Lecithin Constitution

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.)... 

Lysophospholipids, obtained by complete or partial hydrolysis of lecithins, constitute another class of industrially important surfactants that are currently prepared on a large scale. This hydrolytic reaction, catalyzed by phospholipase A2, is typically carried out in 30% phospholipid emulsion in water. However, the process suffers from several complications, one of which is the necessity to inactive phospholipase A2 after completion of the hydrolysis because it is practically impossible to recover and reuse the enzyme from the heterogeneous reaction mixture. Irreversible inactivation of the phospholipase is achieved either by a combination of alkalization and heat treatment... 

Collectively, as found in nature, phospholipids are usually mixtures of different acyl groupings and terminal head groups. These mixtures can be extracted using solvents from, for example, plants (e.g., soybeans) or eggs and constitute the material usually known in commerce ay lecithin. Unfortunately there has been a tendency to call dipalmitoylphosphatidycholine, PC, one of the major constituents of most natural mixtures, lecithin, and it may be better to avoid applying this name to a purified derivative. 

Approximate Constitution of Purified Lecithin Used to Stabilize Injectable Emulsions... 

Normal persons excrete very little TMA in the urine. However, slight TMA excretion may be observed after meals with a high content of TMA precursors like choline or lecithin, or after eating marine fish due to its high TMA N-oxide content. Healthy women may have a short episode of trimethylaminuria at the onset and during menstruation. TMA has also found to be increased in the urine of some patients using carnitine supplementation. Advanced liver and renal disease may result in TMA excretion and this constitutes the so-called secondary trimethylaminurias. 

This lamellar phase is formed of alternate sheets of lipid and water. The lipidic sheets containing the lecithin and the cholesterol are made of two superposed layers of oriented molecules. Each of these two monolayers is mixed and consists of lecithin and cholesterol molecules arranged side by side with their paraffinic ends turned toward the inside of the sheet and their polar groups (phosphatidyl choline group for lecithin and hydroxyl group for the cholesterol) outward—i.e., toward the adjacent sheet of water. This constitution of each of the two mono-layers forming the lipidic sheet is in conformity with the conclusion arising from the study of mixed monolayers of cholesterol and lecithin spread on the free surface of water (1). 

Lecithin can be fractionated from cottonseed as phospholipids and glycolipids. Cottonseed lecithin shows flavor and color deterioration when blended with other vegetable oils. The saturated/unsaturated fatty acid ratio of cottonseed phospholipids is approximately 1 2 (39). Palmitic acid constitutes 90% of the total saturated fatty acids (36%), and linoleic acid is approximately 80% of the total unsaturated fatty acids (64%). Gossypol binds to lecithin during oil extraction from glanded cottonseed (approximately 9% in cmde phospholipids). This economically negates its... 

Diluting the retentate with fresh solvent and its subsequent concentration by ultrahltration will thus allow a substantially oil-free solution of phosphatides to be obtained. A substantially oil-free lecithin powder product can be produced from this solution by evaporating the solvent, for instance, in a spray tower. Conventional oil-free lecithin powder is normally produced by de-oiling conventional standard lecithin, using either acetone or a liquehed hydrocarbon, such as propane. In conventional processes, this de-oiling constitutes a separate process step, and thus... 

Choline is thus a quaternary ammonium hydroxide base, viz., tri-methyl hydroxy-ethyl ammonium hydroxide, as in the formula just given. The constitution of lecithin is therefore as follows ... 

Figure 1. The structure of lecithin represented schematically (A), as a formula (B), as a model (C), and symbolically (D). Such phospholipid bilayers are thought to constitute the basic structure of cell membranes. Illustration by Hans Cassidy. Courtesy of Gale Group. 

Cholesterol is formed in the liver (85%) and intestine (12%) - this constitutes 97% of the body s cholesterol synthesis of 3.2 mmol/day (= 1.25 g/day). Serum cholesterol is esterized to an extent of 70-80% with fatty acids (ca. 53% linolic acid, ca 23% oleic acid, ca 12% palmitic acid). The cholesterol pool (distributed in the liver, plasma and erythrocytes) is 5.16 mmol/day (= 2.0 g/day). Homocysteine stimulates the production of cholesterol in the liver cells as well as its subsequent secretion. Cholesterol may be removed from the pool by being channelled into the bile or, as VLDL and HDL particles, into the plasma. The key enzyme in the synthesis of cholesterol is hydroxy-methyl-glutaryl-CoA reductase (HGM-CoA reductase), which has a half-life of only 3 hours. Cholesterol is produced via the intermediate stages of mevalonate, squalene and lanosterol. Cholesterol esters are formed in the plasma by the linking of a lecithin fatty acid to free cholesterol (by means of LCAT) with the simultaneous release of lysolecithin. (s. figs. 3.8, 3.9) (s. tab. 3.8)... 

Typical solid lipids used are glycerides and/or fatty acids, and may constitute 30% of the formulation. These are from the same family of lipids found in parenteral nutrition emulsions, such as Intralipid, which have been successfully administered intravenously for several decades. Typical excipients are Dynasan 112, composed of short chain fatty acids, Compritol, lecithin, used as an emulsifier, and surfactants such as polysorbate 80, polaxamer 188, PVP, bile salts such as sodium glycocholate, and Span 85. Water can be replaced with oils or PEG 600 to yield dispersions which can be filled into soft gelatin capsules. 

TDC-L system there is an initial decrease in size and in the TC-L system an initial increase in size. Beyond the vertical dashed lines in the center of the plots the micellar sizes are seen to increase strongly to 80 A as the L/BS values reach the phase limit (broad dashed line). It is apparent that these micellar sizes are much larger than the sizes based on the micellar structures proposed by Small and Dervichian [5,6] (Fig. 17). This finding led us to propose a new molecular model (the mixed disk micelle) for these BS-L particles in which BS both coat the perimeter and are solubilized (probably as reverse dimers) within the disk-like fragments of L bilayers (Fig. 17). The marked divergence in micellar sizes as the phase limit is approached provides an explanation for the existence of the limit since at these lipid ratios the micelles become maximally swollen with L. As the L to BS ratio is increased beyond the phase limit, the excess L plus BS constitute a second lecithin-rich liquid ctystalline phase [102,236,251]. 

When an alcoholic solution of lecithin is brought into contact-with hot solution of barium hydroxid it yields barium glycerophosphate, barium stearate, and cholin (see p. 276). This decomposition indicates the constitution of lecithin and its relations to the fats. Glycerophosphoric acid is phosphoric acid in which an atom of hydrogen has been replaced by the univalent i-emalnder CHaOH- CHOH—CHa—left by the removal of OH from, glycerol ... 

Lecithins are derived from glycerol by the replacement of the hydrogen atoms of the hydroxyl groups by certain acid radicals, one of which is the radical of phosphoric acid. When a lecithin is treated with barium hydroxide partial saponification takes place. Choline and palmitic, stearic or oleic acid are obtained, together with glycerophosphoric acid which has the constitution —... 

In this experiment, carried out in 96-well plates, a buffered donor solution containing the compound is separated from a buffered acceptor solution using a fdter support, which has been treated with a solution of egg lecithin (mostly phosphatidylcholine) dissolved in n-dodecane, thus making a kind of sandwich. The wells are usually stirred to minimize the thickness of the unstirred water layer (or aqueous boundary layer), the water-filled area immediately adjacent to the membrane that can sometimes constitute another formidable barrier to the permeation of compounds. The concentration of the compound in the acceptor well is measured, usually by UV detection, over the course of some hours and the permeability, Papp, can be calculated. ... 

In addition to the route via phosphoric acid, bones constitute a natural biosource of the highly important calcium phosphates. Industrially produced natural products such as casein and lecithin are also alternative sources of some phosphorus compounds (Fignre 12.2). They have a growing nnmber of applications in food products and many other areas. 

All the proteins found in the red cell are probably not catalytic some play a structural role, particularly in building up the stroma and membrane. The stroma of the erythrocyte, which constitutes 2-5% of the wet weight of the cell, is made of proteins and lipids. The protein is called stromatin, and it contains at least nine different amino acids. Stromatin apparently is related to keratin and carries the blood group antigenic character. Ten per cent of the stroma seems to be composed of lipids among them are cephalin, lecithin, and cholesterol. 

Like the lecithins and cephalins, the sphingomyelins are surface-active and are important as components of membranes, particularly in nervous tissue. They may constitute up to 25 per cent of the total lipid in the myelin sheath that protects the nerve cells, but they are absent from, or present only in very low concentrations in, energy-generating tissue. 

Figure 3 depicts a model of the functions of multiple proteins and forms of vitamin A that constitute the visual cycle. SRBP delivers atROH to the RPE, possibly through a plasma membrane SRBP receptor. No SRBP receptor has been isolated, however, and molecular characterization remains elusive. As in other tissues, CRBP(I) sequesters atROH and allows its esterification by the 25-kDa endoplasmic reticulum (ER) enzyme lecithin retinol acyltransferase (LRAT). In fact, the amount of CRBP(I) may... 

Lecithin, glyceryl esters containing two fatty acid residues and one phosphate ester (usually as phosphatidylcholine), constitute one of the oldest of the commercially important phosphoric acid derivative surfactants. However, since they contain nitrogen and are normally either nonionic or amphoteric, they are not discussed further in this section. Additional phosphorus-containing surfactants include derivatives of phosphonic and phosphinic acids. 

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