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PubMed1986

Solubilization and reassembly of the mitochondrial benzodiazepine receptor.

We have solubilized and reassembled the peripheral-type benzodiazepine receptor, a component of the mitochondrial outer membrane, from rat adrenal gland mitochondria. The ligand binding site of this receptor undergoes denaturation during solubilization in digitonin, Triton X-100, or 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate at detergent concentrations above 0.1%, which is evident from the loss of high-affinity binding of [3H]PK11195, a ligand selective for the mitochondrial benzodiazepine receptor. The conformation of the binding site for PK11195 can be stabilized during solubilization in sodium cholate by relatively low concentrations of supplementary soybean lipid. Drug displacement studies demonstrate that the pharmacological properties of the receptor are preserved under these conditions. Electron micrographs of the solubilized preparation show a heterogeneous population of many small particles (less than 100 A) and some larger membranous aggregates (up to 500 A). Sucrose gradient centrifugation indicates that these lipoprotein complexes are of high buoyant density. They can be incorporated in liposomes via cholate dialysis in the presence of additional supplementary lipid. The behavior of the mitochondrial benzodiazepine receptor during solubilization and reassembly suggests that it is an integral protein of the outer membrane.

Adrenal Glands↗

PubMed1986

Solubilization and reconstitution of chick renal mitochondrial 25-hydroxyvitamin D3 24-hydroxylase.

Chick kidney mitochondrial 25-hydroxyvitamin D3 24-hydroxylase has been solubilized with sodium cholate and reconstituted with NADPH, beef adrenal ferredoxin, and beef adrenal ferredoxin reductase, each component being essential for maximal 24-hydroxylase activity. The product 24(R),25-dihydroxyvitamin D3 was identified by cochromatography with synthetic compound on straight-phase and reversed-phase high-performance liquid chromatography and by periodate oxidation. The enzyme has an apparent Km for 25-hydroxyvitamin D3 of 0.67 microM. At 1 microM 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3 production is linear with time for up to 15 min and with protein concentrations of up to 2 mg/mL. The antioxidant diphenyl-p-phenylenediamine (1.3 X 10(-4) M) has no effect on this reaction. Reconstituted 24-hydroxylase activity is enhanced by the addition of NaCl and KCl up to 100 mM, with higher concentrations having an inhibitory effect. 1 alpha-Hydroxylase is not present in this preparation from vitamin D replete chicks. The similarities of this reconstituted system to the 25-hydroxyvitamin D3 1 alpha-hydroxylase and the adrenal systems suggest that the 25-hydroxyvitamin D3 24-hydroxylase is also a cytochrome P-450 type mixed-function oxidase.

25-Hydroxyvitamin D3 1-alpha-Hydroxylase↗

PubMed1987

Synthesis of prodan-phosphatidylcholine, a new fluorescent probe, and its interactions with pancreatic and snake venom phospholipases A2.

A new fluorescent probe, prodan-PC, was synthesized by incubating thio-PC, a thiol ester analogue of phosphatidylcholine [1,2-bis(decanoylthio)-1,2-dideoxy-sn-glycero-3-phosphocholine], with acrylodan, a fluorescent thiol-reactive reagent [6-acryloyl-2-(dimethylamino)naphthalene], in the presence of phospholipase A2, which served to generate lysothio-PC in situ. Prodan-PC (PPC) showed maximum absorption in ethanol at 370 nm. The fluorescence emission spectrum showed maximum emission at 530 nm in water and at 498 nm in ethanol. In the presence of a saturating amount of phospholipase A2, the emission maximum shifted to about 470 nm. PPC showed a critical micellar concentration around 5 microM, with evidence of premicellar aggregation above 1 microM. Binding of PPC to Crotalus adamanteus phospholipase A2 was evidenced by an increase in emission at 480 nm and an increase in fluorescence anisotropy. An apparent dissociation constant of 0.323 microM was calculated for this enzyme complex. Binding was dependent on the presence of calcium ion and was abolished by blocking the active site with p-bromophenacyl bromide. Binding was also followed by energy transfer from tryptophan in the enzyme to PPC. Apparent dissociation constants for PPC complexes with phospholipases A2 from Naja naja naja and porcine pancreas and the prophospholipase A2 from porcine pancreas were 0.509, 0.107, and 0.114 microM, respectively. PPC was shown to inhibit the activity of pancreatic phospholipase A2 in thio-PC-sodium cholate mixed micelles. Inhibition studies were complicated because PPC can also serve as an activator of the snake venom enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Naphthylamine↗

PubMed1988

A minimum number of lipids are required to support the functional properties of the nicotinic acetylcholine receptor.

The detergent sodium cholate was used to both solubilize and partially delipidate the nicotinic acetylcholine receptor from Torpedo californica. Using both native membranes and reconstituted membranes, it is shown that the detergent to lipid molar ratio is the most important parameter in determining the effect of the detergent on the functional properties of the receptor. Receptor-lipid complexes were quantitatively separated from detergent and excess lipids by centrifugation through detergent-free sucrose gradients. The lipid to protein molar ratio of the complexes could be precisely controlled by adjusting the cholate and lipid concentrations of the starting membranes. Analyses of both ion influx activity and ligand binding revealed that a minimum of 45 lipids per receptor was required for stabilization of the receptor in a fully functional state. Progressive irreversible inactivation occurred as the lipid to protein mole ratio was decreased below 45, and complete inactivation occurred below a ratio of 20. The results are consistent with a functional requirement for a single shell of lipids around the perimeter of the receptor.

Allosteric Regulation↗

PubMed1988

Solubilization and reconstitution of the D-1 dopamine receptor: potentiation of the agonist high-affinity state of the receptor.

The D-1 dopamine receptor was extracted from rat striatal membranes with sodium cholate and NaCl in the presence of a specific agonist and phospholipids. The soluble receptor then was reconstituted into phospholipid vesicles by further addition of phospholipids prior to detergent removal. Of the total membrane receptors, up to 48% were extracted and 36% were reconstituted into phospholipid vesicles. Yields were greatly reduced if the agonist was omitted or replaced with an antagonist. The solubilized and reconstituted D-1 receptors retained the pharmacological properties of the membrane-bound receptors, including the ability to discriminate between active and inactive enantiomers of specific agonists and antagonists. In this regard, the affinity of the reconstituted receptors for the D-1 specific antagonist 125I SCH 23982 was similar to that of the membrane-bound receptors with a Kd of 1.5 nM. Both the soluble and reconstituted forms of the D-1 receptor exhibited two affinity states for the D-1 specific agonist SK&F R-38393. In contrast to the low proportion of the receptors that had a high affinity for the agonists in striatal membranes (less than 6%), there was a dramatic increase following solubilization (22%) and reconstitution (40%). Similar results were obtained by using dopamine; the proportion of high-affinity sites increased from 4% (membrane-bound) to 48% (reconstituted) of the total receptor population. These high-affinity sites were coupled to G proteins, as guanyl nucleotides completely abolished them. Addition of guanyl nucleotides prior to solubilization or to reconstitution, however, had no effect on the subsequent yield of the reconstituted receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Identification of lung major GTP-binding protein as Gi2 and its distribution in various rat tissues determined by immunoassay.

Antisera were raised in rabbits against the 40-kDa alpha subunit of bovine lung GTP-binding protein, which were identified as the alpha subunit of Gi2 (Gi2 alpha) by the analysis of the partial amino acid sequence. Antibodies were purified with a Gi2 alpha-coupled Sepharose column and then were passed through a Gi1 alpha-coupled Sepharose column to remove antibodies reactive also with 41-kDa alpha. Purified antibodies reacted with Gi2 alpha, but not with Gi1 alpha, Gi3 alpha, or Go alpha in an immunoblot assay. A sensitive enzyme immunoassay method for the quantification of Gi2 alpha was developed by using these purified antibodies. The assay system consisted of polystyrene balls with immobilized antibody F(ab')2 fragments and the same antibody Fab' fragments labeled with beta-D-galactosidase from Escherichia coli. The minimal detection limit of the assay was 1 fmol, or 40 pg. Samples from various tissues were solubilized with 2% sodium cholate and 1 M NaCl, and the concentrations of Gi2 alpha were determined. Gi2 alpha was detected in all the tissues examined in the rat. The highest concentration was found in platelets and leukocytes when the data were expressed as picomoles per milligram of protein. The spleen, lung, and cerebral cortex contained relatively high levels of Gi2 alpha. In the bovine brain, Gi2 alpha was distributed almost uniformly among the various regions. The concentrations of Gi2 alpha were constant in the rat brain throughout ontogenic development, in contrast with those of Go alpha which were markedly increased with age.

Amino Acid Sequence↗

PubMed1990

Mechanism of activation of cholera toxin by ADP-ribosylation factor (ARF): both low- and high-affinity interactions of ARF with guanine nucleotides promote toxin activation.

Activation of adenylyl cyclase by cholera toxin A subunit (CT-A) results from the ADP-ribosylation of the stimulatory guanine nucleotide binding protein (GS alpha). This process requires GTP and an endogenous guanine nucleotide binding protein known as ADP-ribosylation factor (ARF). One membrane (mARF) and two soluble forms (sARF I and sARF II) of ARF have been purified from bovine brain. Because the conditions reported to enhance the binding of guanine nucleotides by ARF differ from those observed to promote optimal activity, we sought to characterize the determinants influencing the functional interaction of guanine nucleotides with ARF. High-affinity GTP binding by sARF II (apparent KD of approximately 70 nM) required Mg2+, DMPC, and sodium cholate. sARF II, in DMPC/cholate, also enhanced CT-A ADP-ribosyltransferase activity (apparent EC50 for GTP of approximately 50 nM), although there was a delay before achievement of a maximal rate of sARF II stimulated toxin activity. The delay was abolished by incubation of sARF II with GTP at 30 degrees C before initiation of the assay. In contrast, a maximal rate of activation of toxin by sARF II, in 0.003% SDS, occurred without delay (apparent EC50 for GTP of approximately 5 microM). High-affinity GTP binding by sARF II was not detectable in SDS. Enhancement of CT-A ADP-ribosyltransferase activity by sARF II, therefore, can occur under conditions in which sARF II exhibits either a relatively low affinity or a relatively high affinity for GTP. The interaction of GTP with ARF under these conditions may reflect ways in which intracellular membrane and cytosolic environments modulate GTP-mediated activation of ARF.

ADP-Ribosylation Factors↗

PubMed1981

Reconstitution of acetylcholine receptor function using purified receptor protein.

Membrane preparations containing only the four acetylcholine receptor polypeptide subunits (40, 50, 60, and 65 x 10(3) daltons) were purified from Torpedo californica electroplax. The receptor protein was extracted from these membranes with 2% aqueous sodium cholate, and complete dissolution into discrete molecular species was confirmed by sedimentation analysis. The solubilized preparation reassociated with exogenous phospholipids when the detergent was removed by dialysis and formed spherically sealed vesicles 400--600 A in diameter. The reconstituted receptor preparations had nearly 90% of their alpha-bungarotoxin binding sites exposed on the exterior surface of the vesicles. In a reproducible manner, the reconstituted acetylcholine receptor responded to carbamoylcholine by exhibiting a rapid efflux of 22Na+ from within the vesicles. Such preparations were successfully reconstituted only from acetylcholine receptor stabilized by asolectin above a minimal level upon dissolution of the original membrane preparation by detergent. In addition to the response to carbamoylcholine, the reconstituted preparations also exhibited pharmacological characteristics that resemble those observed for the original electroplax membranes. In terms of the carbamoylcholine-induced signal, a significantly large fraction of the total receptor was functionally reconstituted. The results also confirm the notion that only the four polypeptides considered to constitute the receptor are essential for acetylcholine-mediated cation translocation and rule out possible roles for other polypeptide species.

Acetylcholine↗

PubMed1981

Dissection of the 66 000-dalton subunit of the acetylcholine receptor.

The 66 000-dalton or delta subunit of the acetylcholine receptor from Torpedo marmorata was covalently labeled in the presence of carbamoylcholine by 5-azido [3H]trimethisoquin (5-A[3H]T), a photoaffinity derivative of the local anesthetic trimethisoquin. After the attack of purified receptor with increasing concentrations of trypsin, the delta chain successively yielded fragments with apparent molecular weights of 50 000 (distinct from the beta subunit and referred to as the 50 000-bis (fragment), 49 000, and 47 000. With nondenatured (sodium cholate solubilized or membrane-bound) receptor, the 47 000-dalton fragment was not sensitive to trypsin and contained all of the covalent 5-A[3H]T label. This fragment was still glycosylated and had the same amino acid N terminus, valine, as the intact delta chain. A specific in vitro phosphorylation site of the delta subunit was located between the 49 000- and 50 000-dalton trypsin cleavage fragment and most likely is exposed to the cytoplasmic side of the membrane. A 16 000-dalton fragment of the delta chain was identified, which carriers a disulfide bond (or bonds) capable of cross-linking nonreduced receptor 9S monomerse into 12S dimers. The fragment did not remain associated with the receptor molecule after trypsin treatment.

Animals↗

PubMed1981

Nucleotide interactions with the dicyclohexylcarbodiimide-sensitive adenosinetriphosphatase from spinach chloroplasts.

The intrinsic nucleotide content of the dicyclohexylcarbodiimide-sensitive ATPase (DSA) from spinach chloroplasts and its interactions with ADP have been studied. Both partially purified and sucrose gradient purified DSA contain at least 1 mol of ADP/mol of enzyme and 1 mol of ATP/mol of enzyme, although considerable variation exists between different preparations. Radioactively labeled ADP is incorporated into DSA in the presence of 5 mM MgCl2 and 10 mM octyl glucoside with a half-life of approximately 30 min. Incorporation of ADP into DSA reconstituted in phospholipid vesicles occurs at about twice this rate, and a slightly slower rate of uptake is observed with [3H]ADP and [3H]ATP in the presence of 2 mM ethylenediaminetetraacetic acid. The [3H]ATP always appears as bound [3H]ADP on the enzyme. Nucleotide analyses indicate that this incorporation represents an exchange with tightly bound ADP. The nucleotide exchange requires binding at another nucleotide site or sites on the enzyme and is essentially a one-turnover process. Even during ATP synthesis less than 20% of incorporated 3H-labeled nucleotide is removed. Binding studies with forced dialysis indicate the presence of a reversible binding site for ADP distinct from the nucleotide exchange. Similar binding isotherms are obtained for the partially purified enzyme stabilized with 10 mM octyl glucoside, the gradient-purified enzyme stabilized with 0.4% sodium cholate, and the reconstituted, partially purified enzyme. The binding stoichiometry is approximately 0.5 mol of ADP/mol of DSA and the dissociation constant is approximately 2 microM, which is similar to the Michaelis constant for ADP estimated from kinetic studies of ATP synthesis.

Adenosine Diphosphate↗

PubMed1981

Effects of detergent micelles on the recombination reaction of opsin and 11-cis-retinal.

When detergent-solubilized proteins interact with hydrophobic or amphiphilic molecules in the presence of detergent micelles, the solubility of the latter species in the micelles must be included in both thermodynamic and kinetic treatments. In this paper, we derive equations which describe the distribution of species present at equilibrium for a system in which a detergent-solubilized protein binds a hydrophobic (or amphiphilic) ligand. We have applied the formalism developed in this paper to the reaction describing the formation of rhodopsin from its apoprotein and 11-cis-retinal. Qualitatively, the results demonstrate that a significant portion of the observed decrease in the extent of recombination for rhodopsin solubilized in either sodium cholate or Tween 80 may be attributed to the partition of retinal into detergent micelles and that a detergent-induced protein denaturation need not be invoked to explain the data. We also discuss results for rhodopsin solubilized in a nonionic detergent (octaethylene glycol n-dodecyl ether) in which the detergent is clearly causing irreversible loss of the capability to recombine with 11-cis-retinal.

Detergents↗

PubMed1982

Solubilization, partial purification, and reconstitution in phosphatidylcholine-cholesterol liposomes of acyl-CoA:cholesterol acyltransferase.

Acyl-CoA:cholesterol acyltransferase (ACAT) was solubilized from pig liver microsomes with a combination of 1:1% deoxycholate and 1 M potassium chloride. This solubilized activity was then reconstituted in lipid vesicles by diluting the extract into a solution of phosphatidylcholine, cholesterol, and sodium cholate, followed by dialysis. The reconstituted activity was shown to be dependent upon cholesterol in the reconstitution mixture and also shown to vary with changes in the phospholipid headgroup: phosphatidylethanolamine was most active, phosphatidylcholine was next, and phosphatidylserine or phosphatidylinositol was inhibitory. The reconstituted activity showed a migration pattern of ficoll gradients that was distinct from that of the unreconstituted enzyme and similar to that of phospholipid-cholesterol liposomes. These method provide a technique to assay the ACAT activity in defined lipid environment. The solubilized ACAT fraction was further purified by ammonium acetate fractionation and Sepharose 4B column chromatography. The entire purification procedure yielded a 150-fold increase in ACAT specific activity with 40% of the original activity recovered.

Acyltransferases↗

PubMed1980

Structural role of phospholipids in ubiquinol-cytochrome c reductase.

The role of phospholipids in ubiquinol-cytochrome c reductase has been studied by the following methods: (1) removal and restoration of phospholipids, (2) circular dichroism measurements, and (3) phospholipase A2 treatment. Over 90% of the phospholipids in the cytochrome b--c1 III complex (a highly purified ubiquinol-cytochrome c reductase) can be removed by repeated precipitation with ammonium sulfate in the presence of 0.5% sodium cholate. The delipidated enzyme complex is inactive. Full restoration of enzymatic activity can only be achieved with a freshly prepared delipidated enzyme complex, made in the presence of 20% glycerol. As the age of the delipidated enzyme increased, the amount of activity restored decreased and the incubation time required to reach maximal activity increased. Removal of phospholipids from the cytochrome b--c1 III complex resulted in an immediate decrease of approximately 15% in molar ellipticities in both the far-UV and the Soret regions. A further decrease in ellipticities was observed upon incubation of the delipidated enzyme at 0 degrees C in 50 mM phosphate buffer, pH 7.4. Replenishing phospholipids to the delipidated enzyme complex restored enzymatic activity and the molar ellipticity in both regions. The absolute requirement for phospholipids in the cytochrome b--c1 III complex was also demonstrated by treatment of the enzyme with purified phospholipase A2. The inactivation of the cytochrome b--c1 III complex by phospholipase A2 was not prevented by the presence of excess exogenous ubiquinone but was prevented by the presence of ethylenediaminetetracetic acid (EDTA). The enzymatic activity of the phospholipase A2 treated complex is fully restorable upon the addition of EDTA and phospholipids.

Circular Dichroism↗

PubMed1979

Formation of unilamellar lipid vesicles of controllable dimensions by detergent dialysis.

Vesicles are formed by solubilizing mixtures of phosphatidylcholine and cholesterol with sodium cholate and removing the detergent by rapid (hollow fiber) dialysis [e.g., Goldin, S. M. (1977) J. Biol. Chem. 252, 5630--5642]. Characterization of the vesicle size distribution by agarose gel filtration, and determination of the intravesicular aqueous compartment, demonstrates that the vesicles are relatively homogeneous in size and are primarily unilamellar. The mean diameter of the vesicles can be varied from 340 to 1280 A by varying the conditions under which they are formed; increasing the mole fraction of cholesterol and lowering the pH of the dialysate tend to produce larger vesicles. The gentle detergent treatment required for vesicle formation and the ability to control vesicle size distribution reproducibly may make this method particularly useful in studies of reconstitution of membrane proteins and in use of vesicles as vehicles for delivery of materials to living cells.

Cholesterol↗

PubMed1979

Solubilization and reconstitution of the catecholamine transporter from bovine chromaffin granules.

The catecholamine transporter from bovine chromaffin granules has been solubilized by using low concentrations of sodium cholate in the presence of phospholipids. The functional solubilized protein has been incorporated into liposomes after removal of the detergent either by gel filtration or by dialysis. Reserpine-sensitive accumulation against a concentration gradient is achieved by artifically imposing a pH gradient across the membrane. In the reconstituted system adenosine 5'-triphosphate (ATP) serves as an energy source only at higher detergent concentrations. The proton-translocating adenosine triphosphatase (ATPase) is solubilized in parallel with the increasing efficiency of ATP as an energy source. Several criteria are proposed to distinguish between carrier-mediated (reserpine sensitive) and unmediated transport in the reconstituted system. The reserpine-sensitive process shows affinity and ss presented in this communication provide further support for the contention that concentrative uptake in biogenic amine storage vesicles is driven by a transmembrane pH gradient, which, in the native system, is generated by a proton-translocating ATPase. Moreover, the assays described provide a tool for the isolation and purification of the transport protein.

Adenosine Triphosphatases↗

PubMed1976

Characterization of a cytochrome P-450 dependent monoterpene hydroxylase from the higher plant Vinca rosea.

A monooxygenase isolated from 5-day old etiolated Vinca rosea seedlings was shown to catalyze the hydroxylation of the monoterpene alcohols, geraniol and nerol, to their corresponding 10-hydroxy derivatives. Hydroxylase activity was inpendent upon NADPH (neither NADH nor combination of NADH, NADP+ and ATP served as substitutes) and O2. Geraniol hydroxylation was enhanced by dithiothreitol (monothiols were less effective) and inhibited by phospholipases, thiol reagents, metyrapone, and cytochrome c, as well as other inhibitors of cytochrome P-450 systems. Geraniol was hydroxylated at a faster rate than nerol, but the alcohols possessed similar apparent Km values. The membrane-bound hydroxylase was solubilized by treatment with sodium cholate, Renex-30, or Lubrol-WX. Cholate-treated enzyme was resolved by DEAE-cellulose chromatography and reconstitution of the hydroxylase was effected utilizing different fractions containing cytochrome P-450, a NADPH-cytochrome c reductase, and lipid.

Binding Sites↗

PubMedDate not supplied

Mimicking a cytoskeleton by coupling poly(N-isopropylacrylamide) to the inner leaflet of liposomal membranes: effects of photopolymerization on vesicle shape and polymer architecture.

Networks of N-isopropylacrylamide (NIPAM) copolymers, coupled to spherical phospholipid bilayers, are suitable as a model for the study of the interaction between the cytoskeleton and cellular membranes, as well as for promising new drug delivery systems with triggerable drug release properties and improved stability. In this article, we describe a simple preparation technique for liposomes from egg phosphatidyl choline (EPC) encapsulating a cross-linked NIPAMminus signTEGDM copolymer skeleton (tetraethylene glycol dimethacrylate, TEGDM) which is coupled only to the inner monolayer by a novel membrane anchor monomer. Polymerization in the lipid vesicles was initiated at the inner membrane surface by the radical initiator 2,2-diethoxy-acetophenone (DEAP) permeating through the membrane from the outside. The effects of photopolymerization and polymer formation on vesicle shape and membrane integrity were studied by transmission electron microscopy (TEM), cryo-TEM, and atomic force microscopy (AFM). Upon UV irradiation, approximately 100% of the vesicles contained a polymer gel and only occasional changes in the spherical shape of the liposomes were observed. The architecture of the polymer network inside the liposomal compartment was determined by the conditions of the photopolymerization. Composite structures of polymer hollow spheres or solid spheres, respectively, tethered to spherical membrane vesicles were produced. The increased stability of the polymer-tethered lipid bilayers against solubilization by sodium cholate, compared to pure EPC vesicles, was determined by radiolabeling the lipid membrane.

Acrylic Resins↗

PubMed2006

Synthesis of a polymer skeleton at the inner leaflet of liposomal membranes: polymerization of membrane-adsorbed pH-sensitive monomers.

We describe the synthesis of liposomes with an artificial membrane skeleton as a model of the native cellular cytoskeleton. Similar to natural conditions, a flat polymer network is coupled to the inner membrane leaflet like a suspended ceiling via membrane-inserted anchor monomers with a spacer. The polymer is composed of DMAPMA (N-(3-N,N-dimethylaminopropyl) methacrylamide) and TEGDM (tetraethylene glycol dimethacrylate) as a linker and is coupled to the membrane anchor DOGM (1,2-distearyl-3-octaethylene glycol glycerol ether methacrylate). In the first step of the synthesis, DMAPMA and TEGDM are encapsulated into liposomes composed of egg phosphatidylcholine (EPC), and free monomers are removed by gel chromatography. At pH 10, DMAPMA adsorbs to the inner membrane surface, as demonstrated in parallel studies with lipid monolayers using a Langmuir film balance. The polymerization by UV irradiation was initiated with DEAP (2,2-diethoxyacetophenone) as the initiator and was shown to be complete after 15 min. At pH 6, polymer was desorbed from the inner membrane surface to form a lamellar structure similar to that of the cellular cytoskeleton, as shown by electron microscopy. In comparison to NIPAM (N-isopropylacrylamide), which was used as a monomer in a recent study (Stauch, O.; Uhlmann, T.; Frohlich, M.; Thomann, R.; El-Badry, M.; Kim, Y.-K.; Schubert, R. Biomacromolecules 2002, 3, 324-32), DMAPMA shows much slower membrane permeation leading to an essential restriction of the formed polymer to the liposomal interior. The DMAPMA-based composite structure stabilizes the lipid membrane against sodium cholate by a factor of 2.5 as compared to plain EPC liposomes. This is discussed in the context of the situation in the liver, where the cytoskeleton probably plays a crucial role in the stabilization of the membrane against high bile salt concentration.

Adsorption↗