Phospholipids defined

The Interaction of Sphingomyelin, Cholesterol and Phospholipids Defines Membrane Structure and Functional Properties... 

Rats fed a purified nonlipid diet containing vitamins A and D exhibit a reduced growth rate and reproductive deficiency which may be cured by the addition of linoleic, a-linolenic, and arachidonic acids to the diet. These fatty acids are found in high concentrations in vegetable oils (Table 14-2) and in small amounts in animal carcasses. These essential fatty acids are required for prostaglandin, thromboxane, leukotriene, and lipoxin formation (see below), and they also have various other functions which are less well defined. Essential fatty acids are found in the stmctural lipids of the cell, often in the 2 position of phospholipids, and are concerned with the structural integrity of the mitochondrial membrane. 

The lag-phase measurement at 234 nm of the development of conjugated dienes on copper-stimulated LDL oxidation is used to define the oxidation resistance of different LDL samples (Esterbauer et al., 1992). During the lag phase, the antioxidants in LDL (vitamin E, carotenoids, ubiquinol-10) are consumed in a distinct sequence with a-tocopherol as the first followed by 7-tocopherol, thereafter the carotenoids cryptoxanthin, lycopene and finally /3-carotene. a-Tocopherol is the most prominent antioxidant of LDL (6.4 1.8 mol/mol LDL), whereas the concentration of the others 7-tocopherol, /3-carotene, lycopene, cryptoxanthin, zea-xanthin, lutein and phytofluene is only 1/10 to 1/300 of a-tocopherol. Since the tocopherols reside in the outer layer of the LDL molecule, protecting the monolayer of phospholipids and the carotenoids are in the inner core protecting the cholesterylesters, and the progression of oxidation is likely to occur from the aqueous interface inwards, it seems reasonable to assign to a-tocopherol the rank of the front-line antioxidant. In vivo, the LDL will also interact with the plasma water-soluble antioxidants in the circulation, not in the artery wall, as mentioned above. 

Lipids may be defined as a large group of molecules with a substantial portion of aliphatic or aromatic hydrocarbon. Included are molecules with diverse chemical characteristics, such as the hydrocarbons, soaps, detergents, acylglycerols, steroids, phospholipids, sphin-golipids, and fat-soluble vitamins, and, subsequently, with diverse physical behavior. One of the most important characteristics of lipids from a biological aspect is their behavior in aqueous environments, as all cells exist in an aqueous milieu. In this respect, the lipids range from almost total insolubility to nearly complete solubility. 

Figure 5.1 shows a tetrad of equilibrium reactions related to the partitioning of a drug between an aqueous environment and that of the bilayer formed from phospholipids. (Only half of the bilayer is shown in Fig. 5.1.) By now, these reaction types might be quite familiar to the reader. The subscript mem designates the partitioning medium to be that of a vesicle formed from a phospholipid bilayer. Equations (4.1)-(4.4) apply. The pAi m in Fig. 5.1 refers to the membrane pKa. Its meaning is similar to that of pAi when the concentrations of the uncharged and the charged species in the membrane phase are equal, the aqueous pH at that point defines pAi em, which is described for a weak base as...

It is possible that nematode-secreted AChEs act on alternative substrates to ACh. We had previously suggested, on the basis of structural similarity, that platelet-activating factor (PAF), a potent phospholipid mediator of inflammation, might represent such an alternative substrate (Blackburn and Selkirk, 1992b) but subsequent studies demonstrated that purified AChEs did not cleave PAF, and the enzyme responsible for this activity in secreted products of N. brasiliensis, PAF acetylhydrolase, was purified and defined as a distinct heterodimeric protein (Grigg et al., 1996). Although an open mind on the subject sould be kept, the strict substrate specificity of the nematode-secreted AChEs suggests that they most likely act on ACh alone. 

Lipids are biological molecules that are soluble in certain organic solvents (whether or not something is a lipid is operationally defined by the solubility). Lipids include a variety of molecules such as triglycerides, phospholipids, and cholesterol. The major type of lipid in membranes is the phospholipid. They re called phospholipids because they all contain a phosphate diester. 

In aqueous systems, membrane lipids may exist in a gel-like solid state or as a two-dimensional liquid. In the case of pure phospholipids, these states interconvert at a well-defined transition temperature, Tc, that increases with alkyl chain length and decreases with introduction of alkyl chain unsaturation. In cell membranes, which have marked heterogeneity in both the polar and nonpolar domains of the bilayer, this state is described as liquid disordered . The presence of sufficient sphingolipids, with... 

Another, but less well defined, class of molecules, some of whose members mediate adhesion interaction, is the four-transmembrane domain family, which shares similar hydropathy plots and may have similar dispositions with respect to the phospholipid bilayer, for example the myelin proteolipid proteins, the connexins of gap junctions, the ryanodine receptor and others. 

A variety of methods have been developed to study exocytosis. Neurotransmitter and hormone release can be measured by the electrical effects of released neurotransmitter or hormone on postsynaptic membrane receptors, such as the neuromuscular junction (NMJ see below), and directly by biochemical assay. Another direct measure of exocytosis is the increase in membrane area due to the incorporation of the secretory granule or vesicle membrane into the plasma membrane. This can be measured by increases in membrane capacitance (Cm). Cm is directly proportional to membrane area and is defined as Cm = QAJV, where Cm is the membrane capacitance in farads (F), Q is the charge across the membrane in coulombs (C), V is voltage (V) and Am is the area of the plasma membrane (cm2). The specific capacitance, Q/V, is the amount of charge that must be deposited across 1 cm2 of membrane to change the potential by IV. The specific capacitance, mainly determined by the thickness and dielectric constant of the phospholipid bilayer membrane, is approximately 1 pF/cm2 for intracellular organelles and the plasma membrane. Therefore, the increase in plasma membrane area due to exocytosis is proportional to the increase in Cm. 

The results summarized above were obtained by using fluorescence based assays employing phospholipid vesicles and fluorescent labeled lipopeptides. Recently, surface plasmon resonance (SPR) was developed as new a technique for the study of membrane association of lipidated peptides. Thus, artificial membranes on the surface of biosensors offered new tools for the study of lipopeptides. In SPR (surface plasmon resonance) systemsI713bl changes of the refractive index (RI) in the proximity of the sensor layer are monitored. In a commercial BIAcore system1341 the resonance signal is proportional to the mass of macromolecules bound to the membrane and allows analysis with a time resolution of seconds. Vesicles of defined size distribution were prepared from mixtures of lipids and biotinylated lipopeptides by extruder technique and fused with a alkane thiol surface of a hydrophobic SPR sensor. 

In model systems for bilayers, one typically considers systems which are composed of one type of phospholipid. In these systems, vesicles very often are observed. The size of vesicles may depend on their preparation history, and can vary from approximately 50 nm (small unilamellar vesicles or SUVs) up to many pm (large unilamellar or LUV). Also one may find multilamellar vesicular structures with more, and often many more than, one bilayer separating the inside from the outside. Indeed, usually it is necessary to follow special recipes to obtain unilamellar vesicles. A systematic way to produce such vesicles is to expose the systems to a series of freeze-thaw cycles [20]. In this process, the vesicles are repeatedly broken into fragments when they are deeply frozen to liquid nitrogen temperatures, but reseal to closed vesicles upon thawing. This procedure helps the equilibration process and, because well-defined vesicles form, it is now believed that such vesicles represent (close to) equilibrium structures. If this is the case then we need to understand the physics of thermodynamically stable vesicles. 

The studies on phospholipid bilayers with defined amounts of charged component are helpful to explain the partition characteristics in biological membranes. Liposome water partition data of propranolol in lipids from kidney epithelial cells (a common model system in pharmaceutical sciences for the uptake into the gastrointestinal tract) have been successfully described with partition models developed for pure bilayers or defined mixtures [159]. Since lipophilic cations and anions can be used as probes for the membrane potential, their interaction with microbial and mitochondrial membranes has been studied... 

The ability of a chiral molecule to distinguish between the enantiomers of a second (different) chiral molecule was defined in Sect. II as a diastereomer discrimination. This phenomenon may be observed in a mixed monolayer of two chiral surfactants and may also occur when a chiral substance is dissolved in the aqueous subphase under the monolayer of a second chiral substance. As before, examples of such chiral discrimination would not include those whose difference in monolayer behavior results only from the gross structural differences of diastereomers such as the different force-area characteristics exhibited by mixed monolayers of l-oleoyl-2-stearoyl-3-s -phospha-tidylcholine with epimeric steroids (120). The relevant experiment, that of comparing the monolayer behavior of mixed monolayers of cholesterol with enantiomeric phospholipids, has been reported (121). As might be anticipated from our previous discussion of... 

Boni, L.T., and Rando, R.R., 1985, The nature ofprotein kinase C activation by physically defined phospholipid vesicles and diacylglycerols. J. Biol. Chem. 260 10819-10825. 

In addition to the differences in phospholipid content between microbial and host cell membranes, it has been demonstrated that disparity exists between the transmembrane potentials of both organisms. The transmembrane potential is defined by the proton flux between the inner and outer bilayers of the cytoplasmic membrane and ranges from —90 to —110 mV in normal mammalian cells in contrast to transmembrane potentials of —130 to —150mV for logarithmic phase microbes. The differences in these electrochemical gradients have been postulated to drive the influx of peptides into the cell and thus act as a crucial barrier for defining host defense peptide selectivity. ... 

A standard procedure for manufacturing liposomes is the film-forming method where the phospholipids are dissolved in an organic solvent. By rotational evaporation of the solvent a thin, multilayered film of phospholipids arises at the inner wall of the vessel. Redispersion of this film in water or aqueous buffer results in the formation of vesieles. The size of these vesicles and the number of bilayers vary. Henee further manufaeturing steps have to follow to obtain defined vesieular dispersions with a suffieiently long shelf life. 

Definity/phospholipid -1- PEG-phospho-lipid liposome aqueous dispersion with gas headspace requires preactivation by high-speed mixing CjFg Approved in US [24]... 

Both the heat-stable SCP protein of Ritter and Dempsey (R2) and the heat-labile SCP protein of Scallen et al. (S2) bind other lipids (e.g., phospholipids and fatty acids) in addition to water-insoluble cholesterol and its precursors (R2, R3, R5, S2). In view of this apparent lack of specificity, Ritter and Dempsey (R5) have suggested that the carrier protein may be more generally called lipid carrier protein (LCP), although its binding to squalene and sterol carrier protein may more directly refiect its functional role in cholesterol biosynthesis. Obviously, more work is needed to clearly define both structural role and functional properties of this protein or proteins. 

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