Other ether lipids

2- acyl-5n-glycerols and l-O-alkyl-2-acyl- n-glycerols stimulate protein kinase C and contribute to signal transduction processes during cell stimulation and trauma (Table 2) (Ford et al, 1989). 

Ether glycerophospholipid species with a saturated ether linkage have been reported as a low molecular mass lipidic modulator of glucocorticoid receptors (Schulman etal., 1992). The occurrence of l-(9-alkyl-T-enyl-2-lyso- n-glycero- 

Involvement of plasmalogens, platelet-activating factor, and other ether lipids in diseases 

Ischemic injury results in a marked decrease in the plasmalogen content of biomembranes. Thus in rabbit myocardium, microsomal plasmalogen-selective PLA activity is markedly increased by 10-fold during ischemic injury (Hazen et al, 1991). A marked stimulation of plasmalogen-selective PLAj also occurs during hypoxic injury to rabbit proximal tubules (Portilla et 

1 -O-alk-1 -enyl-sn-glycero- Lung injury LiliomeZaZ., 1998a 

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Farooqui A. A. and Horrocks L. A. (2004). Plasmalogens, platelet activating factor, and other ether lipids. In Nicolaou A. and Kokotos G. (eds.), Bioactive Lipids. Oily Press, Bridgwater, England, pp. 107-134. 

An improved access to the ilmofosine fluorinated analogue (78) and to oc-tadecyl homologue (79) based on compounds (80) and (81) has been realized in order to find phospholipids exhibiting higher antitumor reactivity in comparison to ilmofosine and other ether lipides. °... 

Fig. 10.10 Differences in the membrane lipids between eukaryotes (including eubacteria) and the archaea. The main components of the membrane in the eukaryotes are fatty acid glycerine esters, while in archaea, the membranes contain mainly di- (or other) ethers of glycerine with phytanol residues...

Triacylglycerols and the ether lipids described in the previous section are classified as neutral lipids. Other neutral lipids are alcohols, waxes, aldehydes, and hydrocarbons derived from fatty acids. These sometimes have specific biological functions. For example, fatty aldehydes are important in the bioluminescence of bacteria (Eq. 23-47). 

Despite its simplicity this route is not easily accessible. Lipases can act enantioselectively on glycerol derivatives, but their enantioselectivity is usually rather low. And, there is no such report for an efficient enough direct acylation of non-substituted glycerol. Lipases of the two type, on the other hand, are known, one of the most efficient being CAL as has been demonstrated in our previously described synthesis of structured MLM-type TAGs and ALM type ether lipids (Halldorsson et al., 2003 Haraldsson, 2005,2007). The third type of lipase is certainly not available when PUFAs such as EPA or DHA are involved. 

The question then arises by what pathways are archaeal ether lipids biosynthesized and how were these pathways selected rather than those used by all other organisms for acyl ester lipid synthesis The available information on lipid biosynthesis in archaea is based largely, with a few exceptions, on labelling studies with whole cells (see previous reviews [4,5,9,10,13,15,85]). Biosynthetic pathways for archaeol, caldarchaeol and their complex lipid derivatives will now be discussed. 

The most important acyclic diterpenoid is phytol. It forms part of the chlorophyll-a molecule and is present in many other chlorophylls (see Section 2.4.4).The saturated analogue of phytol, dihydrophytol (orphytanol), is present in a variety of bacterial glyceride ether lipids (Section 2.4.1b). 

This enzyme is an integral membrane protein exclusively localized to the lumenal side of peroxisomes (A. Poulos, 1993). Reports on the presence of this enzyme in other organelles are likely a result of peroxisomal contamination. This acyltransferase is important in ether lipid biosynthesis (Chapter 9). Once 1-acyldihydroxyacetone-P has been formed it can be used as a substrate for 1-alkyldihydroxyacetone-P synthesis (Chapter 9) or can be reduced to lyso-PA by a peroxisomal acyldihydroxyacetone-P reductase (Fig. 1) that also utilizes 1-alkyldihydroxyacetone-P as a substrate. 

Peroxisomes are present in greater number in the liver than in other tissues. Liver peroxisomes contain the enzymes for the oxidation of very-long-chain fatty acids such as C24 0 and phytanic acid, for the cleavage of the cholesterol side chain necessary for the synthesis of bile salts, for a step in the biosynthesis of ether lipids, and for several steps in arachidonic acid metabohsm. Peroxisomes also contain catalase and are capable of detoxifying hydrogen peroxide. 

The polarity of a lipid affects its volatility, solubility, and nonspecific binding to other polar compounds. Lipids are often functionally classified as neutral or polar on the basis of their mobility on thin-layer chromatography using a neutral or polar solvent system. Neutral lipids such as wax esters, steryl esters, ether lipids, and TAG are chemically neutral while other neutral lipids, such as free FA, fatty alcohols, and monoacylglycerols, are actually slightly polar due to the presence of hydroxyl or carboxyl groups. Table 3.8 lists the relative polarities of some common lipid classes. 

The lipid fraction of milk is dominated by triacylglycerols (TAG), with small amounts of various other lipid classes (Table 13.1). Milk lipids also contain small quantities of glycolipids, fat-soluble vitamins, ether lipids, and flavor components, mainly lactones, aldehydes, and ketones. 

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