resistance to trypsin digestion. The relative amount of each entrapped protein species was then compared to that of the original protein solution. In multilamellar vesicles and large unilamellar vesicles, proteins of molecular weight up to 97 000 had the same trapping efficiency as sucrose. In small unilamellar vesicles generated by either sonication or ethanol injection, however, the relative trapping efficiency of protein decreased progressively as the molecular weight of the protein became greater. For example, the trapping efficiency of alpha-amylase (Mr 97 000) was only half of that for sucrose. The apparent decrease in trapping efficiency with the protein's molecular weight in small unilamellar vesicles canbe accounted for by the combination of the bound water layer at the vesicle's internal surface and the steric hindrance when protein is captured during vesicle formation.

Synthesis and characterization of a new fluorescent phospholipid

A novel fluorescent phospholipid, whose structure was tentatively assigned as 1-(2'-thio-1'-hydroxyethyl)-2-(ethylphosphatidyl)isoindole), was synthesized by reacting O-phthalaldehyde and beta-mercaptoethanol with phosphatidylethanolamine. The fluorescent lipid product was purified by silicic acid chromatography. The purity was demonstrated by thin-layer chromatography. This fluorescent phospholipid could not form stable lipid vesicles. However, a mixture of phosphatidylcholine and this fluorescent phospholipid did form stable vesicles after sonication, as demonstrated by Sepharose 4B column chromatography and electron microscopy. The absorption and fluorescence properties of this lipid, both as aqueous micelles or incorporated into vesicles, have been determined. The potential usage of this new fluorescent phospholipid in membrane studies is discussed.

Transport of 5-hydroxytryptamine by dense granules from porcine platelets

A method is described for the isolation of a homogeneous preparation of dense granules from procine platelets. The purified dense granule fraction contained approximately 400 nmol of 5-hydroxytryptamine/mg of protein and appeared to be homogeneous when examined by electron microscopy. Isolated dense granules transport exogenously added 5-hydroxytryptamine via two mechanisms: 1) a carrier-mediated process predominating at low substrate concentrations and 2) a diffusion-controlled process predominating at high substrate concentrations. Temperature studies revealed an apparent energy of activation of 14.9 kcal/mol for the carrier-mediated transport. Kinetic data yielded a Km of 3.3 micron and a Vmax of 0.79 nmol/min/mg of protein for the mediated transport process. Steady state uptake was sensitive to changes in medium osmotic pressure and a decline in uptake below 300 mosM was correlated with release of endogenous 5-hydroxytryptamine. The transport was inhibited by a number of structural analogs of 5-hydroxytryptamine. These results demonstrate the existence of a carrier-mediated transport system for 5-hydroxytryptamine in the membranes of the platelet dense granules.

Interactions of phospholipid vesicles with murine lymphocytes. II. Correlation between altered surface properties and enhanced proliferative response

The effect of unilamellar lipid vesicles composed of dioleoyl lecithin (DOL), egg yolk lecithin (EYL), 1:1 EYL:cholesterol (Chol), dipalmitoyl lecithin (DPL), and dimyristoyl lecithin (DML) on the mitogenic response in mouse lymphocytes was tested. Cortisone-resistant thymocytes were briefly treated with lipid vesicles and subsequently stimulated with concanavalin A (con A). All of the lipid vesicles induced an enhanced mitogenic response on day 3 as tested by [3H]TdR incorporation and by counting total cells. The order of enchanced [3H]TdR incorporation (less than or equal to 5.3 times the control) was DML greater than DPL greater than 1:1 EYL:Chol greater than EYL congruent to DOL greater than untreated control cells. These increases were paralleled by increased numbers of total cells. The response of spleen cells to a B-cell mitogen, bacterial lipopolysaccharide, was similarly enhanced by vesicle pretreatments in the same order. Vesicle treatments alone were not mitogenic Pretreatment of cells with lipid vesicles modified lectin binding: DML and DPL increased the binding of [125I]con A by three to four times the control, whereas 1:1 EYL:Chol, EYL, or DOL had little or no effect. The binding of [125I]phytohemagglutinin-P (PHA-P) to vesicle-treated cells was indistinguishable from untreated cells. The lectin (con A; PHA-P)-induced agglutination of vesicle-treated cells was also modified by different lipid vesicles in the same order as the mitogenic response. Based on the results presented in the accompanying report [6], we find that the cell surface adsorption properties of the applied lipid vesicles correlate with their ability to enhance the mitogenic response, and that they modify agglutinability and lectin binding. These results are further discussed in terms of the possible alteration of membrane properties and subsequent cellular activity.

Interactions of phospholipid vesicles with murine lymphocytes. I. Vesicle-cell adsorption and fusion as alternate pathways of uptake

The interactions of mouse thymocytes with unilamellar phospholipid vesicles comprised of dimyristoyl lecithin (DML), dipalmitoyl (DPL), dioleoyl lecithin (DOL), and egg yolk lecithin (EYL) were examined in vitro. In cells treated with [3H]DML or [3H]DPL vesicles, electron croscope (EM) autoradiographic analysis showed most of the radioactive lipids to be confined to the cell surface. Transmission EM studies showed the presence of intact vesicles (DPL) and collapsed or ruptured vesicle fragments (DML) adsorbed to the surfaces of treated cells. In cells treated with DPL vesicles containing a water-soluble dye (6-carboxyfluorescein; 6-CF), most of the fluorescent vesicles were localized at the periphery of the treated cells. Furthermore, substantial fractions of the cell-associated DPL and DML could be released by a mild trypsinization without damaging the cells. These results suggest that the uptake of DML and DPL is primarily due to vesicle-cell adsorption. Such an adsorption process appears to be enhanced at or below the thermotropic-phase transition temperature of the vesicle lipid. Under certain conditions these adherent vesicles also formed patches or caps on the cell surface. In cells treated with DOL or EYL vesicles, transmission EM and EM autoradiography showed relatively little exogenous vesicle lipid located at the cell surface. Thymocytes incubated (37 degrees C) with [14C] EYL vesicles containing a trapped marker, [3H]inulin, incorporated both isotopes at identical rates. In separate experiments it was found that this marker was located inside the treated cells. Thymocytes treated with DOL vesicles containing 6-CF exhibited a uniform and diffuse distribution of dye in the internal volume of the cells. Little cell-associated EYL or DOL could be released by trypsinization. Evidence against endocytosis of intact vesicles as a major pathway of vesicle uptake is also presented. These observations, coupled with the demonstration of vesicle-cell lipid exchange as a minor component of vesicle uptake suggest that incorporation of EYL and DOL vesicles by thymocytes is primarily by vesicle-cell fusion.

Accelerated calcium ion uptake in murine thymocytes induced by concanavalin A

The mechanism of enhancement of Ca2+ uptake by the T cell mitogen concanavalin A (Con A) was studied in murine thymocytes. Native Con A enhanced the rate of Ca2+ uptake as much as 9-fold, an increase being observed within five minutes after Con A addition. The effect of Con A was reversed completely by alpha-methyl mannopyranoside (alpha-MM). Increased Ca2+ uptake was observed with increasing concentrations of Con A, between 2 and 400 microgram/ml, indicating that the stimulation of Ca2+ uptake is not restricted to mitogenic lectin concentrations (0.5-2 microgram/ml). Succinyl Con A exhibits only a slight effect in the same concentration ranges as native Con A. Ca2+ uptake, both in the absence and presence of Con A, is strongly dependent on energy metabolism and is carrier mediated. The augmentation of Ca2+ uptake by native Con A is due to an enhanced Vmax. Uptake of the anion, CrO42-, by thymocytes, found to be a non-saturable process, was also enhanced by Con A. The effect of Con A on CrO42- permeability appears to be independent of its effect on Ca2+ uptake.

Mechanism of maleic acid-induced glucosuria in dog kidney

The multiple indicator-dilution technique in vivo and isolated brush-border membranes in vitro have been used to explore the mechanism of maleic acid-induced glucosuria in dog kidney. The interaction of D-glucose with the antiluminal membrane from the peritubular fluid surface is unaltered. It is demonstrated that alpha-methyl-D-glucoside (alpha MG) enters and exits from the proximal tubular cell only across the brush-border membrane. Then using alphaMG as a reference indicator, it is shown that maleic acid does not cause complete inhibition of D-glucose interaction with the antiluminal membrane from the cytoplasmic surface. The binding of [3H]phlorizin both in vivo and in vitro is not affected by prior administration of maleic acid, indicating that D-glucose interaction with the outside surface of the brush border is also not affected by maleic acid. The data are therefore consistent with the concept that maleic acid-induced glucosuria is due either to i) partial inhibition of D-glucose movement from cytoplasm across the antiluminal membrane into the blood, ii) stimulated movement back across the brush-border membrane into urine, or iii) a combination of the two effects.

The identification of fibrous proteins in fetal rat epidermis by electrophoretic and immunologic techniques

Two proteins have been identified in extracts of fetal rat skin which are related to the two major fibrous proteins of newborn rat stratum corneum. The relative amount of these proteins increases daily from the 16th to the 20th day (d) of gestation when judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and immunoelectrophoresis using antibody to the purified fibrous protein. Two-dimensional analysis by SDS-polyacrylamide gel electrophoresis and immunoelectrophoresis demonstrates that these two proteins are the only cross-reactive species in the fetal skin from 16d to 19d development. Some additional lower-molecular-weight components can be detected at 20d and 21d. In double-diffusion analysis, cross-reactive proteins in 19d fetal extracts show partial identity but have fewer antigenic sites than proteins in 20d extracts. The 20d protein shows a reaction of identity with purified newborn fibrous protein. Immunofluorescence studies on fetal skin support the prescence of cross-reacting components at 16d development related to the newborn fibrous protein. Intensity of fluorescence increases at 18d and 20d in the spinous and granular cell cytoplasm and in the keratohyaline granules. The stratum corneum, first seen at 20d, is intensely fluorescent. The cellular localization and time of appearance of the cross-reactive proteins suggest that they may be associated with tonofilaments.

Purification and characterization of shikimate kinase enzyme activity in Bacillus subtilis

In Bacillus subtilis shikimate kinase enzyme activity can be demonstrated when a small polypeptide forms a trifunctional complex with the bifunctional enzyme 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase-chorismate mutase. The shikimate kinase polypeptide whoch carries the catalytic site has been purified to homogeneity by a five-step procedure. The skikimate kinase was determined to have a molecular weight of 10,000 by superfine Sephadex G-75 thin layer chromatography and by calculation of the minimum chemical molecular weight from its amino acid composition. This number corresponds closely to the molecular weight determined by the mobility of the protein following electrophoresis on polyacrylamide gels containing sodium dodecyl sulfate. The enzyme aggregates with itself forming larger molecular weight proteins. Thes aggregational pattersn depend on protein concentration and sulfhydryl bridges. The enzyme activity is completely inhibited by EDTA and the requirement for Mg2+ can be partially replaced by Mn2+, Ca2+, and Co2+. The inhibition of shikimate kinase activity by p-hydroxymercuribenzoate is reversed completely when the enzyme complex is treated with dithiothreitol, suggesting the sulfhydryl groups may be involved with the active site. The trifunctional complex is relatively unstable, and the nonidentical subunits dissociate readily. This dissociation results in a 99% loss in shikimate kinase activity and a 30% decrease in the chorismate mutase-DAHP synthetase activities. Shikimate kinase activity is subject to a variety of controls. It is inhibited by the allosteric effectors chorismate and prephenate, the products of the reaction, ADP, and shikimate 5-phosphate. The activity responds to changes in the energy charge of the cell. Because of the variety of controls exerted on this enzyme, this member of the regulatory complex may represent the key enzyme in the allosteric control of the synthesis of the common precursors of aromatic acid synthesis.

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

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

Interaction of phospholipid vesicles with cultured mammalial cells. I. Characteristics of uptake

The interaction of monolayer cultures of Chinese hamster V79 cells with artificially generated, unilamellar lipid vesicles (approximately 500 A diameter) was examined. Vesicles prepared from a variety of natural and synthetic radiolabeled phosphatidyl cholines (lecithins) were incubated with V79 cells bathed in a simple balanced salt solution. After incubation, the cells were analyzed for exogenous lipid incorporation. Large quantities (approximately 10(8) molecules/cell/h) of lecithin became cell associated without affecting cell viability. The effects of pH, charged lipids, and the influence of the vesicle lipid phase transition on the uptake process were examined. Glutaraldehyde fixation of cells before vesicle treatment, or incubation in the presence of metabolic inhibitors, failed to reduce the lecithin uptake by more than 25-50%, suggesting that the lipid uptake is largely energy independent. Cells in sparse culture took up about ten times more lipid than dense cultures. Prolonged incubation (greater than 15 h) of sparse cell cultures with lecithin vesicles resulted in significant cell death while no deleterious effect was found in dense cultures, or with 1:1 lecithin/cholesterol vesicles. When vesicle-treated cells were homogenized and fractionated, about 20-30% of the exogenous lipid was found in the plasma membrane fraction, with the remainder being distributed into intracellular fractions. Electron microscope radioautography further demonstrated that most of the internalized lipid was present in the cytoplasm, with little in the nucleus. These results are discussed in terms of possible modification of cell behavior by lipid vesicle treatment.

Lipid and temperature dependence of membrane-bound ATPase activity of Acholeplasma laidlawii

Arrhenius plots (15–45 °C) of the ATPase activity of Acholeplasma laidlawii membranes enriched with either arachidoyl or saturated short-chain groups exhibited a pronounced discontinuity in slope around 25–30 °C, absent in oleoyl-enriched membranes. The membrane lipid fluidity was measured with a stearic acid spin label. At the growth temperature (37 °C) the membrane lipid fluidity was identical for all three kinds of enriched membranes. For membranes enriched with saturated acyl chains, a plot of the electron paramagnetic resonance anisotropy parameter 2T11vs. the reciprocal of temperature showed a biphasic profile with a discontinuity in slope around 25–30 °C. For oleoyl-group enriched membranes this plot yields only a straight line between 15 and 50 °C. Thus, lipids do play a role in the functioning of the membrane-bound ATPase. This enzyme is probably localized in the more fluid regions of the membrane.