1. Basic Chemistry
Lipids with ether bonds to long-chain alkyl moieties in addition to having ester bonds to fatty acids are not important constituents of many lipids of commercial value, but they are very common in nature, especially as membrane constituents. Usually the ether bond is to position sn-1 of a glycerol moiety, which may be part of a nonpolar lipid or more often a phospholipid in animal tissues or in anaerobic bacteria. At one time, they were considered to be little more than a biological novelty. However, findings of elevated levels of ether lipids in cancer tissues, followed by the discovery of distinctive ether lipids, such as platelet-activating factor, with important biological activities have greatly stimulated the interest in these compounds.
Two main types of glycerol ether bonds exist in natural lipids - ether and vinyl ether as illustrated. The double bond adjacent to the oxygen atom in the latter has the Z or cis configuration.
The terms "plasmanyl-" and "plasmenyl-" lipids for alkyl and alkenyl ethers, respectively, are recommended by IUPAC-IUB, but they do not appear to have been widely taken up in the literature. Ether bonds are stable to both alkaline and acidic hydrolysis under most practical conditions, but vinyl ether bonds open readily under acidic conditions to form aldehydes.
|For example, when acidic transesterification methods are used to prepare fatty acid methyl esters from lipid extracts that contain a proportion of vinyl ether bonds, free aldehydes are generated that are rapidly converted to dimethyl acetals and can be analysed in this form.|
In animal tissues, the alkyl and alkenyl moieties in both nonpolar and phospholipids tend to be rather simple in composition with 16:0, 18:0 and 18:1 (double bond in position 9) predominating. Other alkyl groups may be present, but other than in fish lipids they are found at low levels only. The trivial names - chimyl, batyl and selachyl alcohols - are given to the glycerol ethers with 16:0, 18:0 and 18:1 alkyl groups, respectively, in the sn-1 position.
Pure ether lipids are rarely easy to separate from the fully acylated forms, and often their presence in tissues is inferred from isolation of their hydrolysis products or from spectroscopic methods.
Ether analogues of triacylglycerols, i.e. 1-alkyldiacyl-sn-glycerols, are present at trace levels only if at all in most animal tissues, but they can be major components of some marine lipids. For example, they can make up 50% of the total lipids in dogfish (Squalus acanthias) and in ratfish (Hydrolagus colliei), and they can comprise 30% of the liver lipids of other sharks. In such species, alkyldiacylglycerols must be a form of storage lipid, and they appear to be stored intracellularly in liver in lipid vacuoles. It has been suggested that they have a function in density control affecting buoyancy. The alkyl moieties are the conventional saturated and monoenoic components, though usually with a wider range of chain lengths than in other animal tissues (ruminants may be a further exception). For example in dog fish, the composition of alkyl groups is reported to be 10:0 (6%), 14:0 (2%), 16:0/16:1 (24%), 18:0 (18%), 18:1 (44%) and 22:0/22:1 (2%). In terrestrial mammals, 1-alkyldiacyl-sn-glycerols have been found in liver and adipose tissue, and in tumours, though usually in low proportions relative to triacylglycerols.
Neutral plasmalogens, i.e. related compounds with vinyl ether bonds in position 1, have rarely been found at greater than trace levels in animal tissues, though again they can be more abundant in some marine species (up to 5% of ratfish lipids, for example). In terrestrial mammals, they have been found in liver, adipose tissue and tumours. 1-Alkenyldiacylglycerols, together with the corresponding 1-alkyl lipids and ether-containing phospholipids, are reported to be major components of lipid droplets or ‘adiposomes’ in cultured CHO K2 cells. Reports of their occurrence in plants now appear to have been discounted.
In bovine heart muscle, 1-alkyldiacyl-sn-glycerols and 1-alkenyldiacylglycerols comprised 1.6% and 0.25-0.8% of the simple lipids, respectively. The compositions of the fatty acids and alkyl substituents of each of these lipids are listed in Table 1.
|Table 1. Composition (wt %) of aliphatic moieties of 1-alkyldiacyl-sn-glycerols, 1-alkenyl-sn-diacylglycerols and triacylglycerols of bovine heart muscle|
|Alkenyl ethers||Fatty acids||Alkyl ethers||Fatty acids||Fatty acids|
|Data from Schmid, H.H.O. and Takahashi, T. Biochim. Biophys. Acta, 164, 141-147 (1968).|
The fatty acid components of the ether lipids are more similar to those of the phospholipids in composition than to those of the triacylglycerols (see below).
Small amounts of methoxy-substituted glyceryl ethers have been found in alkyldiacylglycerols and alkylacyl phospholipids in liver oils from sharks and other cartilaginous fish species. The main components have C16 saturated and C16/18 monounsaturated alkyl chains with a cis double bond in position 4, although one isomer with an alkyl group analogous to that of docosahexaenoic acid is known. They are claimed to have potent antibacterial and anticancer activities.
On hydrolysis with alkali, the ether bond of 1-alkyldiacyl-sn-glycerols is stable, and 1-alkylglycerols and free (unesterified) acids are the products.
Under the same basic conditions, neutral plasmalogens yield 1-alkenylglycerols and free fatty acids. On the other hand, acidic hydrolysis yields aldehydes (or dimethyl acetals), as discussed above, together with glycerol and free fatty acids.
In addition to diacyl forms, the membrane phospholipids of many animal and microbial species contain high proportions of molecular species with ether and vinyl ether bonds in position sn-1, and often the vinyl ether or plasmalogen form predominates. The annamox bacteria, which contain ladderane fatty acids, are unusual in that the alkyl moieties are in position sn-2 of their phospholipids.
In animal tissues, the highest proportion of the plasmalogen form is usually in the phosphatidylethanolamine class with rather less in phosphatidylcholine, and commonly little or none is in other phospholipids such as phosphatidylinositol. In phosphatidylcholine of most tissues, a higher proportion is often of the O-alkyl rather than the O-alkenyl form, but the reverse tends to be true in heart lipids.
In bovine heart muscle, alkylacyl-, alkenylacyl and diacyl forms of phosphatidylcholine comprised 1%, 16% and 24% of the phospholipids, respectively, and the corresponding proportions in phosphatidylethanolamine were 0.5%, 11% and 16%, respectively. The compositions of the fatty acids and alkyl substituents of the phosphatidylcholine forms are listed in Table 2 .
|Table 2. Composition (wt %) of aliphatic moieties of alkylacyl-, alkenylacyl- and diacyl-forms of phosphatidylcholine of bovine heart muscle|
|1-Alkenyl ethers||Fatty acids position 2||1-Alkyl ethers||Fatty acids position 2||Fatty acids position 1||Fatty acids position 2|
|Data from Schmid, H.H.O. and Takahashi, T. Biochim. Biophys. Acta, 164, 141-147 (1968). This paper should be consulted for information on the phosphatidylethanolamine forms.|
The corresponding forms of phosphatidylethanolamine differ in having much higher proportions of 18:0 components in position 1 and much more arachidonate (20:4(n-6)) in position 2. In brain, a high proportion of the fatty acid constituents are 20:4, 22:4 and 22:6 species. Diplasmalogens (1,2-di-(O-1′-alkenyl) forms) have been reported to constitute a high proportion of the phosphatidylethanolamine in epididymal spermatozoa from rabbits.
In bacteria, the most common plasmalogens are forms of phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine and cardiolipin, but many other phospholipids and even glycosyldiacylglycerols have been found with vinyl ether bonds. Plasmalogens do not occur in aerobic aerobic or facultatively anaerobic bacteria, fungi or plants, and it has been suggested that during evolution there has been an appearance, disappearance and reappearance of plasmalogens. This theory is supported by differences in the biosynthetic mechanisms between bacteria and animals.
The biosynthesis of glycerol ethers including plasmalogens has been studied in animal tissues mainly and resembles that of the corresponding diacyl-phospholipids (see the appropriate web pages) with a few key differences. The first steps are carried out in the peroxisomes with the process being completed in the endoplasmic reticulum.
Dihydroxyacetone phosphate (DHAP) is first esterified with a long-chain acyl-CoA ester, before the ether bond is introduced by replacing the acyl group with a long-chain alcohol, a reaction catalysed by an alkyl-DHAP synthase. A remarkable feature of this reaction is that the oxygen atom comes from the alcohol moiety not glycerol. The enzyme fatty acyl-CoA reductase 1, outside the peroxisome, supplies most of the fatty alcohols used for the purpose, and this appears to be the rate-limiting enzyme in plasmalogen biosynthesis.
At this point, the intermediate is transferred to the cytosolic face of the endoplasmic reticulum. Following reduction of the ketone group at the sn-2 position to 1-alkyl-sn-glycero-3-phosphate, 1-alkyl-2-acyl-sn-glycero-3-phosphate is synthesised by a distinctive alkyl/acyl-glycero-3-phosphate acyltransferase. A phosphohydrolase removes the phosphate group, and the resulting 1-alkyl-2-acyl-sn-glycerol is converted to the ethanolamine/choline phospholipids by the enzyme systems used to produce the diacyl forms. 1-Alkyl-2-acyl-sn-glycero-3-phosphoethanolamine is then the main substrate for a delta-1’-desaturase to yield the plasmalogen 1-alk-1’-enyl-2-acyl-GPE, which can then be converted to the 1-alk-1’-enyl-2-acyl-sn-glycero-3-phosphocholine. It should be noted that this pathway is very different and is separated spatially from that producing diacyl-phosphatidylethanolamines (or phosphatidylcholine) via the CDP-ethanolamine pathway.
A further route to glycerol ethers and plasmalogens involves phosphorylation of alkylglycerols with an alkylglycerol kinase. The mechanism for biosynthesis of the 2-methoxy ethers has yet to be established.
As with other phospholipids, the final fatty acid compositions of ether lipids are attained by remodelling processes. While this can occur by reacylation after removal of the fatty acids of position sn-2 by the action of a phospholipase A2, with formation of lysophosphatides (which may have messenger functions), much of the arachidonate and other polyunsaturated fatty acids are introduced by exchange reactions from diacyl phospholipids that are catalysed by CoA-independent transacylases.
Plasmalogen biosynthesis in bacteria appears to be accomplished by a different route that does not require DHAP, but many of the details have still to be elucidated.
Catabolism: O-Alkyl linkages can be cleaved oxidatively by a microsomal alkyl monooxygenase, present in liver and intestinal tissue, to yield fatty aldehydes, which can be oxidized to fatty acids or reduced to fatty alcohols. Plasmalogenases in cells are less well characterized, but produce similar products from plasmalogens.
5. Functions of Ether Lipids
Within a membrane, the acyl chain in plasmalogens is oriented perpendicularly to the membrane surface as in diacyl phospholipids, but the head group lacks a carbonyl oxygen in the sn-1 position and is much more lipophilic. As a consequence, there is stronger intermolecular hydrogen bonding between head groups, leading to changes to the arrangement of lipids within membranes with a high propensity to form an inverse hexagonal phase (non-bilayer forming), which is a requirement for membrane fusion. This occurs at lower temperatures than the diacyl analogues and they have a larger dipole moment, so plasmalogen-containing cell membranes are less fluid than those deficient in plasmalogens. In spite of the relatively high concentrations of polyunsaturated fatty acids, they have a tendency to accumulate in membrane raft domains.
As well as being structural components of cell membranes, plasmalogens may have a number of other functions. The information is based partly on their distribution and properties in various types of cell and partly on their physical properties, but also on the effects of changes that occur in plasmalogen metabolism in certain mutant cells.
Plasmalogens serve as a store of polyunsaturated fatty acids that can be released by specific stimulant molecules, especially in membranes that are stimulated electrophysiologically, and they may act as intracellular signalling compounds. Thus, at least two plasmalogen-selective enzymes of the phospholipase A2 type are involved in the degradation of plasmalogens, releasing arachidonic and docosahexaenoic acids from position sn-2, for eicosanoid or docosanoid production as part of signalling mechanisms. The other product is a lysoplasmalogen, which can be reacylated or further degraded by a lysoplasmalogenase with formation of aldehyde and phosphoglycerol moieties. However, lysoplasmalogen may also have a signalling function as it is known to activate cAMP-dependent protein kinase.
Claims that plasmalogens protect membranes against oxidative stress have been more difficult to substantiate. Indeed, there are counter-suggestions that polyunsaturated fatty acids protect plasmalogens against oxidative damage. However, there does appear to be good evidence from studies in rat brain that plasmalogens do function as endogenous antioxidants in this tissue at least. There is also experimental evidence to suggest that plasmalogens could protect unsaturated membrane lipids against oxidation by singlet oxygen, on the assumption that the oxidation by-products are not cytotoxic.
In contrast, there are unwanted side effects with myeloperoxidase, an abundant protein in leukocytes such as neutrophils, monocytes, and macrophages. On activation, it converts hydrogen peroxide to hypochlorous acid (HOCl). This contributes to the antimicrobial and cytotoxic properties of leukocytes, but it can also react adventitiously with other cellular constituents, and in particular it reacts with the vinyl ether bond of choline and ethanolamine plasmalogens to generate lysophospholipids with the fatty acid component in position sn-2 and 2-chloro-fatty aldehydes. The latter can be reduced in tissues to α-chloro-fatty alcohols or oxidized to α-chloro-fatty acids, which can be further metabolised to a family of products.
Activated neutrophils and monocytes together with infarcted myocardium and human atherosclerotic lesions have been shown to produce significant amounts of 2-chlorohexadecanal and related lipids, which have been shown to have a number of deleterious effects. The other products lysophospholipids are cytotoxic and pro-atherogenic. It seems likely that proposed protective action of plasmalogens as antioxidants is in competition with the damaging effects of the reaction with myeloperoxidase.
In the human peroxisomal disorder Rhizomelic Chondrodysplasia Punctata, which is characterized clinically by defects in eye, bone and nervous tissue, there are defects in the biosynthesis of plasmalogens. This is also true of Zellweger’s syndrome and certain other rare genetic conditions. In Alzheimer’s disease, there is a significant loss of plasmalogens in brain tissue that appears to be correlated with the progression of the illness. Plasmalogens are involved in aspects of cholesterol metabolism. High concentrations in male reproductive tissues suggest that plasmalogens have a role in spermatogenesis and fertilization, while deficiencies in plasmalogens can lead to cataract formation in the eye. Studies of these phenomena are now being aided by the use of genetically modified mice lacking specific enzymes involved in the biosynthesis of ether lipids.
1-O-Alkylglycerols in the diet are rapidly absorbed from the gastrointestinal tract from which they are transported to other tissues and utilized for synthesis of a full range of ether-containing lipids (3-O-alkylglycerols are absorbed equally rapidly but are then oxidized to fatty acids). There is a tradition in Scandinavian folk medicine for the use of shark liver oils for the treatment of cancers and other ailments, including wound healing, gastric ulcers and arthritis, and there appears to be some substance to the claims that are under active investigation. The alkylglycerol constituents, and the 2-methoxy constituents especially, appear to be the key ingredients. The mechanism for the biological effects is uncertain, but they are believed to increase the permeability of membranes and there is evidence for direct effects on the enzyme protein kinase C, which has vital functions in signal transduction. Synthetic ether analogues of lysophospholipids are also being tested as anticancer agents.
6. Other Ether Lipids
Platelet-activating factor - this important lipid, related to phosphatidylcholine, now has its own web page. Di- and tetra-alkyl ether lipids of the Archaea – these are unique lipids based on 2,3-dialkyl-sn-glycerol backbones and also have their own web page.
Many other types of ether lipids have been reported in tissues. These include cholesterol ethers and vinyl ethers, glycerol thio-ethers, and dialkylglycerophosphocholines, which have been found in bovine heart. Small amounts of ether analogues of galactosyldiacylglycerols, including seminolipid or 1-O-hexadecyl-2-O-hexadecanoyl-3-O-β-D-(3'-sulfo)-galactopyranosyl-sn-glycerol, have been found in brain and nervous tissue, and in testis and spermatozoa. Similarly, in mammalian tissues, the glycosylphosphatidylinositol (GPI) component of GPI-anchored proteins frequently contain a 1-O-alkyl-2-O-acyl-sn-2-glycerol residue. Very recently, a new biologically active ether lipid, 2-arachidonyl-glyceryl ether, was detected in porcine brain; it is an endogenous agonist of a cannabinoid receptor.
7. Analysis of Ether Lipids
Intact ether lipids are not easily purified for detailed analysis. However, 1-alkyldiacylglycerols and neutral plasmalogens can be separated from triacylglycerols by exacting thin-layer chromatography techniques. It is usually necessary to convert phospholipids to nonpolar forms, either by modifying or removing the polar head group, before alkylacyl-, alkenylacyl- and diacyl-forms can be isolated by TLC or HPLC. Another common approach to analysis consists in isolation and derivatization of the alkyl or alkenyl moieties for analysis by gas chromatography-mass spectrometry. In the native form, they can be quantified readily by 31P-NMR spectroscopy, although mass spectrometry is often the preferred methodology nowadays.
Although it is now somewhat out of date, a book Ether Lipids: Biochemical and Biomedical Aspects (edited by H.K. Mangold and F. Paltauf, Academic Press, 1983) can be recommended as a general source of information. More recent reviews include -
- Braverman, N.E. and Moser, A.B. Functions of plasmalogen lipids in health and disease. Biochim. Biophys. Acta, 1822, 1442-1452 (2012) (DOI: 10.1016/j.bbadis.2012.05.008).
- Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Bridgwater, U.K. and Woodhead Publishing Ltd, now Elsevier) (2010).
- Farooqui, A.A. and Horrocks, L.A. Plasmalogens, platelet-activating factor, and other ether glycerolipids. In: Bioactive Lipids. pp. 107-134 (edited by A. Nicolaou and G. Kokotos, The Oily Press, Bridgwater) (2004).
- Ford, D.A. Lipid oxidation by hypochlorous acid: chlorinated lipids in atherosclerosis and myocardial ischemia. Clin. Lipidol., 5, 835–852 (2010) (DOI: 10.2217/clp.10.68)
- Goldfine, H. The appearance, disappearance and reappearance of plasmalogens in evolution. Prog. Lipid Res., 49, 493-498 (2010) (DOI: 10.1016/j.plipres.2010.07.003).
- Gorgas, K., Teigler, A., Komljenovic, D. and Just, W.W. The ether lipid-deficient mouse: tracking down plasmalogen functions. Biochim. Biophys. Acta, 1763, 1511-1526 (2006) (DOI: 10.1016/j.bbamcr.2006.08.038).
- Iannitti, T. and Palmieri, B. An update on the therapeutic role of alkylglycerols. Mar. Drugs, 8, 2267-2300 (2010) (DOI: 10.3390/md8082267).
- Magnusson, C.D. and Haraldsson, G.G. Ether lipids. Chem. Phys. Lipids, 164, 315-340 (2011) (DOI: 10.1016/j.chemphyslip.2011.04.010).
- McIntyre, T.M., Snyder, F. and Marathe, G.K. Ether-linked lipids and their bioactive species. In: Biochemistry of Lipids, Lipoproteins and Membranes (5th Edition), pp. 245-276 (edited by D.E. Vance and J.E. Vance, Elsevier Science) (2008).
- Wallner, S. and Schmitz, G. Plasmalogens the neglected regulatory and scavenging lipid species. Chem. Phys. Lipids, 164, 573-589 (2011) (DOI: 10.1016/j.chemphyslip.2011.06.008).
Updated: May 5th, 2014