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Ether Lipids



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 non-polar lipid or more often a phospholipid in animal tissues or in anaerobic bacteria. At one time, ether lipids were considered to be little more than a biological novelty, although they comprise nearly 20% of the human phospholipidome. 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 its own web page here), with important biological activities have greatly stimulated the interest in these compounds.


1.   Basic Chemistry

Two main types of glycerol ether bonds exist in natural lipids - ether (alkyl) and vinyl (alk-1-enyl) ether as illustrated. The double bond adjacent to the oxygen atom in the latter has the Z or cis configuration.

Formulae of ethers and vinyl ethers

The terms "plasmanyl-" and "plasmenyl-" lipids for alkyl and alk-1-enyl ethers, respectively, are recommended by IUPAC-IUB, but they do not appear to have been widely taken up in the literature.

In animal tissues, the alkyl and alkenyl moieties in both non-polar 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.

Alkyl ether bonds are stable to both alkaline and acidic hydrolysis under most practical conditions, but alk-1-enyl-ether bonds open readily under acidic conditions to form aldehydes (or depending on conditions, acetal derivatives - see the Analysis section below). On hydrolysis with alkali, the ether bonds of 1-alkyldiacyl-sn-glycerols and its 1-alk-1'-enyl equivalent are stable, and 1-alkyl- and 1-alk-1'-enylglycerols, respectively, and free (unesterified) acids are the products.

Acidic and basic hydrolysis of ether lipids


2.   Alkyldiacylglycerols and Neutral Plasmalogens

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. Similarly, 1-alkyldiacyl-sn-glycerols can be a major component of lipids of marine invertebrates (80% of squid liver lipids); they are present in the lipids of all corals, where it is proposed that they confer resistance to lipases. 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 have been discounted. As an example, 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.
Component 1-Alkenyldiacylglycerols 1-Alkyldiacylglycerols Triacylglycerols
Alkenyl
ethers
Fatty acids Alkyl
ethers
Fatty acids Fatty acids
 
14:0 2 1 4 3 2
15br 11 3 2 3 -
15:0 5 - 3 - 1
16:0 38 14 32 23 19
16:1 4 2 - 2 2
17br 5 1 4 2 -
17:0 24 - 2 1 2
18:0 7 16 34 21 19
18:1 - 27 21 29 45
18:2 - 22 - 4 7
18:3(n-3) - 1 - 1 1
20:3(n-6) - 3 - - -
20:4(n-6) - 6 - 1 -
22:4(n-6) - 2 - 3 -
22:5(n-3) - 3 - 5 -
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). In marine invertebrates, polyunsaturated fatty acids tend to be concentrated in position sn-2.

A methoxy alkylglycerolSmall amounts of methoxy-substituted glyceryl ethers have been found in alkyldiacylglycerols and alkylacyl phospholipids in liver oils from sharks and other cartilaginous fish species. In addition to the methoxyl group in position 2, 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. It is claimed that they have potent antibacterial and anti-cancer activities and that they boost the immune system.


3.   Phospholipids with Ether-Linked Substituents

While neutral ether lipids tend to be encountered only rarely, the membrane phospholipids of many animal and microbial species usually contain appreciable proportions of molecular species with ether and vinyl ether bonds in position sn-1, in addition to diacyl forms, 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. Diplasmalogens (1,2-di-(O-1'-alkenyl) forms) have been reported to constitute a high proportion of the phosphatidylethanolamine in epididymal spermatozoa from rabbits.

Formulae of plasmanylcholine and plasmenylcholine

In adult humans, approximately 20% of the total phospholipids have an ether linked moiety, but the proportions in different tissues vary greatly. For example, liver phospholipids contain less than 5% of ether lipids, while spermatozoa can contain up to 40%. Other than the heart, plasmenylethanolamine tends to be the main ether lipid (illustrated above), with as much as 80% of the total ethanolamine-phospholipids in some brain tissues in this form, while in kidney, skeletal muscle and retina it can amount to 20-40%. Much less of the phosphatidylcholine and commonly little or none of other phospholipids such as phosphatidylinositol is in this form. In phosphatidylcholine of most tissues, a higher proportion is often of the O-alkyl rather than the O-alkenyl form (neutrophils contain over 40% as the O-alkyl form), but the reverse tends to be true in heart lipids.

As an example, 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.
Component Alkenylacyl- Alkylacyl- Diacyl-
1-Alkenyl
ethers
Fatty acids
position 2
1-Alkyl
ethers
Fatty acids
position 2
Fatty acids
position 1
Fatty acids
position 2
 
C14-15 6 - 10 - 1 -
16:0 62 2 5 11 62 5
16:1 2 1 - 1 3 3
17br 8 - 9 - 7 -
17:0 3 - 1 - 2 -
18:0 11 1 10 2 14 -
18:1 7 12 18 13 8 29
18:2 - 53 - 51 3 46
18:3(n-3) - 1 - 1 - 1
20:3(n-6) - 6 - 5 - 5
20:4(n-6) - 21 - 14 - 10
20:5(n-3) - 1 - - - -
22:4(n-6) - 2 - 2 - 1
22:5(n-3) - 1 - 1 - -
Data again from Schmid, H.H.O. and Takahashi, T. Biochim. Biophys. Acta, 164, 141-147 (1968), and 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 sn-1 and much more arachidonate (20:4(n‑6)) in position sn-2. In brain and retina, a high proportion of the fatty acid constituents are 20:4, 22:4 and 22:6 species. The nematode Caenorhabditis elegans, now considered a useful model for studies of ether lipid function, contains high proportions of alkylacyl and alkenylacyl forms of phosphatidylethanolamine but not of other phospholipids; it has mainly C18 constituents in both positions of the glycerol moiety.

In anaerobic 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 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.


4.   Biosynthesis of Ether Lipids

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 by enzymes associated with the membranes of peroxisomes, an organelle normally associated with the catabolism of lipids, with the process being completed in the endoplasmic reticulum.

Biosynthesis of ethers and plasmalogens

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 (alkylglyceronephosphate 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 by an exchange reaction. 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. Similarly, the biosynthesis of alkyl and alkenyl lipids in anaerobic bacteria appears to be accomplished by a different route that does not require DHAP, but many of the details have still to be elucidated.

As with other phospholipids, the final fatty acid compositions of ether lipids are attained by remodelling processes. While this can occur by re-acylation after removal of the fatty acids of position sn-2 by the action of a phospholipase A2, with formation of lysophospholipids (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. Presumably, 1-alkyl,2-acyl-glycerols derived from phospholipids are the source of the 1-alkyl,2,3-diacyl-sn-glycerols in animal cells; acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is reported to be the main enzyme responsible for introducing the fatty acid into position sn-3.

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).

Catabolism: The fatty acid in position sn-2 of an alkylacyl phospholipid, including platelet activating factor, is first released by the action of a phospholipase A2, before the O-alkyl linkage is cleaved oxidatively by a microsomal alkylglycerol monooxygenase, present in liver and intestinal tissue. The aliphatic product is a fatty aldehyde, which is then further oxidized to the corresponding acid by a fatty aldehyde dehydrogenase. Although plasmalogenases in cells are less well characterized, plasmalogens yield similar products.


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 for the diacyl analogues and as they have a larger dipole moment, 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.

Scottish thistlePlasmalogens 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 re-acylated 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 by acting as sacrificial antioxidants in vivo have been more difficult to substantiate, although it has been demonstrated that singlet oxygen interacts more rapidly with ether lipids than with other lipids in vitro at least. 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 and retina that plasmalogens do function as endogenous antioxidants in these tissues at least. It is believed that the oxidation by-products of plasmalogens are less toxic than the free aldehydes and hydroperoxides produced by oxidation at other unsaturated centres. A study with genetic mutants of the nematode C. elegans demonstrated that ether lipids were important for optimal fertility, lifespan, survival at cold temperatures, and resistance to oxidative stress.

In contrast, it has long been known that greatly elevated levels of ether lipids are found in cancers, and there appears to be a strong correlation with the promotion of aggressive forms of the disease. It is known that the peroxisomal alkylglyceronephosphate synthase is up-regulated appreciably in cancer cells, leading to substantial changes in the content and composition of many types of lipid including signalling lipids such as eicosanoids, which favour the development of cancers.

There are also 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 metabolized to a family of products.

Reaction of plasmalogens with active oxygen species produced by myeloperoxidase

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 for which a number of deleterious effects have been demonstrated. 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. Also, there are reports that 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.

There is a tradition in Scandinavian folk medicine for the use of shark liver oils, which are rich in ether lipids, 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, are considered 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. Neutral ether lipids but not the analogous phospholipids accumulate in the tissues of those who suffer from Wolman’s disease, a rare genetic disorder caused by a deficiency in lysosomal acid lipase. Synthetic ether analogues of lysophospholipids are also being tested as anticancer agents.


6.   Other Ether Lipids

Platelet-activating factor - this biologically active lipid, related to phosphatidylcholine, has its own web page. Di- and tetra-alkyl ether lipids of the Archaea are unique lipids based on 2,3-dialkyl-sn-glycerol backbones and also have their own web page, as does seminolipid or 1-O-hexadecyl-2-O-hexadecanoyl-3-O-β-D-(3'-sulfo)-galactopyranosyl-sn-glycerol, which is an important constituent of the lipids of testis and spermatozoa.

In recent years an unusual group of branched glycerol dialkyl glycerol tetraether lipids has been discovered in peat bogs and soils. In general, they consist of octacosane (C28) alkyl units with either 13,16-dimethyl- or 5,13,16-trimethyl substituents, and forms with 4 to 6 methyl groups attached to the n-alkyl chains and/or with 0 to 2 cyclopentyl moieties in the alkyl chain. Related membrane-spanning diabolic acids are also present. In addition to being non-isoprenoid in nature, these lipids differ from those of the Archaea in that they have a 1,2-di-O-alkyl-sn-glycerol rather than the 2,3-di-O-alkyl-sn-glycerol configuration typical of the latter.

Glycerol dialkyl glycerol tetraether lipid from soil anaerobic bacteria

It is believed that these branched membrane lipids are produced by anaerobic soil bacteria, probably of the genus Acidobacteria. The nature of the intact lipids from which they are derived has also to be fully elucidated, although some components have been identified with glucuronosyl or glucosyl units attached to the glycerol ether backbone.

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, have been found in brain and nervous tissue. Also in mammalian tissues, the glycosylphosphatidylinositol (GPI) component of GPI-anchored proteins frequently contain a 1-O-alkyl-2-O-acyl-sn-2-glycerol residue. Sponges can contain highly unusual glycosyldiacylglycerols with ether bonds, though these are believed to originate in symbiotic bacteria. A biologically active ether lipid, noladin ether an analogue of 2-arachidonylglycerol, has been detected in porcine brain; it is an endogenous agonist of a cannabinoid receptor. In addition, an unusual galactoglycerolipid with phytol ether-linked to position 1 of glycerol has been partially characterized from algae and cyanobacteria, and it may occur at trace levels in some higher plants. Cyclic ether lipids, e.g. epoxy and furanoid fatty acids, and betaine lipids in which the polar moiety is linked to glycerol via an ether bond are also discussed in separate pages of this website. Similarly, bacterial proteolipids contain a diacylglycerol unit linked from position sn-3 via a thio ether bond to a protein.


7.   Analysis of Ether Lipids

Formula of a dimethylacetalPure 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. 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 non-polar 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. 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 (see Figure 2) that are rapidly converted to dimethyl acetals and can be analysed in this form. In the native form, ether-containing phospholipids of all types can be quantified readily by 31P NMR spectroscopy, although mass spectrometry is often the preferred methodology nowadays.


Suggested Reading

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 publications include -



Lipid listings Credits/disclaimer Updated: June 2nd, 2017 Author: William W. Christie LipidWeb icon