Lecithins synthesis

The major pathway of phosphatidylcholine (lecithin) synthesis is via preformed choline (Fig. 15.5). Phosphotidylethanolamine can be converted to phosphatidylcholine in a minor pathway by the addition of 3CH3 groups (from methionine). Thus, phosphatidylcholine can be synthesized de novo if choline is not available and there is a source of "CH3" groups. In a major pathway, phosphatidylcholine can also be synthesized more directly starting with choline. Choline can be phosphorylated with ATP to form phosphocholine. Phosphocholine can react, via phosphocholine cytidyltransferase, in the presence of CTP, to form CDP choline + pyrophosphate, which is pulled in the forward direction by hydrolysis of the pyrophosphate. The CDP choline then can react with diacylglycerol to form phosphatidylcholine and CMP. This reaction is catalyzed by the enzyme phosphocholinetransferase. The major role of phospholipids in cell membranes is discussed in Chapter 4. 

Lands, W. E. M., and I. Merkl Metabolism of glycerolipids. III. Reactivity of various acyl esters of co-enzyme A with a -acylglycerophosphorylcholine, and positional specificities in lecithin synthesis. J. biol. Chem. 238, 898—904 (1963). 

Radioautography was applied also to the study of lecithin and phosphatidylethanolamine metabolism in the liver. In those studies use was made of liver slices, which permitted to carry out a pulse-chase type of experiment (O. Stein and Stein, 1969). Before discussing the results obtained it is pertinent to point out certain restrictions which were imposed on the experimental design. In order to label selectively lecithin molecules, use had to be made of labeled choline, which can represent only one pathway of lecithin synthesis and in addition suffers from the drawback that it supposedly may also be incorporated into the lecithin molecule not by de novo synthesis, but by exchange. This view is held by Treble et al. (1970), but has not been supported by experimental data of Nagley and Hallinan (1968) and of O. Stein and Stein (1969). 

Fig. 7.5. Diagrammatic representation of the hypothesis of Bennet-CIark that a cyclic formation and breakdown of the phospholipid, lecithin, within the membrane would enable it to act as an amphoteric carrier. Energy as ATP is consumed in lecithin synthesis. (From Sutcliffe, as Fig. 7.2.)...

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Glycerophospholipids are used for membrane synthesis and for producing a hydrophilic surface layer on lipoproteins such as VLDL. In cell membranes, they also serve as a reservoir of second messengers such as diacylglycerol, inositol 1,4,5-triphosphate, and arachidonic acid. Their structure is similar to triglycerides, except that the last fatty acid is replaced by phosphate and a water-soluble group such as choline (phosphatidylcholine, lecithin) or inositol (phosphatidyl-inositol). 

Fig. 2 Synthesis of facetted nanoparticles showing change in the color and nature of the sample with time and showing the phase separation observed in the isooctane/AOT (0.8 M)/Lecithin (0.4 M)/HAuCl4 (0.01 M) sample after 4 months...

The internal synthesis of lecithin in lecithin liposomes would be a signihcant step forwards. In parhcular, it would be very intereshng to see, given a certain excess of the enzymes, for how many generahons the cell self-reproduction could go on. It is clear, however, that after a certain number of generations, the system would undergo death by dilution. ... 

The metabolism of HDL probably involves interaction with both hepatic and peripheral cells, as well as with other lipoproteins. HDL may remove cholesterol from tissues, the "scavenger hypothesis (11,12). The cholesterol may then be esterifed by the action of lecithin cholesterol acyl transferase. HDL may provide cholesterol to the liver for bile acid synthesis (13) and some HDL may be catabolized by the liver in the process. HDL has not been found to interfere with the binding of LDL in cultured human fibroblasts (6). However, in cultured human arterial cells, porcine or rat hepatocytes, and rat adrenal gland, there appears to be some competition of HDL with LDL binding sites, suggesting the presence of a "lipoprotein-binding" site (14). 

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