Liposomes acid vesicles

To demonstrate polymerase activity in a model cell, Chakrabarti et al. [79] encapsulated polynucleotide phosphorylase in vesicles composed of dimyris-toylphosphatidylcholine (DMPC). This enzyme can produce RNA from nucleoside diphosphates such as adenosine diphosphate (ADP) and does not require a template, so it has proven useful for initial studies of encapsulated polymerase activity (Fig. 10a). Furthermore, DMPC liposomes are sufficiently permeable so that 5-10 ADP molecules per second enter each vesicle. Under these conditions, measurable amounts of RNA in the form of polyadenylic acid were synthesized and accumulated in the vesicles after several days incubation. The enzyme-catalyzed reaction could be carried out in the presence of a protease external to the membrane, demonstrating that the vesicle membrane protected the encapsulated enzyme from hydrolytic degradation. Similar behavior has been observed with monocarboxylic acid vesicles [80], and it follows that complex phospholipids are not required for an encapsulated polymerase system to function. 

Figure 2 L/p/d aggregates. K) Micelle, spheroidal aggregate of single tailed fatty acid, the polar heads being in contact outside with water molecules. B Bi layer, two dimensional fluid, composed of glycerophospholipids or sphingolipids, each molecule possessing two hydrophobic tails. C) Liposome, spheroidal vesicle filled with water, bounded by a single bilayer. D) LDL, lipids/proteln/antioxidants aggregate, carrier of cholesterol in blood plasma.

Introduction - Liposomes are vesicles composed of one or more lipid bilayers completely surrounding an internal aqueous space. They are usually composed of phospholipids either in pure form or In combination with other amphipathic molecules such as sterols, long chain bases or acids, or membrane proteins. The structure of liposomes varies from large (0.5->5y) multllamellar vesicles to small ( 300 A) unilamellar vesicles.2,3 More recently, new methods have been reported describing the formation of unilamellar vesicles of intermediate size. >5.6 xhe general properties of liposomes and their interaction with various macromolecules have been described in several reviews. ... 

Two important aspects of Uposome formation must be emphasized here. First, in some cases we observe the formation of ordered supramolecular structures starting from a chaotic disordered mixture of surfactants (as in the ethanol injection method . As noticed before, this increase of order is attended by a simultaneous increase of water entropy and a decrease of overall free energy (Upids and solvent). Secondly, every time a liposome forms, there is the anergence of a division, with an inside world that is different from the external environment, even if the two worlds actually interact with each other. The discrimination between inside and outside, appUcable to lipid vesicles, is the first structural pre-requisite for the living cell. It is therefore clear that lipid or fatty acid vesicles may be considered relevant experimental model of simplified cells, and their role on... 

Liposomes can be prepared according to different methods. Classical discussions" focus on the preparation of phospholipid vesicles, whereas the preparation of fatty acid vesicles has been reviewed only recently. "... 

P. Walde, T. Namani, K. Morigaki, H. Hauser, Formation and properties of fatty acid vesicles (liposomes), in Liposome Technology, 3rd ed., Vol. I, G. Gregoriadis (Ed.), Informa Healthcare,... 

Liposomes are vesicles produced from phospholipids, cholesterol, fatty acids, etc. In typical liposome immunoassays, a fluorophore, mostly carboxy-fluorescein, is encapsulated in the liposomes. The fluorophore can be released by addition of a lysing component such as serum complement or a surfactant. In several liposome based immunoassay formats the hapten is linked to a cytolysin such as mellitin. After the immunoreaction the only free portion of the conjugate is capable of disrupting the liposomes releasing the fluorophore. 

Glycerol-containing phospholipids are used for the preparation of liposomes and vesicles phosphatidylcholine (13.1), phosphatidylserine, phosphatidylethanol-amine, phosphatidylanisitol, phosphatidylglycerol, phosphatidic acid and cholesterol. In most preparations, a mixture of lipids is used to obtain the optimum structure. 

Liposomes are multilamellar structures consisting of several bilayers of lipids (several pm) - they are produced by simply shaking an aqueous solution of phospholipids, e.g. egg lecithin. When sonicated, these multilayer structures produce unilamellar structures (with size range of 25-5 nm) that are referred to as liposomes. A schematic picture of liposomes and vesicles is given in Fig. 1.40. Glycerol-containing phospholipids are used for the preparation of liposomes and vesicles phosphatidylcholine phosphat-idylserine phosphatidylethanolamine phosphatidylanisitol phosphatidylglycerol phosphatidic acid cholesterol. In most preparations, a mixture of lipids are used to obtain the most optimum structure. 

Liposomes are vesicles with a bilayer lipid sheet formed by cationic lipids in aqueous solutions. When liposomes get in contact with nucleic acids they undergo a rearrangement into nucleic acid lipid complexes called lipoplexes. Lipoplexes can be actively taken up by eukaryotic cells by endocytosis, and the lipoplex is then internalized into the cell cytosol within endosomes. The endo-somal complex is finally destroyed by increasing the osmotic pressure created by the lipids buffering action and by the fusion of the lipid with the endosomal membrane. The ability of a lipid to destroy endosomes, also referred to as endosomal escape, is one of the main characteristics of a good synthetic transfection reagent, as it indicates the capability of the vector to release its nncleic acid load into cells once having crossed the cell membrane. 

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

Liposomes, which are lipid bilayer vesicles prepared from mixtures of lipids, also provide a useful tool for studying passive permeability of molecules through lipid. This system has, for example, been used to demonstrate the passive nature of the absorption mechanism of monocarboxylic acids [131]. Liposome partitioning of... 

The work with Cl Acid Yellow 129 used only unilamellar vesicles. The liposomes again suppressed exhaustion but increased dye-fibre bonding, leading to better fastness properties. It is claimed that liposomes can be used to control the rate of exhaustion. 

Fig. 10.5 Schematic diagrams a micelle consisting of ionized fatty acid molecules, a phospholipid bilayer and the vesicle bilayer of a liposome...

The permeability coefficient of 2.6x 10 locm/s at 296 K measured by Deamer is sufficient to supply the enzyme in the liposomes with ADP. How could it be shown that RNA formation actually does take place in the vesicles The increase in the RNA synthesis was detected by observing the fluorescence inside the vesicles. In the interior of the liposomes, the reaction rate is only about 20% of that found for the free enzyme, which shows that the liposome envelope does limit the efficiency of the process. The fluorescence measurements were carried out with the help of ethidium bromide, a fluorescence dye often used in nucleic acid chemistry. 

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