Diacyl plasmalogenic

Figure 2. Phospholipid subclasses. Phospholipid subciasses are categorized by the type of covalent attachment of aliphatic constituents to the sr>l carbon of the glycerol backbone. The aliphatic chain of diacyl, plasmalogen, and alkyl-ether subclasses of phospholipids contain an ester, vinyl-ether, or alkyl-ether bond at the sn-l position, respectively.

Figure 5. Biosynthetic pathways for diacyl, plasmalogen and alkyl-ether molecular subclasses of phospholipids. Monoacyl dihydroxyacetone phosphate is the key branch-point intermediate whose utilization determines the phospholipid subclass distribution of newly synthesized phospholipids. Reduction of monoacyl dihydroxyacetone phosphate leads to the biosynthesis of diacyl phospholipids. Fatty alcohol exchange, catalyzed by alkyl dihydroxyacetone phosphate synthase, is the first committed step in the biosynthesis of alkyl-ether and plasmalogen subclasses of phospholipids.

Studies by Johnston and Roots (1964), Roots (1968) and Kreps (1981) have revealed an increased ratio between the plasmalogenic and diacyl forms of phosphatidyl ethanolamine in oceanic fish from low-temperature waters. During cold adaptation, the ratios between the main phospholipid fractions alter the relative proportion of phosphatidyl choline decrease and phosphatidyl ethanolamine, phosphatidyl serine and sphingomyelin, all of which contain large amounts of polyenoic acids, increase (Caldwell and Vemberg, 1970 Miller etal, 1976 Wodke, 1978 Hazel, 1979 Brichon et al., 1980 van den Thillart and de Bruin, 1981 Zabelinsky and Shukolyukova, 1989). 

Similar to aUcyl forms, the mono- and dialk-l-enyl ethers of glycerol have been identified in natural samples, whereas no trialk-l-enyl ethers have been isolated so far. Acylated (mono- and diacyl derivatives) alk-l-enyl ethers (or neutral plasmalogens) have been also reported (2). 

Ether analogs of triacylglycerols have been described. l-Alkyl-2,3-diacyl-sn-glycerols are characteristically elevated in tumor lipids and l-alk-l -enyl-2,3-diacyl-sn-glycerols (neutral plasmalogens) have also been detected in tumors, in adipose tissue of mammals, and in fish liver oil. Alkylacetylacylglycerols, RAF precursors, also have been described in human leukemic cells. 

In model membranes, ether-linked lipids decrease ion permeability and surface potential, and lower the phase temperature of membrane bilayers when compared to their diacyl counterparts. Ethanolamine plasmalogen is enriched, along with cholesterol, in membrane lipid rafts. These domains have a distinct hydrophobic environment with ordered lipid packing, and plasmalogen can serve to stabilize the interaction of specific raft proteins (Chapter 1). 

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. 

The other sources of the arachidonic acid released by platelets in response to thrombin are phosphatidylcholine and phosphatidylethanolamine (Table 1.1). However, each of these phospholipids exists as three species in platelets, namely as the diacyl-, 1-0-alkyl-, or 1-0-alk-l enyl- forms (Table 1.2). Table 1.2 also shows that l-O-alkyl-2-acyl-PC and l-O-alk-l enyl-2-acyl-PE are enriched in arachidonic acid relative to the other classes of PC or PE, giving rise to the idea that they could serve as important sources of arachidonic acid in stimulated platelets. Purdon and Smith " prelabelled platelets with pH]-arachidonic acid and followed the changes in the different species of PC and PE following thrombin stimulation. It was found that while there was a decrease in radiolabel of both 1,2-diacyl-PC and 1,2-diacyl-PE at all times studied, there was no decrease in the other species of PC or PE, and, indeed, radiolabel in l-O-alkyl-2-acyl-PC and l-0-alk-l -enyl-2-acyl-PE increased at later time points (3-5 min) after thrombin. The thrombin-induced incorporation of arachidonic acid in plasmalogen PE was observed previously by others. Purdon and Smith " concluded that, upon stimulation of human platelets, arachidonic acid is released from both 1,2-diacyl-PC and 1,2-diacyl-PE for metabolism by cyclo-oxygenase and lipoxygenase, while certain other pools of phosphatidylcholine and phosphatidylethanolamine act to collect arachidonic acid. 

Plasmalogens are a group of phosphoglycerides which are phosphorylated derivatives of 1-alk-cw-l-enyl-2-acyl-5n-glyceroL The similarity in properties between plasmalogens and the corresponding 1,2-diacyl phospholipids has made isolation procedures difficult. Moreover, chemical synthesis is made... 

Tkn interesting feature of brain tissue is its considerable content of alkyl- and alkenyl-ether linked PLs indeed, to the present authors knowledge, only spermatozoa possess a comparable significant content of ether lipids [11]. These PLs are detectable with sensitivities comparable to those for diacyl PLs, but with characteristic mass differences (—16 amu compared to the corresponding diacyl-PC). The extreme sensitivity of plasmalogens to even traces of acids is the reason why lyso-Upids are always detectable when TLC separation of brain lipids is performed the acidic groups on the TLC plate (normally unmodified silica gel) are sufficient to induce an unwanted hydrolysis of plasmalogens [134, 135]. 

Table 2. Incorporation of Cvtidine-5 -Diphosphate Ethanolamine into Phosphatidylethanolamine and Ethanolamine Plasmalogen of Dispersions of Neuronal and Glial Cells from Rabbit Brain in Absence and Presence of Added Diacyl Glycerol or Alkenylacyl Glycerol...

Ford, DA and Gross, RW (1989) Differential accumulation of diacyl and plasmalogenic diglyceiides during myocardial ischemia. Circ Res, 64, 173-177. 

Plasmalogen species play many roles in cellular functions in addition to serving as a biomembrane component. Previous studies demonstrated that membranes comprised of plasmalogen species possess a compact membrane conformation and unique membrane dynamics in comparison to membranes comprised of its diacyl counterpart [75-78]. A central role of plasmalogen PE in facilitating membrane fusion and cell-cell communication [79, 80] was demonstrated and the propensity for membrane fusion is directly linked with the quantity of the double bonds present in the GPL acyl chains of the membrane [81]. The role of plasmalogen as an antioxidant... 

The linkage of aliphatic chains at the sn-1 position (i.e., the changed content of plasmalogen vi. diacyl counterparts). 

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