Phospholipid aggregates

Closed, spherical, single-bilayer, 300- to 600-A-diameter surfactant and/or phospholipid aggregates dispersed in aqueous solutions. Ultrasonic dispersal of multilamellar vesicles (MLVs) or employing procedures such as French Press filtration result in SUV formation. 

Another possible explanation for the fact that transfersomes are able to deliver molecules only under nonoccluded conditions could be that as a result of water evaporation from the applied sodium cholate-phospholipid aggregates system, concentrated micellar systems of cholates or cholates-phospholipids, or both, are generated. The micelles may further delipi-dize the stratum corneum creating small pores through which the drug could penetrate. 

For all the PI-PLC enzymes examined thus far, PI hydrolysis occurs in two sequential reactions as shown in Figure 7 (Griffith and Ryan 1999) (i) an intramolecular phosphotransferase reaction at a phospholipid/aggregate surface to produce DAG and water-soluble 1,2-cyclic phosphate, cIP (or cIPx in the case of a phosphoinositide substrate), followed by (ii) a phosphodiesterase reaction where the cIP is hydrolyzed to inositol-1 -phosphate (or IPX). The second reaction can be studied separately it occurs with a soluble monomeric substrate and is much slower (both lower kcat and higher Km) than the first step (Volwerk et al., 1990 Zhou et al., 1997). It is an excellent probe for allosteric effects of membranes on catalysis (Zhou et al., 1997 Wu et al., 1997). 

A classical desktop centrifuge will be OK for this purpose. At this stage, carefully control the bottom of the tube as the presence of a white precipitate points to removal of phospholipid aggregates this may occur with phospholipids which, for geometrical reasons, are not apt to form small unilamellar vesicles (22, 23). 

One can encounter difficulties regarding the extrusion of liposomes at this high lipid concentrations due to phospholipid aggregates clogging the filters. To avoid this, the extruder needs to be heated to attain a temperature of at least 40°C during the extrusion. 

The amphipathic nature of phospholipids, which governs their interactions, is critical to the structure of biomem-branes. When a suspension of phospholipids is mechanically dispersed in aqueous solution, the phospholipids aggregate into one of three forms spherical micelles and liposomes and sheetlike, two-molecule-thick phospholipid hilayers (Figure... 

A membrane is a large macromolecular complex, about 5 nm thick, in which two sheets of phospholipids aggregate so that their hydrophobic lipid portions are packed together between the outer layers, which are... 

FIGURE32.12 Phospholipid aggregation in planar bilayer and vesiculation in liposomal structure. 

Two papers have dealt with model membrane systems. Shintani and co-workers reported an NOE and MD simulation study of phospholipid aggregates in the form of micelles and vesicles. In the case of these systems, the usual approximation in the NOE analysis - separability of the information on distances and dynamics - are not applicable and the MD data provide an important support for the analysis of experimental data. Uchino et described characterization of rigid and elastic vesicles, loaded with the drug keterolac. Among other techniques, they applied T2 (really line-width) studies for suitable isolated proton spins in the drug. 

Leontidis E (2010) Phospholipid aggregates as model systems to understand ion-specific effects experiments and models. In Kunz W (ed) Specific ion effects. World Scientific, Singapore, pp 55-84... 

Association of the hydrophobic tails of membrane lipids leads, as depicted in Fig. 4.47a, to what Watkins [341] has described as polar discontinuities. Transition to the micellar state is considered to be essential to allow cell fusion to occur as the biomolecular leaflet is thermodynamically a stable system which would resist coalescence with similar structures. External influences can, however, induce phospholipid aggregation and can thus alter the permeability of cell membranes to water-soluble and oil-soluble species. Calcium ions, for example, induce inverse micelle formation in phospholipid systems [342]. Other metal ions also result in this transformation (Fig. 4.47b) addition of adenosine triphosphate (ATP) removes the metal and leads to a reversion to the normal micellar pattern. Maas and Coleman [343] have postulated that such transitions may have significance in nerve membrane operation. Metal-ATP-phospholipid complexes... 

Other nuclei can be useful in determining structure of lyotropic liquid crystals. In fact, N and NMR have been applied successfully to investigation of the structure of phospholipid aggregates in water. 

Because of their relatively large hydrophobic surface area, apolipoproteins, in the absence of lipids, readily self-associate in aqueous solution (Stone and Reynolds, 1975 Vitello and Scanu, 1976). The rate of desorption of apolipoproteins from lipoprotein surfaces has not been studied systematically. Extensive studies of the reverse process, which is the assembly of lipid apoprotein complexes, have been conducted in considerable detail. The dynamic of lipid-protein interactions have been studied primarily with in vitro model systems. Analysis of the association of apolipoproteins with various phospholipid aggregates have provided important clues about the nature of the kinetically important steps in the transfer of apolipoproteins between lipoproteins (Pownall et al., 1977 1978a Massey et al., 1981a Mantulin et al., 1981). 

Phospholipid Aggregates as Modei Systems to Understand Ion-Specific Effects Experiments and Modeis... 

Phospholipid Aggregates to Understand Ion-Specific Effects the partitioning model ... 

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