Ether phospholipids functions

The r-alkyl desaturase system, a microsomal mixed-function oxidase responsible for the biosynthesis of ethanolamine plasmalogens from alkyl lipids (Fig. 6), was initially characterized in the early 1970s (F. Snyder, 1971 A. Paltauf, 1973). The reverse of this reaction (i.e., conversion of an alk-l -enyl moiety to an alkyl chain via a reductase) has not been observed. The alkyl desaturase is a unique activity since it can specifically and stereospecifically abstract hydrogen atoms from C-1 and C-2 of the 0-alkyl chain of an intact phospholipid molecule to form the cis double bond of the O-alk-l -enyl moiety. Only intact l-alkyl-2-acyl-in-glycero-3-phosphoethanolamine is known to serve as a substrate for the alkyl desaturase. As with other reactions in complex ether phospholipid synthesis, the molecular identity of the responsible enzyme is unknown. 

Such compounds may form up to 50 per cent of the phospholipids of heart tissue, but their function is unclear. An ether phospholipid called platelet activating factor is a highly potent aggregator of blood platelets. 

In the late 1980s, efforts initiated by Fyles in Canada (Carmichael el al., 1989) and by our own group (Nakano et al., 1990) resulted in two different channel models, both of which incorporated crown ethers. Both of these efforts resulted in channels that show significant transport of alkali metal cations through phospholipid bilayers. Both groups (Gokel and Murillo, 1996) have undertaken extensive stmctme-activity studies to characterize their function. 

Lipids are of special concern in comparing plastic metabolism between the two groups of fish. Triacyl-glycerols, cholesterol ethers and non-esterified fatty acids, which are the direct sources of energy, have already been discussed in the previous section. We now turn to phospholipids and cholesterol, which are essential to the structure of cell membranes, and to polyunsaturated fatty acids, which determine to a large extent the functional activity of these membranes. 

Butyl ether has the ability to dissolve lipids. As a result, it may cause irritation and pain upon contact with eyes and nose mucosa. It also causes dermal irritation and dermatitis upon contact with the skin. Damage caused by butyl ether appears to be scattered loss of epithelial cells due to dissolution of phospholipid cell membranes. At the CNS level, butyl ether, like other volatile organic solvents, depresses the CNS by dissolving in the cell lipid membrane and disrupting the lipid matrix. These effects are known as membrane fluidization. At the molecular level, membrane fluidization disrupts solute gradient homeostasis that is essential for cell function. 

One of the main functions of peroxisomes is to detoxify the cell by splitting hydrogen peroxide. They contain the enzyme catalase. Catalase converts H2O2 (hydrogen peroxide, a toxic by-product of cellular metabolism) to H2O and O2, with 4H202 4H2O-I-2O2. Peroxisomes also degrade fatty acids and toxic compounds and catalyze the first two steps required in the synthesis of ether-linked phospholipids (which are later used to build membranes) and sterols. In addition, it plays a role in isoprenoid biosynthesis and amino acid metabolism. 

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