Liposomes such as large unilamellar vesicles obtained by extrusion technique (LUVET) are generally accepted as close models of cell membranes. Figure 6.3 represents a sketch of a vesicle, where it can be seen that the system consists of two distinct compartments - the aqueous and lipid phases. The phospholipid polar heads face the aqueous internal and external phases, whereas the fatty acid chains form the hydrophobic bilayer of the model membrane.
Fig. 6.3 Schematic representation of Large Unilamellar Vesicles made by Extrusion Technique (LUVET) and two potential phosphatidylcholines. Thiyl radicals (RS*) are initially generated in the aqueous compartment and may enter the lipid bilayer.
The CTI of unsaturated fatty acid residues in LUVET catalyzed by thiyl radicals was also studied in some detail [31,35,37]. Trends of the reactivity indicated the overall picture of geometric isomerization in model membranes by the action of diffusible thiyl radicals. In particular, using vesicles made of egg yolk lecithin, it was possible to demonstrate that the double bonds located closest to the membrane polar region are the most reactive towards the attack of diffusing thiyl radicals . In the case of linoleic acid residues in vesicles, the double bond in position 9 was more reactive than that in position 12. Also arachidonic acid residues in vesicles were more reactive than oleic and linoleic acids, and two positions (i.e. the double bonds in 5 and 8) out of the four present in this compound were transformed preferentially. From the studies carried out so far, arachidonic acid residues in membrane phospholipids emerge as very important components to be investigated, because they allow endogenous trans isomers, formed by radical processes, to be distinguished from exogenous trans isomers derived from dietary contribution.
In particular, investigations should focus on the erythrocyte membrane phos-pholipids, which are the preferential storage place for arachidonic acid after biosynthesis. As shown in a previous section, nutritional investigations indicated that trans fatty acids are incorporated in cell membranes because the trans dietary pre cursors can be processed in vivo. In the case of arachidonic acid, as shown in the biosynthetic paths of Scheme 6.6, two double bonds (positions 11 and 14) originate from linoleic acid, the precursor taken from the diet, whereas the two other double bonds (positions 5 and 8) are formed by desaturase enzymes, which produce selectively the cis unsaturation. It is evident that the 5 and 8 double bonds of arachidonic acid, stored in membrane phospholipids, can only have a cis configuration, unless these positions have been involved in an isomerization process and converted to trans isomers.
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