Membranes Natural Lipids

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Figure 13.2 Activated G protein receptors, here represented as seven red transmembrane helices, catalyze the exchange of GTP for GDP on the Gapy trimer. The then separated Ga-GTP and Gpy molecules activate various effector molecules. The receptor is embedded in the membrane, and Ga, Gpy and G py are attached to the membrane by lipid anchors, and they all therefore move in two dimensions. (Adapted from D. Clapham, Nature 379 297-299, 1996.)...

Finkelstein, A. and Cass, A. (1967). Effect of cholesterol on the water permeability of thin lipid membranes, Nature, 216, 717-718. 

The method of introduction of the fluorophore into the membrane is also important. Many probes are introduced into preexisting vesicles, natural membranes, or whole cells by the injection of a small volume of organic solvent containing the fluorophore. For DPH, tetrahydrofuran is commonly used, while methanol is often employed for other probes. The amount of solvent used should be the absolute minimum possible to avoid perturbation of the lipids, since the solvent will also partition into the membrane. With lipid vesicles this potential problem can be avoided by mixing the lipids and fluorophore followed by evaporation of the solvent and codispersing in buffer. For fluorophores attached to phospholipids, this is the only way to get the fluorophore into the bilayer with natural membranes, phospholipid exchange proteins or other techniques may have to be employed. 

Sretcher, M.S., 1972a, Asymmetrical lipid bilayer structure for biological membranes. Nature New Biol, 236 11-12. 

Although the present finding that BLM formed from simple lipids alone can possess intrinsic low yb without the presence of protein layers, it in no sense invalidates the bimolecular leaflet model. Our study does suggest, however, that natural lipids such as lecithin when in a bilayer configuration could exist in natural membranes to give the results of low yt observed by earlier workers, which has been thought essential in the development of the concept of the bimolecular lipo-protein model based upon interfacial tension measurements. 

Incorporation of Natural Lipids into Polymerizable Membranes... 

What reasons are there for mixing polymerizable lipids with natural ones Polymerized membrane systems, especially those based on diacetylenic lipids, have proven to be excessively rigid and to show no phase transition. Addition of natural lipids could help to retain a certain membrane mobility even in the polymerized state, with almost unaffected stability. Furthermore, natural lipids can provide a suitable environment for the incorporation of membrane proteins into polymerizable membranes (see 4.2.3). Besides this, enzymatic hydrolysis of the natural membrane component can be used for selectively opening up a vesicle in order to release entrapped substances in a defined manner (see 4.2.2). Therefore, it is interesting to learn about the miscibility of polymerizable and natural lipids and also about the polymerization behavior of these mixtures. Investigations on this subject have thus far focused on mixtures of natural lipids with polymerizable lipids carrying diacetylene moieties. 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

Enzymatic Hydrolysis of Natural Lipids in Polymeric Membranes... 

The action of phospholipase A2 on mixed monolayers of natural and polymerizable lipids can be measured under constant surface pressure by the contraction of the monolayer as a function of time as depicted schematically in Fig. 39. It turns out that the chief parameter influencing the enzymatic activity is the miscibility of the lipid components and not the fact whether the film is polymerized or not. In mixed and demixed membranes the enzyme is able to hydrolyze the natural lipid component, but with considerable differences in the hydrolizing rate (Fig. 40). A pure dilauroyllecithin (DLPC) monolayer is completely hydrolyzed in a few minutes after injecting the enzyme... 

In addition to enzymatic hydrolysis of natural lipids in polymeric membranes as discussed in chapter 4.2.2., other methods have been applied to trigger the release of vesicle-entrapped compounds as depicted in Fig. 37. Based on the investigations of phase-separated and only partially polymerized mixed liposomes 101, methods to uncork polymeric vesicles have been developed. One specific approach makes use of cleavable lipids such as the cystine derivative (63). From this fluorocarbon lipid mixed liposomes with the polymerizable dienoic acid-containing sulfolipid (58) were prepared in a molar ratio of 1 9 101115>. After polymerization of the matrix forming sulfolipids, stable spherically shaped vesicles are obtained as demonstrated in Fig. 54 by scanning electron microscopy 114>. 

The membrane-bound catalysts for water oxidation can also be obtained with other transition metal hydroxides. Gerasimov et al. [272] have shown that illumination of a Ru(bpy) + — persulfate system in the presence of Co(II) and lipid vesicles results in the formation of a colloid catalyst for water oxidation, viz. Co(III) hydroxide, immobilized on the lipid membranes. The same catalyst can be obtained without illumination by Co(II) oxidation with a Ru(bpy)3+ complex in the vesicle suspension. The selectivity of water oxidation with the catalysts thus obtained depends on the nature of the membrane-forming lipid. Switching from the synthetic DPL to the natural eggL the process selectivity decreases by about two orders of magnitude due to consumption of the oxidant for oxidation of organic impurities contained in lipids of natural origin [113]. 

Membrane lipids are amphiphatic molecules they contain both a hydrophilic and a hydrophobic moiety. Molecules of this kind can lead to various types of interface. A natural example is the cellular membrane, a bilayer arrangement of such molecules, that marks the frontier between cells. The principal constituents of this membrane are lipids and proteins (Fig. 17.1). 

Membranes serve to delineate cellular entities, in which case they are termed plasma membranes, and various subcellular particles, such as mitochondria, nuclei, and lysosomes. Such subcellular structures as endoplasmic reticulum and the Golgi apparatus also consist, in large part, of membranous materials. The basic structure of membranes is lipid in nature, and it is the amphipathic nature of such lipid that permits them to aggregate into bilayers. Lipid is not the only component of membranes, however there are protein and carbohydrate components as well. It is these components that function as the means by which hydrophilic substances can cross membranes or that serve as recognition beacons that permit various substances to affect the metabolic activities of cellular and subcellular particles. 

As it is well known [36,37], the natural lipid/protein mixtures (such as amniotic fluid) can undergo different phase transitions due to variation in temperature or composition. Of special importance for the natural bilayer lipid membranes is the so-called main phase transition between the lipid crystalline and gel states at which a melting of the hydrocarbon tails of the lipid molecules occurs. For example, it has been demonstrated [36] that there exists an upper limit of the gel phase content in membranes above which the membrane morphology and permeability change dramatically thus making the execution of the physiological functions of the membrane impossible. 

---