Artificial lipid vesicles

Fig. 5. (a) Detergent solubilization and (b) reconstitution into artificial lipid vesicles of an integral... 

Artificial lipid vesicles, termed liposomes, are colloid particles in which phospholipid bilayers or tetraether monolayers encapsulate an aqueous medium. Because of their physicochemical properties, liposomes are widely used as model systems for biological membranes and as delivery systems for biologically active molecules. In general, water-soluble molecules are encapsulated within the aqueous compartment whereas water insoluble substances may be intercalated into the liposomal membrane [147]. 

Like the spinach enzyme, the pea ATPase is activated equally by Mg2+ and Mn2+ and hydrolyzes a broad range of nucleoside triphosphates, but not ADP, AMP, or monophosphorylated substrates. Although pea chloroplast envelope membranes have ADPase and pyrophosphatase activity, we conclude that the activities are distinct from the ATPase activity. The envelope ATPase differs from putative transport ATPases characterized in other plant membranes in that it is not inhibited by vanadate or DCCD, nor is it stimulated by potassium. However, a role for this activity in proton efflux and ion transport cannot be ruled out, because the envelope vesicles may be sufficiently leaky that protons and ions can diffuse freely across the membrane. This might limit any stimulatory effect of K+ and uncouplers. Evidence supporting a role for the ATPase in proton transport will depend on further characterization of the envelope vesicles, and/or purification and reconstitution of the ATPase into artificial lipid vesicles. 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir-Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11-16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17[. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. 

As well as fluorescence-based assays, artificial membranes on the surface of biosensors offered new tools for the study of lipopeptides. In a commercial BIA-core system [231] a hydrophobic SPR sensor with an alkane thiol surface was incubated with vesicles of defined size distribution generating a hybrid membrane by fusion of the lipid vesicles with the alkane thiol layer [232]. If the vesicles contain biotinylated lipopeptides their membrane anchoring can be analyzed by incubation with streptavidine. Accordingly, experiments with lipopeptides representing the C-terminal sequence of N-Ras show clear differences between single and double hydrophobic modified peptides in their ability to persist in the lipid layer [233]. 

Fig. 16.2 Schematic representation of cellular and artificial membrane nanotubes. (A) Two cells are connected by a tunneling nanotube (arrowhead) containing a bundle of filamentous actin (red line). N (grey), nucleus M (purple), mitochondrium ER (green), endoplasmic reticulum G (blue), Golgi apparatus. (B) Lipid nanotube connecting two lipid vesicles formed by pulling a membrane tether. (C) Membrane tether pulled from the plasma membrane of a cell (see Color Plates)...

Satoh T, Kobayashi K, Sekiguchi S, et al. Characteristics of artificial red cells. Hemoglobin encapsulated in poly-lipid vesicles. ASAIO J 1992 38 M580. 

The scientific community was attracted to the study of liposomes due to the relatively simple procedure of their preparation. Moreover, if prepared from natural phospholipids, they are biocompatible, and possess low cytotoxicity, low immunogenicity, and biodegradability [304], Liposomes, however, have two main disadvantages the structural instability both in vitro and in vivo, and low cell specificity [304], To increase the stability, the structure of the phospholipid layer has been modified to include artificial lipids and/or cholesterol. Polymerizable vesicles have also been prepared [305]. It is obvious that the biocompatibility of these modified systems has to be addressed. 

Nonviral systems have been developed and used to deliver genes in vivo. Delivery of genes by means of liposomes—artificial lipid bilayer vesicles—overcomes the potential safety hazards associated with viral gene sequences required for packaging. DNA-Uposome complexes have been... 

Fig. 20. The surface of a hydrophobic SPR-sensor (covered with long chain alkanethiols) was enlarged to an artificial membrane by application of lipid vesicles with a defined size distribution. Application of a Ras-lipopeptide construct with both a farnesyl- and a palmitoyl-modification leads to increase in resonance signal assumed to indicate membrane insertion (grey trace). Washing with buffer induces the slow decrease in signal. A Ras protein without hydrophobic modification (black trace) does not lead to signal increase...

Keywords Artificial cells Encapsulation Lipid vesicles Membranes... 

For this purpose liposomes are used as lipid phase. Unilamellar liposomes are artificial lipid bilayer vesicles. They can be considered as real model bilayer membranes as they ideally consist of a circular bilayer membrane. The hydrophobic acyl chains are assembled in the hydrophobic core of the liposome whereas the hydrophilic head groups point to the water in the inside and outside of the vesicle. Liposomes can be produced from a variety of lipids and from mixtures of lipids. This possibility allows studying the influence of membrane constituents on the partition of solutes. Kramer et al. (1997) studied the influence of the presence of free fatty acids in membranes on the partition behaviour of propranolol. The influence on a-Tocopherol in membranes on the partition behaviour of desipramine has been reported recently (Marenchino et al. 2004) using a liposome model. 

The use of artificial lipid membranes and isolated membrane fragments and vesicles has been of great value in the study of membrane function, particularly transport and membrane-bound enzyme reactions. Current research in this area uses sophisticated techniques for determining the molec-... 

Figure 4.24. Examples of artificial lipids that form vesicles...

K. Katagiri, K. Ariga, J. Kikuchi, Preparation of Organic-Inorganic Hybrid Vesicle Cerasome Derived from Artificial Lipid with Alkoxysilyl Head , Chem. Lett, 661 (1999)... 

Liposomes are artificial, spherical vesicles consisting of amphiphilic lipids (mostly phospholipids), enclosing an aqueous core. Depending on the processing conditions and the chemical composition, liposomes can either be unilamellar or multilamellar. 

The highly complex and variable composition of natural cell membranes makes them a difficult subject for experimental studies. Artificial lipid membranes have consequently been prepared and studied for many years as models of cell membranes [1,3-7], A diverse array of geometries has been developed, including small and large unilamellar vesicles, giant lipid vesicles, lipid membranes supported on solid and polymer-coated substrates, and BLMs. These have been used to study the physical and chemical properties of lipids and lipid mixtures as well as membrane-associated proteins, including reconstituted transmembrane receptors. 

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