Figure 1 Molecular structures of phospholipids. The class of phospholipid is defined by the nature of the nitrogenous base or polyol esterified to the phosphate group (X). The species distribution within any phospholipid class is determined by the fatty acyl substituents at the sn-1 and sn-2 positions of the glycerol backbone. The dipalmitoyl species would be designated PC16:0/16:0 if X was choline. If arachidonic acid was esterified at sn-2, then the molecule would be designated as PC1 6:0/20:4. In the diacyl species, fatty acids are attached by ester linkages. For sn-1-alkyl-sn-2-acyl species, the sn-1 fatty acid is attached by an ether bond. For sn-1-alkenyl-sn-2-acyl species, the sn-1 fatty acid is attached by a vinyl ether linkage.

In addition to diacyl species, with both fatty acids attached by ester bonds, there are a number of species with ether-linked fatty acids, principally in the sn-1 position. These ether phospholipids include 1-alkyl-2-acyl species, largely present in PC, and 1-alk-1-enyl species (plasmalogens), largely present in PE. These ether lipids are major components of many cell membranes, particularly neuronal and inflammatory cells, and there have been significant advances in understanding the biochemical pathways for their synthesis and catabolism. Some alkyl acyl PC species are substrates for the generation of the potent bioactive lipid platelet-activating factor (1-alkyl-2-acetyl-glycero-3-phosphocholine (PAF)), but the function of most ether lipids is largely unclear. One possibility is that generation of 1-alkyl-2-acyl-glycerol as a second messenger rather than diacylglycerol may contribute to differential regulation of protein kinase C isoforms, and antioxidant properties have been reported for plasmalogens.

N Lipid rafts

Outside cell

Plasma ;; '{¡; ;; ;; ¡; u :< ^ ;; u n :.; ■■! :i ;; ;'

membrane;; y. :■) il i; )■ >) !1« )) )) i; ¡1 K V: ¡; ¡1 !! ;( Cytoplasm

Neutral phospholipid \\ Acidic phospholipid SI (uncharged) - PC, PE ' (negatively charged - PS, PI, PA)

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