Lipid vesicles, cholesterol effects

Pyrene has been used to investigate the extent of water penetration into micelles and to accurately determine critical micellar concentrations (Kalyanasundaram, 1987). Polarity studies of silica or alumina surfaces have also been reported. In lipid vesicles, measurement of the ratio Ii/Iui provides a simple tool for determination of phase transition temperatures and also the effect of cholesterol addition. 

Criado M, Eibl H, Barrantes Ff. Effects of lipids on acetylcholine receptor. Essential need of cholesterol for maintenance of agonist-induced state transitions in lipid vesicles. Biochemistry. 1982 21(15) 3622-3629. 

Discuss the effects on the lipid phase transition of pure dimyris-toyl phosphatidylcholine vesicles of added (a) divalent cations, (b) cholesterol, (c) distearoyl phosphatidylserine, (d) dioleoyl phosphatidylcholine, and (e) integral membrane proteins. 

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. 

The lipid content of the membranes can be varied, allowing systematic examination of the effects of varying lipid composition on certain functions. For instance, vesicles can be made that are composed solely of phosphatidylchohne or, alternatively, of known mixtures of different phospholipids, glycohpids, and cholesterol. The fatty acid moieties of the lipids used can also be varied by employing synthetic lipids of known... 

Yeagle, P. L. Hutton, W. C. Huang, C.-H. Martin, R. B., Headgroup conformation and lipid-cholesterol association in phosphatidylcholine vesicles A 31P 1H nuclear Over-hauser effect study, Proc. Natl. Acad. Sci. 72, 3477-3481 (1975). 

Figure 2 (A) Effect of incubation temperature on uptake of doxorubicin into 200 nm EPC/cholesterol (55 45 mol/mol) large unilamellar vesicles (LUVs) exhibiting a transmembrane pH gradient (pH 4 inside, 7.8 outside). Doxorubicin was added to LUVs (D/L = 0.3 wt wt) equilibrated at 21°C, 37°C, and 60°C. (B) Effect of cholesterol on the uptake of doxorubicin at 20 into lOOnm LUVs exhibiting a transmembrane pH gradient (pH 4.6 inside, 7.5 outside). Lipid compositions were EPC and EPC/cholesterol (1 1 mol/mol). The initial drug-to-lipid ratio was 100 nmol/pmol. Source Prom Refs. 12 (A), 21 (B).

In the previous two sections we discussed the electrodeformation and electroporation of vesicles made of single-component membranes in water. In this section, we consider the effect of salt present in the solutions. The membrane response discussed above was based on data accumulated for vesicles made of phosphatidylcholines (PCs), the most abundant fraction of lipids in mammahan cells. PC membranes are neutral and predominantly located in the outer leaflet of the plasma membrane. The inner leaflet, as well as the bilayer of bacterial membranes, is rich in charged lipids. This raises the question as to whether the presence of such charged lipids would influence the vesicle behavior in electric fields. Cholesterol is also present at a large fraction in mammalian cell membranes. It is extensively involved in the dynamics and stability of raft-hke domains in membranes [120]. In this section, apart from considering the response of vesicles in salt solutions, we describe aspects of the vesicle behavior of fluid-phase vesicles when two types of membrane inclusions are introduced, namely cholesterol and charged lipids. 

In order to understand the complex behavior of cellular membranes and their response to external perturbations Uke electric flelds, one has to elucidate the basic mechanical properties of the lipid bilayer. The signiflcant expansion in recent years of the field of membrane raft-hke domain formation [11, 132, 133] imposes the compelling need for understanding the effect of hpid bilayer composition on membrane properties. Cholesterol, a ubiquitous species in eukaryotic membranes, is an important component in raft-hke domains in cells and in vesicles, which mohvates studies aimed at understanding its influence on the mechanical properties and stability of membranes. 

In a subsequent study, van Hal et al. [40] reported that a decrease in cholesterol content in liquid state bilayers, which increases bilayer fluidity, resulted in an increase in estradiol transport across SC. With confocal laser scanning microscopy, Meuwissen et al. examined the diffusion depth of gel- vs. liquid-state liposomes labeled with fluorescein-dipalmitoylphosphatidylethanolamine (fluorescein-DPPE) with human skin in vitro [41] (Figure 3) and rat skin in vivo [42] and found that the lipophilic label when applied in liquid-state bilayers onto the skin penetrated deeper into the skin than when applied in gel-state liposomes. Recently, Fresta and Puglisi [43] reported that corticosteroid dermal delivery with skin-lipid liposomes was more effective than delivery with phospholipid vesicles, both with respect to higher drug concentrations in deeper skin layers and therapeutic effectiveness. This is a very surprising result, because skin lipid liposomes are rigid and form stacks of lamellae on the surface of the skin [44]. From the previously mentioned studies it seems clear that the thermodynamic state of the bilayer plays a crucial role in the effect of vesicles on dmg transport rate across skin in vitro. 

It is of interest not only to perforate vesicle membranes but also to destroy them after they have served their purpose as transport vehicles, in particular for DNA. Natural vesicles, so-called endosomes, contain about 50% cholesterol. The disruption of such cholesterol-containing lipid bilayers by Triton XI00 or sodium deoxycholate, examples of artificial and natural detergents, results in a leaky membrane at low concentration and in a catastrophic rupture process above the cmc of the amphiphiles. Vesicles made of fluid phospholipid bilayers devoid of cholesterol showed only leakiness under the same conditions. Amphiphiles with a carboxylate end group and a very bulky hydrophobic end (e.g., with two tert. butyl groups) disrupt membranes at pH 5 and have no effect above pH 7 (harpoons). For an example, see Figure 6.5.3. 

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