The Ether-linked Lipids

Closely related to both the triacylglycerols and phospholipids, the ether-linked lipids contain in place of one ester group an alkoxy (-OR) or alkenyl (-0-CH = CH-R) group.Phospholipids containing the alk-... 

Some animal tissues and some unicellular organisms are rich in ether lipids, in which one of the two acyl chains is attached to glycerol in ether, rather than ester, linkage. The ether-linked chain may be saturated, as in the alkyl ether lipids, or may contain a double bond between C-l and C-2, as in plasmalogens (Fig. 10-9). Vertebrate heart tissue is uniquely enriched in ether lipids about half of the heart phospholipids are plasmalogens. The membranes of halophilic bacteria, ciliated protists, and certain invertebrates also contain high proportions of... 

Most cells continually degrade and replace their membrane lipids. For each hydrolyzable bond in a glycerophospholipid, there is a specific hydrolytic enzyme in the lysosome (Fig. 10-15). Phospholipases of the A type remove one of the two fatty acids, producing a lysophospholipid. (These esterases do not attack the ether link of plasmalogens.) Lysophospholipases remove the remaining fatty acid. 

Inclusion of other molecules of irregular shape within membranes also lowers Tm. However, a molecule of cholesterol can pack into a bilayer with a cross-sectional area of 0.39 nm2, just equal to that of two hydrocarbon chains.49 It tends to harden membranes above Tm but increases mobility of hydrocarbon chains below Tm.97 -100 A complex of cholesterol and phosphatidylcholine may form a separate phase within the membrane.101102 The ether-linked plasmalogens may account for over 30% of the phosphoglycerides of the white matter of the brain and of heart and ether linked phospholipids are the major lipids of many anaerobic bacteria.103 Their Tm values are a few degrees higher than those of the corresponding acyl phospholipids.104... 

Ether-linked lipids constitute up to 35% of the total phospholipid in molluscs. Although they are usually regarded as animal constituents, small amounts of ether-linked lipids have been identified in plants. The major phospholipids of archaebacterial membranes are ether-linked derivatives of the polyprenyl phytanyl group and of the dimeric biphytanyl group (Chapter... 

As mentioned in Chapter 2, the alkylacylcholine phosphoglycerides predominate in the choline-containing fraction in the majority of mammalian cells. On the other hand, the alkenylacyl form predominates in the ethanolamine-containing fraction. It is important to stress that these ether-linked lipids are found essentially only in the choline- and ethanolamine-containing phospho-glyceride. The reason for this high specificity of distribution is not known at present. 

Debuch, H. and Seng, P. (1972) The history of ether-linked lipids through 1960, in Ether Lipids Chemistry and Biology, F. Snyder, editor, Academic Press, New York, pp. 1-24. 

The Halobacteriaceae, commonly referred to as the halobacteria, are a family of extremely halophilic archaebacteria [113]. As in other archaebacteria, their membranes contain ether-linked lipids. The primary lipids present are diphytanyl phospholipids [113]. Their cell walls are also unique in structure and lack muramic acid. There are several species of halobacteria that vary considerably in their physiological characteristics. The halobacteria are unicellular rods or cocci. More recently flat, square and box-shaped cells have been described. Halobacteria are found growing in salterns or natural salt lakes and on the surface of salted fish. They often form dense planktonic blooms and can form massive accumulations on solid substrates. They may be involved in mat communities in hypersaline environments. 

The early literature concerning ether-linked lipids has been covered in detail [5,16,17]. Perhaps the first evidence, albeit circumstantial, to suggest the existence of O-alkyl lipids in nature was reported in publications by C. Doree (1909) in England and A. Kossel and... 

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). 

A significant literature describing the enzymes involved in the metabolism of ether-linked lipids [8-10,24,25] and PAF [13,14] is available. 

Compound-specific A C values for 31 different lipid biomarker molecules are shown in Figure 8 for sedimentary horizons corresponding to pre-bomb (before ad 1950) and post-bomb (1950-1996) eras. These organic compounds represent phytoplank-tonic, zooplanktonic, bacterial, archaeal, terrestrial higher plant, and fossil carbon sources. The lipid classes include long-chain n-alkanes, alkanoic (fatty) acids, -alcohols, C30 mid-chain ketols and diols, sterols, hopanols, and C40 isoprenoid side chains of the ether-linked glycerols of the Archaea. 

Mueller, H.W., Purdon, A.D., Smith, J.B. and Wykle, R.L. (1983). 1-0-Alkyl-linked phosphoglycerides of human platelets Distribution of arachidonate and other acyl residues in the ether-linked and diacyl species. Lipids, 18, 814-881... 

Archaeal lipids have been exploited for its use in drug delivery system in therapeutics. Liposomes easily assimilate with the cells and introduce the substance directly into it. They can also deliver non biological agents into the human body without any degradation. These ether linked lipids are resistant to high temperature and oxidation events than ester-linked core lipids, hence it contributes to its thermostability (Van de Vossenberg, 1998). Thermostability is essential as they have to go through the steps of sterility... 

Another type of lipid, the ether-linked phospholipids (e.g., plasmalogens), comprises about 20% of membrane phospholipids (Figure 7). Plasmalogen synthesis requires enzymes present in both peroxisomes and the endoplasmic reticulum. These lipids are thought to be part of the cellular defense mechanism against oxidative injury. 

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