Lysolecithin structure

At their critical micelle concentrations, surface active agents (such as sodium dodecyl sulfate, Triton X-100, lysolecithin, and bile salts) self-associate into spherical or rod-shaped structures. Because dilution to below the c.m.c. results in rapid disassembly or dissolution of these detergent micelles, micelles are in dynamic equilibrium with other dissolved detergent molecules in the bulk solution. 

A more complex case is the serum lipoprotein (74), shown in Figure 13. When sonicated into water, total lipids from both the low density (/ ) and high density (a) lipoproteins give rise to the high resolution spectra expected of molecules which have a high degree of motion. The spectra of the native lipoproteins show line widths nearly identical to those of the lipids alone, so that no additional motional constraints of the apolar portions of the phospholipids occur when the lipids are bound to the apoproteins of the blood lipoproteins. All the obvious peaks observed in the native lipoproteins can be accounted for by lipid protons, and no upheld shift of the methylene protons occurs. We can conclude that unlike the case of the lysolecithin-serum albumin system, the bonding of lipids to proteins is not apolar. In the serum lipoproteins the NMR results are consistent with a micellar structure and not with extensive apolar association of lipid with protein. 

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. 

Neutral fats behave differently at similar concentrations mainly because they lack the strongly polar head so that their behaviour is determined by their non polar residue and in solution they tend to form small round fat droplets - it is this particular feature that makes them so suited as a food store. Monoacyl phospholipids like lysolecithin tend to behave more like the neutral fats - they prefer to form micelles even at quite low concentration. This means that the insertion of just a few lysolecithin molecules into a lipid bilayer will greatly disrupt its organized structure. 

The lysolecithin treatment removes all or most of the plasma membrane and NE from the sperm, but a small amount of material can sometimes be seen at the ends of the sperm heads when they are stained with the membrane dye 3,3 -dihexyloxacarbocyanine iodide (DiOQ cat. 14414 Eastman Kodak Co., Rochester, NY). However, most of the surface of the sperm chromatin is devoid of any membrane. Centrioles and even short pieces of flagella frequently remain attached to the nuclei. Therefore, technically the structures that are isolated by this procedure are permeabilized sperm rather than sperm nuclei. 

It has been known for a long time that mono-olein and lysolecithin have strong fusogenetic effects on membranes. The cubic phase of mono-olein has been discussed above lysolecithin systems also exhibit cubic phases (Arvidson et aL, 1985). It is therefore tempting to assume that fusion is induced by local occurrence in the membrane of the cubic structure which is a bilayer in a three-dimensional arrangement. Cubic tube systems connecting planar lipid bilayers have in fact been observed (Harbich et aL, 1978). The occurrence of an isotropic phase has also been demonstrated in connection with mono-olein-induced fusion (Tilcock and Fisher, 1982). 

When P < 1/3, individual molecules are conically shaped, as shown in Figure 10. This results in the formation of spherical micelles in solution. As the volume of the hydrophobic tail is increased, P increases. For 1/3 < P < 1/2, nonspherical (cylindrical) micelles are formed. As P increases further, bilayer structures are formed. At P > 1, inverted structures are formed. This packing parameter can be used to rationalize why sodium dodecyl sulfate (SDS) forms spherical micelles in solution, while lysolecithin forms wormlike micelles. 

In mammals the introduction of new double bonds into mono- and polyunsaturated fatty acids exclusively occurs in the carboxyl end and is never directed toward the terminal methyl-group. Therefore no transition of fatty acids belonging to the linoleic acid family into those of the linolenic acid type has been observed. This has been shown by means of terminally labeled synthetic polyunsaturated fatty acids (Stoffel 1961, Klenk 1964). The complete enzyme system for polyunsaturated fatty acid synthesis is arranged on the cytoplasmic membranes. In view of the importance of polyunsaturated fatty acids for the structure of glycero-phospholipids, it is interesting to mention the acyl-transferases catalyzing the acylation of the j8-position of lysolecithin, lysophosphatidic acid and L-a-glycero-phosphate. These and other enzymes of phospholipid biosynthesis are located in the cytoplasmic reticulum, which therefore appears to be the main site of lipid synthesis of the cell. 

Phospholipase D occurs widely in the plant kingdom, but the enzyme has not been reported in animal tissues. Although it shows greater activity towards glycerophosphatides with the L-a structure, apparently the enzyme is able to release choline from jS-lecithin, but less rapidly (Davidson and Long 1968). The enzyme does not liberate choline from lysolecithin, GPC or phosphorylcholine (Kates 1966). 

Figure 6. Because the solubility of cholesterol in aqueous systems is low, its absorption depends on the formation of detergent structures (mixed micelles) in the small intestine. Initially, when dietary fat enters the stomach and passes into the small intestine, it takes the form of relatively large lipid droplets (shown in grey). Bile acids (shown in black) reduce the surface tension in the hpid droplets, leading to the formation of smaller structures (mixed micelles). Mixed micelles consist of an outer sheU of bile acids, monoacylglycerols, phospholipids and lysolecithin, and an inner core of digestion products of fats such as fatty acids, monoacylglycerols, cholesterol (of which 90% infreeform), andfat-soluble micronutrients.

Figure 4.41 Structure of phosphatidylcholine (lecithin), lysolecithin, phosphatidyl-ethanolamine and phosphatidyl inositol.

The presence of sialic acids in Amoeba remains uncertain. Treatment of Amoeba proteus with bacterial sialidase led to a reduction in pinocytotic activity compared to non-treated organisms (Chatterjee and Ray 1975). The same authors could also enhance lysolecithin-induced intercellular adhesion in A. proteus by sialidase treatment (Ray and Chatterjee 1975), although the release of Neu5Ac was not measured. These studies stand in opposition to those conducted with A. discoides (Allen et al. 1974, 1976), where no sialic acid was detected in biosynthetic and structural analytical experiments. 

Phospholipase A2 has diverse biological effects it disrupts the electron transport chain and the integrity of the mitochonrial structure. It increases the membrane permeability of the nerve axon and breaks down brain synaptic vesicles. Lysolecithin formed by the action of venom phospholipase A2 is actually responsible for the disruption of red cell membranes, hence hemolysis. 

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