Platelet ethanolamine-containing

The choline- and ethanolamine-containing phospholipids of human platelets can be subdivided, based on the nature of the linkage to the fatty acid residue at the sn-1 position of the glycerol backbone, into 1,2-diacyl, l-O-alkyl-2-acyl and l-0-alk-l -enyl-2-acyl species (Table 1.2). The majority of arachidonic acid in the phosphatidylcholines is present in the diacyl species while 1-0-alk-l enyl-2-acyl-PE (plasmalogen) contains most of the arachidonic acid in the phosphatidylethanolamines. The amount of arachidonic acid present in the diacyl species of phosphatidylethanolamine is similar to that in diacyl phosphatidylcholine. Ferret et al have shown that the distribution of arachidonate in the phospholipids of the platelet plasma membrane is asymmetric, the majority of it being present in the inner leaflet. 

Plasma membrane lipids are asymmetrically distributed between the two monolayers of the bilayer, although the asymmetry, unlike that of membrane proteins, is not absolute. In the plasma membrane of the erythrocyte, for example, choline-containing lipids (phosphatidylcholine and sphingomyelin) are typically found in the outer (extracellular or exoplasmic) leaflet (Fig. 11-5), whereas phosphatidylserine, phosphatidyl-ethanolamine, and the phosphatidylinositols are much more common in the inner (cytoplasmic) leaflet. Changes in the distribution of lipids between plasma membrane leaflets have biological consequences. For example, only when the phosphatidylserine in the plasma membrane moves into the outer leaflet is a platelet able to play its role in formation of a blood clot. For many other cells types, phosphatidylserine exposure on the outer surface marks a cell for destruction by programmed cell death. 

There are also differences in the composition of the phospholipids in human platelets. These cells contain a little over twice as much phosphatidylcholine as sphingomyelin. Another dramatic difference is noted in the ether phospholipid content in the human platelets, where there is approximately 37% vinyl ether-containing components in the ethanolamine fraction whereas there only are 10% or less in the other phosphoglyceride species. The vinyl ether linkage has not been detected in the sphingolipids of these cells. 

Fig. 9. Biosynthesis of platelet-activating factor (PAF) via the remodeling pathway. Lyso-PAF, the immediate precursor of PAF, can be formed from l-alkyl-2-acyl-sn-glycero-3-phosphocholine through the direct action of (I) PLAj or (II) CoA-independent transacylase. The lysoplasmenylethanolamine (or other potential ethanolamine- and choline-containing lysoglycerophospholipids) is thought to be generated by (III) a PLAj that exhibits a high degree of selectivity for substrates with an atachidonoyl moiety at the sn-2 position. The transacylase (II) appears to possess both acyl transfer and PLAj hydrolytic activities. Lyso-PAF produced by either the transacylation (II) or direct PLAj (I) action can be acetylated to form PAF by (IV) an acetyl-CoA acetyltransferase.

Approximately 30% of position sn-2 in human platelet phospholipids consists of arachidonate. Of course, the percentage of arachidonate varies within the different phospholipids species and the amounts of the different species of phospholipid vary in the platelet. This is illustrated in Table 1.1. which shows that sphingomyelin contains essentially t-o arachidonic acid while ethanolamine-phospholipids contain the most arachidonic acid. Although phosphatidylinositol is the phospholipid species that is most enriched in arachidonic acid, its contribution to the total content of arachidonate in blood platelets is smalP. ... 

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