Lecithin moieties

The identity of the moiety (other than glycerol) esterified to the phosphoric group determines the specific phosphoHpid compound. The three most common phosphoHpids in commercial oils are phosphatidylcholine or lecithin [8002-45-5] (3a), phosphatidylethanolamine or cephalin [4537-76-2] (3b), and phosphatidjlinositol [28154-49-7] (3c). These materials are important constituents of plant and animal membranes. The phosphoHpid content of oils varies widely. Laurie oils, such as coconut and palm kernel, contain a few hundredths of a percent. Most oils contain 0.1 to 0.5%. Com and cottonseed oils contain almost 1% whereas soybean oil can vary from 1 to 3% phosphoHpid. Some phosphoHpids, such as dipaLmitoylphosphatidylcholine (R = R = palmitic R" = choline), form bilayer stmetures known as vesicles or Hposomes. The bdayer stmeture can microencapsulate solutes and transport them through systems where they would normally be degraded. This property allows their use in dmg deHvery systems (qv) (8). 

Phosphatidylcholine, commonly known as lecithin, is the most commonly occurring in natnre and consists of two fatty add moieties in each molecule. Phosphati-dylethanolamine, also known as cephahn, consists of an amine gronp that can be methylated to form other compounds. This is also one of the abundant phospholipids of animal, plant, and microbial origin. Phosphatidylserine, which has weakly acidic properties and is found in the brain tissues of mammals, is found in small amounts in microorganisms. Recent health claims indicate that phosphatidylserine can be used as a brain food for early Alzheimer s disease patients and for patients with cognitive dysfunctions. Lysophospholipids consist of only one fatty acid moiety attached either to sn-1 or sn-2 position in each molecule, and some of them are quite soluble in water. Lysophosphatidylchohne, lysophosphatidylserine, and lysophos-phatidylethanolamine are found in animal tissues in trace amounts, and they are mainly hydrolytic products of phospholipids. 

Studies of the reaction of ozone with simplified lipid systems have shown that malonaldehyde can be produced by direct ozonolysis. The use of malonaldehyde assay as an index of lipid peroxidation is therefore invalid in ozone studies. Liposomes formed from egg lecithin and prepared in aqueous media were quite resistant to ozone, but the contribution of polyconcentric spheres to this resistance has not been fully assessed. However, the bilayer configuration, with the susceptible unsaturated fatty acids shielded from ozone by the hydrophilic areas of the molecule, may be resistant. In hexane, where the fatty acid moieties are exposed, ozone reacts stoichiometrically with the double bonds. The experiments with aqueous suspensions of phosphatidylcholine gave no evidence of the formation of lipid peroxides,nor did experiments with films of fatty acids exposed to ozone. ... 

A model that is consistent with these observations of the action of trypsin and phospholipase A and with the discontinuities in the All-composition curves (Figures 2 and 3) is one in which the lipid monolayer is not a continuous palisade of uniformly oriented lipid molecules but rather an assembly of surface micelles. In this model, proposed by Colacicco (4, 5), the protein first comes into contact with the lipid molecules at the periphery of the surface micelles and then inserts itself as a unit between them. This is the basis for the generalized nonspecific interaction between lipids and proteins which results in increase of surface pressure. One may thus explain the identical All values obtained with films of lecithin and 80 mole % lactoside by picturing the lecithin molecules outside and the lactoside molecules inside the surface micelles. In this model lecithin prevents the bound lactoside from interacting nonspecifically with globulin and produces the same increase in pressure as with a film of pure lecithin. In the mixed micelle the lactose moiety of the lactoside protrudes into the aqueous subphase. Contact of the protein with these or other nonperipheral regions of the surface micelle would not increase the surface pressure. 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

La water-lecithin is a lamellar structure in which the polar heads (the phosphatidyl choline group of lecithin) constitute two-dimensional disordered arrays in contact with water, whereas the chains are in the molten state in between water layers in disordered moieties (see Figure 1). (For a review of x-ray studies of lecithin-water phases, see Ref. 11.)... 

A surfactant molecule is an amphiphile, which means it has a hydrophilic (water-soluble) moiety and a hydrophobic (water-insoluble) moiety separable by a mathematical surface. The hydrophobic tails of the most common surfactants are hydrocarbons. Fluorocarbon and perfluorocarbon tails are, however, not unusual. Because of the hydrophobic tail, a surfactant resists forming a molecular solution in water. The molecules will tend to migrate to any water-vapor interface available or, at sufficiently high concentration, the surfactant molecules will spontaneously aggregate into association colloids, i.e., into micelles or liquid crystals. Because of the hydrophilic head, a surfactant (with a hydrocarbon tail) will behave similarly when placed in oil or when put in solution with oil and water mixtures. Some common surfactants are sodium or potassium salts of long-chained fatty acids (soaps), sodium ethyl sulfates and sulfonates (detergents), alkyl polyethoxy alcohols, alkyl ammonium halides, and lecithins or phospholipids. 

Transesterification. Transesterification allows for the incorporation of free fatty acids into lecithin molecules. Unhydrolyzed lecithin contains two fatty acids, and the fatty acid moiety can be different at the two positions on the phospholipid molecule. The fatty acid composition can have an effect on the stability and functionality of the lecithin. Changes in the fatty acid composition can be done through transesterification (165). Transesterification using lipases can be used for the addition of polyunsaturated fatty acids to lecithin to enhance the essential fatty acid profile, or to improve functionality (166). 

Oi-lipoproteins based on their electrophoretic migration. HDL transports cholesterol esters derived from the action of lecithin-cholesterol-acetyl transferase (LCAT). They are further differentiated into HDL2a, HDL2b and HDL3 on the basis of their varying densities and protein moiety. (29) (s. fig. 3.8)... 

For molecular modeling of the lipoproteins, values for the partial specific volumes of the lipoprotein components are required. The partial specific volume of an aqueous egg yolk lecithin suspension is 0.984 ml/g (Hauser and Irons, 1972), and this provides a reasonable approximation for the partial specific volume of the phospholipid occupying the surface monolayer of a lipoprotein. The reciprocal of the density of liquid triolein (Small, 1986) yields its partial specific volume, 1.102 ml/g, and provides a reasonable approximation for triglyceride dissolved in the cholesteryl ester-filled core of the LDL. For cholesterol, the partial specific volume of 1.021 ml/g measured in benzene (Haberland and Reynolds, 1973) has been employed. The value of 0.740 ml/g employed for the partial specific volume of apoBlOO was determined from its amino acid composition (Lee et al., 1987). A value of 0.60 ml/g was used for the partial specific volume of the carbohydrate moiety. One important parameter, the partial specific volume of cholesteryl ester, remains to be determined. As will be shown below, its value is estimated to be 1.058 ml/g. 

Phosphatidylcholines, or lecithins, are zwitterionic over a wide pH range because of the presence of a quaternary ammonium group and a phosphate moiety. Phosphatidylcholines are the most abundant phospholipids in animal tissues and typically contain palmitic, stearic, oleic, linoleic, or arachidonic acid, usually with saturated fatty acids in the sn- position and unsaturated fatty acids at sn-2. 

This reaction is responsible for formation of most of the cholesteryl ester in plasma. The preferred substrate is phosphatidylcholine, which contains an unsaturated fatty acid residue on the 2-carbon of the glycerol moiety. HDL and LDL are the major sources of the phosphatidylcholine and cholesterol. Apo A-I, which is a part of HDL, is a powerful activator of LCAT. Apo C-I has also been implicated as an activator of this enzyme however, activation may depend on the nature of the phospholipid substrate. LCAT is synthesized in the liver. The plasma level of LCAT is higher in males than in females. The enzyme converts excess free cholesterol to cholesteryl ester with the simultaneous conversion of lecithin to lysolecithin. The products are subsequently removed from circulation. Thus, LCAT plays a significant role in the removal of cholesterol and lecithin from the circulation, similar to the role of lipoprotein lipase in the removal of triacylglycerol contained in chylomicrons and VLDL. Since LCAT regulates the levels of free cholesterol, cholesteryl esters, and phosphatidylcholine in plasma, it may play an important role in maintaining normal membrane structure and fluidity in peripheral tissue cells. 

To summarize, the lecithin studies provide a qualitative picture of how the solvent affects the phase behavior for a zwitterionic surfactant with a large hydro-phobic moiety. Lecithin forms lamellar lyotropic liquid crystals with a wide variety of solvents a sufficiently hydrophilic solvent — and indeed even amphiphilic compounds with a hydrophilic moiety — stabilizes lamellar phases. 

N+ trimethylammonium group P phosphate group P — N+ = polar moiety of the lecithin molecule... 

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