Phosphatidylethanolamine and Related Lipids

1. Phosphatidylethanolamine – Structure and Occurrence

Phosphatidylethanolamine or 1,2-diacyl-sn-glycero-3-phosphoethanolamine (once given the trivial name 'cephalin') is usually the second most abundant phospholipid in animals and plants after phosphatidylcholine, and it is frequently the main lipid component of microbial membranes. It can amount to 20% of liver phospholipids and as much as 45% of those of brain; higher proportions are found in mitochondria than in other organelles. As such, it is obviously a key building block of membrane bilayers, where it is present exclusively in the inner leaflet of the plasma membrane in animal cells. It is a neutral or zwitterionic phospholipid (at least in the pH range 2 to 7), with the structure illustrated (of one molecular species).

Structural formulae of phosphatidylethanolamine

In animal tissues, phosphatidylethanolamine tends to exist in diacyl, alkylacyl and alkenylacyl forms, and data for the compositions of these various forms from phosphatidylcholine from bovine heart muscle as an example are listed in our web page on ether lipids. As much as 70% of the phosphatidylethanolamine in some cell types (e.g., inflammatory cells, neurons and tumour cells) can have an ether linkage, but in liver, the plasmalogen form of phosphatidylethanolamine, i.e., with an O‑alk-1′-enyl linkage, accounts for only 0.8% of total phospholipids. Generally, there is a much higher proportion of phosphatidylethanolamine with ether linkages than of phosphatidylcholine. If biosynthesis of the plasmalogen form is inhibited by physiological conditions, it is replaced by the diacyl form so that the overall content of the phospholipid remains constant.

In general, animal phosphatidylethanolamine tends to contain higher proportions of arachidonic and docosahexaenoic acids than the other zwitterionic phospholipid, phosphatidylcholine. These polyunsaturated components are concentrated in position sn-2 with saturated fatty acids most abundant in position sn-1, as illustrated for rat liver and chicken egg in Table 1. In most other species, it would be expected that the structure of the phosphatidylethanolamine in the same tissues would exhibit comparable features.

Position	Fatty acid
Table 1. Positional distribution of fatty acids in phosphatidylethanolamine in animal tissues.
	14:0	16:0	18:0	18:1	18:2	20:4	22:6

Rat liver [1]
sn-1		25	65	8
sn-2	2	11	8	8	10	46	13

Chicken egg [2]
sn-1		32	59	7	1
sn-2		1	1	25	22	29	12
1, Wood, R. and Harlow, R.D., Arch. Biochem. Biophys., 131, 495-501 (1969); DOI. 2, Holub, B.J. and Kuksis, A. Lipids, 4, 466-472 (1969); DOI.

The O-alkyl and O-alkenyl groups at the sn-1 position of the analogous ether lipids generally have 16:0, 18:0 or 18:1 chains, whereas arachidonic and docosahexaenoic acids are the most abundant components at the sn-2 position.

The positional distributions of fatty acids in phosphatidylethanolamine from the leaves of the model plant Arabidopsis thaliana are listed in Table 2. Here again, saturated fatty acids are concentrated in position sn-1, and there is a preponderance of di- and triunsaturated components in position sn-2. The pattern is somewhat different for the yeast Lipomyces lipoferus, where the compositions of the two positions are comparable.

Position	Fatty acid
Table 2. Composition of fatty acids (mol %) in positions sn-1 and sn-2 in the phosphatidylethanolamine from leaves of Arabidopsis thaliana and from Lipoferus lipoferus .
	16:0	16:1	18:0	18:1	18:2	18:3

A. thaliana [1]
sn-1	58	trace	4	5	15	18
sn-2	trace	trace	trace	2	60	38

L. lipoferus [2]
sn-1	29	18	4	28	13	6
sn-2	23	15	3	34	17	6
1, Browse, J., Warwick, N., Somerville, C.R. and Slack, C.R. Biochem. J., 235, 25-31 (1986); DOI. 2, Haley, J.E. and Jack, R.C. Lipids, 9, 679-681 (1974); DOI .

2. Phosphatidylethanolamine – Biosynthesis

The two main pathways employed by mammalian cells for the biosynthesis of phosphatidylethanolamine are the CDP-ethanolamine pathway, i.e., one of the general routes to phospholipid biosynthesis de novo in plants and animals, and the phosphatidylserine decarboxylase pathway, which occur in two organelles, the endoplasmic reticulum and mitochondria (although there can be contact points between the two). Ethanolamine is obtained by decarboxylation of serine in plants, but in animals most must come from dietary sources and requires facilitated transport into cells. A small amount of ethanolamine phosphate comes from catabolism of sphingosine-1-phosphate, and this is essential for the survival of the protozoon Trypanosoma brucei.

The initial steps in phosphatidylethanolamine biosynthesis occur in the cytosol with first the phosphorylation of ethanolamine by two ethanolamine kinases to produce ethanolamine phosphate; the reverse reaction can occur by means of the enzyme ethanolamine phosphate phospholyase and this may be regulatory in some tissues. In the second and rate-limiting step, the reaction of ethanolamine phosphate with cytidine triphosphate (CTP) to form cytidine diphosphoethanolamine is catalysed by CTP:phosphoethanolamine cytidylyltransferase. In the seeds of higher plants, a deficiency of this last enzyme has profound effects upon the viability and maturation of embryos.

Main pathway of phosphatidylethanolamine biosynthesis

Figure 1. Biosynthesis of phosphatidylethanolamine de novo in cells.

In the final step, a membrane-bound enzyme CDP-ethanolamine:diacylglycerol ethanolamine-phosphotransferase catalyses the reaction of cytidine diphosphoethanolamine with diacylglycerol to form phosphatidylethanolamine. There are two such enzymes, ethanolamine phosphotransferase 1 (EPT1) in the Golgi and a choline/ethanolamine-phosphotransferase (CEPT1) in the endoplasmic reticulum, but EPT1 is more active in the biosynthesis of the plasmalogen form, 1‑alkenyl-2-acyl-glycerophosphoethanolamine, and especially those molecular species containing polyunsaturated fatty acids, while CEPT1 produces species with shorter-chain fatty acids. The diacylglycerol precursor is formed from phosphatidic acid by the action of the enzyme phosphatidic acid phosphohydrolase (see our web pages on triacylglycerols and phosphatidylcholine). An enzyme that degrades ethanolamine phosphate, i.e. ethanolamine phosphate phospholyase, is part of the regulatory mechanism for hepatic phosphatidylethanolamine synthesis and plasma lipoprotein metabolism in mice.

The second major pathway is the decarboxylation of phosphatidylserine to form phosphatidylethanolamine in mitochondria (as discussed further in our web pages on phosphatidylserine), and conservation of this pathway from bacteria to humans suggests that it has been preserved to optimize mitochondrial performance. As mitochondria are not connected with the rest of the cell's membrane network by classical vesicular routes, they must receive and export small molecules through non-vesicular transport at zones of close proximity with other organelles at membrane contact sites such as a specific domain of the endoplasmic reticulum termed the mitochondria-associated membrane (MAM). Transport of phosphatidylserine is then enabled by tethers that bridge two membranes and by lipid transfer and recruitment proteins. In this process, the lipid must traverse two aqueous compartments, the cytosol and the mitochondrial intermembrane space, to reach the inner mitochondrial membrane.

The phosphatidylserine decarboxylase is located on the external aspect of the mitochondrial inner membrane, and most of the phosphatidylethanolamine in mitochondria is derived from this pathway. While various isoforms of the phosphatidylserine decarboxylase exist in prokaryotes, yeasts and mammals, the main forms designated 'PSD1' are found only in mitochondria and are related structurally. An additional isoform designated 'PSD2' is located in the endosomal membranes of yeasts, and the phosphatidylethanolamine formed from this is the preferred substrate for phosphatidylcholine biosynthesis. It is evident that cellular concentrations of phosphatidylethanolamine and phosphatidylserine are interdependent and tightly regulated.

Figure 2. Phosphatidylethanolamine synthesis from phosphatidylserine in mitochondria.

This pathway is important in mammalian cells and yeasts, although the relative contributions of the two main pathways for phosphatidylethanolamine synthesis in mammalian cells appears to depend on the cell type. In cells in culture, more than 80% of the phosphatidylethanolamine is reported to be derived from the phosphatidylserine decarboxylase pathway, but in hamster heart and rat hepatocytes, only ~5% of phosphatidylethanolamine synthesis comes from this route, and most is from the CDP-ethanolamine pathway. In yeasts, 70% of the phosphatidylethanolamine is generated by PSD1. The spatially segregated pools within the cell are functionally distinct, but both are essential to life. In embryonic mice, disruption of the phosphatidylserine decarboxylase gene causes misshapen mitochondria and has lethal consequences, although phosphatidylethanolamine synthesis continues for a time in other cellular regions. Similarly, elimination of the endoplasmic reticulum route is embryonically lethal.

In prokaryotic cells, such as Escherichia coli, in which phosphatidylethanolamine is the most abundant membrane phospholipid, all of it is derived from phosphatidylserine decarboxylation. In this instance, the enzyme undergoes auto-cleavage for activation and utilizes a pyruvoyl moiety to form a Schiff base intermediate with phosphatidylserine to facilitate decarboxylation.

Three further minor biosynthetic pathways are known. Phosphatidylethanolamine can be formed by re-acylation of lysophosphatidylethanolamine in the mitochondria-associated membrane where the phosphatidylserine synthase II is located or by an enzymatic exchange reaction of ethanolamine with phosphatidylserine, while the bacterial plant pathogen Xanthomonas campestris is able to synthesise phosphatidylethanolamine by condensation of cytidine diphosphate diacylglycerol with ethanolamine. The various biosynthetic mechanisms produce different pools of phosphatidylethanolamine species, which are often in different cellular compartments and have distinctive compositions. Studies with mammalian cell types in vitro suggest that the CDP-ethanolamine pathway produces molecular species with mono- or di-unsaturated fatty acids on the sn-2 position preferentially, while the phosphatidylserine decarboxylation reaction generates mainly species with polyunsaturated fatty acids on the sn-2 position. In addition, two proteins designated TLCD1/2 promote the incorporation of monounsaturated fatty acids into phosphatidylethanolamine.

Remodelling by the Lands’ cycle: As with other phospholipids, the final fatty acid composition in animal tissues is attained by a process of remodelling known as the Lands’ cycle (see the web page on phosphatidylcholine for a more detailed discussion). The first step is hydrolysis by a phospholipase A₂ to lysophosphatidylethanolamine, followed by reacylation by means of various acyl-CoA:lysophospholipid acyltransferases. The enzymes LPLAT1, 2 and 3, which are involved in phosphatidylcholine biosynthesis, remodel phosphatidylethanolamine also; LPLAT1 utilizes lysophosphatidylethanolamine mainly and is specific for oleoyl-CoA. Some of these isoforms appear to be confined to particular tissues, and there are independent acyltransferases in macrophages and other cells/tissues that transfer such fatty acids as arachidonic from phosphatidylcholine to ethanolamine-containing phospholipids. In lysosomes, lysophosphatidylethanolamine and lysophosphatidylcholine formed during degradation are exported into the cytosol by a transporter Spns1 for recycling.

Ether lipids: It should be noted that all of these pathways for the biosynthesis of diacyl-phosphatidylethanolamine are very different and are separated spatially from that producing alkylacyl- and alkenylacyl-phosphatidylethanolamines and described in a separate web page, suggesting that there may be differences in their cellular roles. In the protozoon T. brucei, it has been demonstrated that the diacyl and ether pools of phosphatidylethanolamine are used for separate purposes and cannot substitute for each other.

3. Phosphatidylethanolamine – Biological Function

Physical properties: Although phosphatidylethanolamine has sometimes been equated with phosphatidylcholine in biological systems, there are significant differences in the chemistry and physics of these lipids, and they are utilized in different ways in biochemical processes. Both are crucial components of membrane bilayers and the ratio of the two may be a relevant factor, although phosphatidylethanolamine has a smaller head group, which gives the lipid a cone shape. On its own, it does not form bilayers but inverted hexagonal phases. With other lipids in a bilayer, it is believed to exert a lateral pressure that modulates membrane curvature and stabilizes membrane proteins in their optimum conformations. It can hydrogen bond intermolecularly to adjacent lipids and to proteins through its polar head group, and the phosphate can be stabilized at the binding site by interactions with lysine and arginine side chains, or with hydroxyls from tyrosine side chains; there can be strong interactions too with the acyl chains of the phospholipid.

In contrast to phosphatidylcholine, phosphatidylethanolamine is concentrated with phosphatidylserine in the inner leaflet of the plasma membrane. On the other hand, it is present in both the inner and outer membranes of mitochondria, and in the former, it is present at much higher levels than in other organelles and facilitates membrane fusion and protein movement across membranes in addition to many other essential mitochondrial functions, including oxidative phosphorylation via the electron transport chain. It is a component of membrane contact sites between the endoplasmic reticulum and mitochondria. In eukaryotic cells in general, but especially those of insects, studies suggest that like cholesterol it increases the rigidity of the bilayer to maintain membrane fluidity, while in bacterial membranes, it appears that a primary role is simply to dilute the high negative charge density of the anionic phospholipids.

Protein Interactions: Membrane proteins amount to 30% of the genome, and they are involved in transport, energy production, biosynthesis, signalling and communication. Within a membrane, most integral proteins consist of hydrophobic α-helical trans-membrane domains that zigzag across it and are connected by hydrophilic loops. Of those parts of the proteins outwith the bilayer, positively charged residues are much more abundant on the cytoplasmic side of membrane proteins as compared to the trans side (the positive-inside rule). Phosphatidylethanolamine is believed to inhibit location of negative amino acids on the cytoplasmic side, and it has an appropriate charge density to balance that of the membrane surface and the protein, supporting the positive-inside rule. However, it can permit the presence of negatively charged residues on the cytosolic surface in some circumstances in support of protein function.

Phosphatidylethanolamine is vital for mitochondrial efficiency, as demonstrated by defects in the operation of oxidative phosphorylation in the absence of PSD1 and by the role of the lipid in the regulation of mitochondrial dynamics in general and in the biogenesis of proteins in the outer mitochondrial membrane. Synthesis of phosphatidylethanolamine in the inner membrane of mitochondria is critical for the cytochrome bc₁ complex (III), where there is a conserved binding site for the lipid in a specific subunit.

Scottish thistle Phosphatidylethanolamine binds non-covalently to a superfamily of cytosolic proteins with multiple functions termed 'PE-binding proteins'. While four members have been identified in mammals (PEBP1 to 4), more than 400 members of the family that are conserved during evolution are known from bacteria to higher eukaryotes, and are required for such tasks as lipid binding, neuronal development, serine protease inhibition and the regulation of several signalling pathways; in plants, they control shoot growth and flowering. Of these, PEBP4 is of particular interest as it is highly expressed in many different cancers and can increase their resistance to therapy. In animal tissues, phosphatidylethanolamine is important in the sarcolemmal membranes of the heart during ischemia, it participates in secretion of the nascent very-low-density lipoproteins (VLDL) from liver and in membrane fusion and fission. It is required by the Ca²⁺-ATPase in that one molecule of phosphatidylethanolamine is bound in a cavity between two transmembrane helices, acting as a wedge to keep them apart. This is displaced when Ca²⁺ is bound to the enzyme. Many other proteins bind non-covalently to phosphatidylethanolamine in a comparable way, including rhodopsin and aquaporins.

Although the mechanism has yet to fully elucidated, effects on protein conformation are believed to be behind a finding that phosphatidylethanolamine is the primary factor in brain required for the propagation and infectivity of mammalian prions. Host defence peptides are antimicrobial agents produced by both prokaryotic and eukaryotic organisms, and many of these have a high affinity for phosphatidylethanolamine as a lipid receptor to modulate their activities. The peptide antibiotics cinnamycin and duramycins from Streptomyces have a hydrophobic pocket that fits around phosphatidylethanolamine such that the binding is stabilized by ionic interaction between the ethanolamine group of the lipid and the carboxylate moiety of the peptide to form a complex that inhibits Gram-positive organisms such as Bacillus species.

Much of the evidence for the unique properties of phosphatidylethanolamine comes from studies of the biochemistry of the bacterium E. coli, where this lipid is a major component of the membranes. Gram-negative bacteria have two membrane bilayers in the cell wall (see our web page on lipid A), and as much 90% of the phospholipid in the inner leaflet of the outer membrane is phosphatidylethanolamine, together with a high proportion in the cytoplasmic leaflet of the inner membrane. Phosphatidylethanolamine is required to support transport of lactose across membranes by the enzyme lactose permease, and other transport systems may require or be stimulated by it, while there is evidence that phosphatidylethanolamine acts as a 'chaperone' during the assembly of this and other membrane proteins to guide the folding path for the proteins and to aid in the transition from the cytoplasmic to the membrane environment. In the absence of this lipid, the transport membranes may not have the correct tertiary structure and so will not work properly. Whether the lipid is required once the protein is correctly assembled is not fully understood in all cases, but it may be needed to orient enzymes correctly in the inner membrane. In contrast, it can inhibit folding of some multi-helical proteins. Some studies suggest that life in this organism can be maintained without phosphatidylethanolamine, but that crucial processes are inhibited.

The content of phosphatidylethanolamine in newly secreted VLDL particles and in apo B-containing particles isolated from the lumen of the Golgi is much higher than that in circulating VLDLs, suggesting that this lipid participates in VLDL assembly and/or secretion, although it is rapidly and efficiently removed from VLDL in the circulation. With lipid droplets in cells, phosphatidylethanolamine is believed to promote coalescence of smaller droplets into larger ones; its synthesis is increased in brown adipocytes (and in liver) during cold exposure.

Autophagy and ferroptosis: A covalent conjugate of phosphatidylethanolamine with a protein designated 'Atg8' is formed by the action of cysteine protease ATG4 (belonging to the caspase family) and various other proteins and influences the process of autophagy (controlled degradation of cellular components) in yeast by promoting the formation of membrane vesicles containing the components to be degraded (phosphatidylinositol 3-phosphate is essential to this process). Similarly, oxidatively modified phosphatidylethanolamine is a factor in ferroptosis, a form of apoptosis in which disturbances to iron metabolism lead to an accumulation of hydroperoxides.

Precursor of other lipids: Phosphatidylethanolamine is a precursor for the synthesis of N-acyl-phosphatidylethanolamine (see below) and thence of anandamide (N‑arachidonoylethanolamine), and the plasmalogen form is the precursor for the 1-alkenyl phosphatidylcholine. In liver, it is the substrate for the hepatic enzyme phosphatidylethanolamine N-methyltransferase, which provides about a third of the phosphatidylcholine. It is the donor of ethanolamine phosphate during the synthesis of the glycosylphosphatidylinositol anchors that attach many signalling proteins to the surface of the plasma membrane, and in bacteria, it is used in the same way for the biosynthesis of lipid A and other lipopolysaccharides.

Other miscellaneous functions: Phosphatidylethanolamine is the precursor of an ethanolamine phosphoglycerol moiety bound to two conserved glutamate residues in eukaryotic elongation factor 1A, which is a crucial component in protein synthesis. This unique modification appears to be of great importance for the resistance of plants to attack by pathogens.

Francisella tularensis bacteria, the cause of tularemia, suppresses host inflammation and the immune response when infecting mouse cells, an effect due to a distinctive phosphatidylethanolamine species containing 10:0 and 24:0 fatty acids. As the synthetic lipid produces the same effects in vitro in human cells infected with dengue fever virus, it is hoped that this lipid will prove to be a potent anti-inflammatory therapeutic agent. Akkermansia muciniphila, a bacterial species from human gut, produces a phosphatidylethanolamine species with two different branched chain components (anteiso-15:0 and iso-15:0) that has remarkable specificity for immune signalling in its host via a toll-like receptor TLR2-TLR1 heterodimer.

4. Lysophosphatidylethanolamine

Formula of lysophosphatidylethanolamine Lysophosphatidylethanolamine (LPE), with one mole of fatty acid per mole of lipid, is found in trace amounts only in animal tissues, other than plasma (10 to 50µM, or ~1% of total serum phospholipids), and it is formed mainly by hydrolysis of phosphatidylethanolamine by the enzyme phospholipase A₂ as part of the de-acylation/re-acylation cycle that controls its overall molecular species composition as discussed above. There are reports of the participation of LPE in cellular functions, such as differentiation and migration of certain neuronal cells but also of various cancer cells. For example, oleoyl-LPE in brain stimulates neurite outgrowth and protects against glutamate toxicity. Some further biological effects have been reported in animal tissues in vitro, and it may be an agonist for the G-protein-coupled receptor for lysophosphatidic acid LPAR₁.

In plants, lysophosphatidylethanolamine is an inhibitor of phospholipase D, a key enzyme in the degradation of membrane phospholipids during the early stages of plant senescence, where it retards the senescence of leaves, flowers and post-harvest fruits. Indeed, it has horticultural applications when applied externally, e.g., to stimulate ripening and extend the shelf-life of fruit, delay senescence and increase the vase life of cut flowers.

In bacteria, lysophosphatidylethanolamine displays chaperone-like properties to promote the folding of citrate synthase and other enzymes. With other lysophospholipids, it is produced in the envelope membranes by many different endogenous and exogenous factors and must be transported back into the bacterial cell by flippases for conversion back to the diacyl forms by the action of a peripheral enzyme, acyl-ACP synthetase/LPL acyltransferase. Lysophosphatidylethanolamines produced by certain bacteria act synergistically with sulfonolipid rosette-inducing factors (RIFs) to induce choanoflagellates to move from a unicellular to a multicellular state.

5. N-Acyl Phosphatidylethanolamine

Formula of N-acyl phosphatidylethanolamine N-Acyl phosphatidylethanolamine with the free amino group linked to a further fatty acid has been detected in rather small amounts in several animal tissues (~0.01%), including the brain, nervous tissues and epidermis, when the N-acyl chain is often palmitic or stearic acid (human plasma: N16:0-PE (40%), N18:1-PE (23.3%), N18:0-PE (19%), N18:2-PE (16.6%) and N20:4-PE (1.4%)). In animals, it is the precursor of anandamide (see our web pages on endocannabinoids for a more detailed discussion of N‑acyl phosphatidylethanolamine synthesis and metabolism) and of other ethanolamides (e.g., N‑oleoylethanolamide) in brain, the intestines and other tissues. In brief, it is formed biosynthetically by the action of a transferase, such as cytosolic phospholipase A₂ε (Ca²⁺-dependent), which exchanges a fatty acid from the sn-1 position of a phospholipid (probably phosphatidylcholine) to the primary amine group of phosphatidylethanolamine without a hydrolytic step and with both diacyl- and alkenylacyl-species of phosphatidylethanolamine serving as acceptors. In addition, some transfer can occur from phosphatidylethanolamine per se by an intramolecular reaction.

Under conditions of degenerative stress, N-acyl phosphatidylethanolamine can accumulate in significant amounts as the result of ischemic injury, infarction or cancer, and it may be involved in neuroprotection, anti-inflammation and membrane stabilization. It is present in plasma after feeding a high fat diet to rats, and then it can cross into the brain where it accumulates in the hypothalamus. It should be noted that some N-acyl phosphatidylethanolamine can be formed artefactually because of faulty extraction procedures during analysis.

In plants, N-acyl phosphatidylethanolamine is a common constituent of cereal grains (e.g., wheat, barley and oats) and of some other seeds (1.9% of the phospholipids of cotton seeds but 10-12% of oats), but in other plant tissues, it is detected most often under conditions of physiological stress when it is hydrolysed with release of N-acylethanolamines and phosphatidic acid. In contrast to animals, synthesis is by direct acylation of phosphatidylethanolamine with a free fatty acid with catalysis by a membrane-bound transferase in a reverse serine-hydrolase catalytic mechanism, although lyso-N-acyl phosphatidylethanolamines have been detected in tomato plants as possible intermediates. Both N-acyl lipid classes have been implicated in such physiological processes as the elongation of main and lateral roots, regulation of seed germination, seedling growth and defence from attacks by pathogens.

N-Acyl phosphatidylethanolamine has been found in many microbial species, while N-acetyl phosphatidylethanolamine was detected in a filamentous fungus, Absidia corymbifera, where it comprised 6% of the total membrane lipids and was accompanied by an even more unusual lipid 1,2‑diacyl-sn-glycero-3-phospho(N-ethoxycarbonyl)-ethanolamine. The Gram-positive bacterial species Bacillus subtilis contains aminoacyl phosphatidylethanolamines with alanine or lysine attached to the ethanolamine moiety.

6. Mono- and Dimethylphosphatidylethanolamines

Mono- and dimethylphosphatidylethanolamines are formed by sequential methylation of phosphatidylethanolamine by the enzyme phosphatidylethanolamine N‑methyltransferase as intermediates in one of the mechanisms for the biosynthesis of phosphatidylcholine. This is a minor pathway in general in animals, although it is significant in liver when choline is deficient in the diet, but it is the major route in yeasts and bacteria, although these intermediate lipids do not seem to be vital components of yeast membranes.

Formulae of mono- and dimethylphosphatidylethanolamines

They are never found at greater than trace levels in animal or plant tissues, and as might be expected, they are more abundant in many species of bacteria that interact with plants.

7. Non-Enzymatic Modification of Phosphatidylethanolamine by Carbonyl-Amine Reactions

Phosphatidylethanolamine can react non-enzymatically to form imine and/or Michael adducts with the hydroxy-alkenals and related compounds that are products of hydroperoxidation of unsaturated fatty acids, including malondialdehyde, acrolein, epoxyalkenals, hydroxyalkenals (e.g., 4-hydroxy-trans-2-nonenal, HNE), oxoalkenals and γ‑ketoaldehydes. The chemistry and biochemistry of bioactive aldehydes is discussed in more detail in a separate web page. As an example, the reaction with glycoxal is illustrated.

Figure 3. Michael reaction of phosphatidylethanolamine with glycoxal (partial structures).

The adduct with HNE, for example, can react further to generate toxic pyrroles. Such products accelerate the peroxidation of membrane lipids, and they are believed to be factors in the generation of oxidative stress both in foods and in tissues. In vivo, they are inflammatory mediators and they have been implicated in a several disease states, such as atherogenesis and diabetes, and during aging. Alkyl-modified phosphatidylethanolamines induce a negative membrane curvature in lipid vesicles in vitro, and they have the potential to modify membrane properties of membrane transporters, channels, receptors and enzymes under conditions of oxidative stress.

Levuglandins and isolevuglandins (oxylipins) are reactive cyclo-oxygenase metabolites of arachidonic and docosahexaenoic acids, which react rapidly with the free amine group of phosphatidylethanolamine (and with proteins) in vivo to form cytotoxic hydroxy-lactam derivatives.

In recent years, the concept of the Maillard reaction has been expanded to include glycation of amino phospholipids. Phosphatidylethanolamine reacts with glucose and other sugars to form first unstable Schiff bases, which rearrange to produce Amadori products of phosphatidylethanolamine, as illustrated below for glucose with formation of deoxy-D-fructosyl-phosphatidylethanolamine, especially under hyperglycaemic conditions. Indeed, there are suggestions that Amadori-phosphatidylethanolamine may be a useful predictive marker for hyperglycaemia in the early stages of diabetes.

Figure 4. Formation of Amadori adduct of phosphatidylethanolamine by reaction with glucose.

Once Amadori-phosphatidylethanolamine is formed, it can undergo further reactions, for example with reactive oxygen species to form carboxymethyl- and carboxyethyl-adducts, which have the potential to trigger pathological processes, including the neuropathy and retinopathy associated with diabetic complications. Thus, it has been demonstrated that glycated phosphatidylethanolamine and its oxidation products induce the production of pro-inflammatory cytokines and that they have a role in apoptotic cell signalling. Unsurprisingly, such changes to the phospholipid head group change the biophysical properties of membrane bilayers appreciably and can lead ultimately to the disintegration of cell membranes. Phosphatidylserine might be expected to form analogous adducts, but these have proved harder to detect in tissues in vivo.

In the visual cycle, phosphatidylethanolamine reacts with all-trans-retinal in the photoreceptor outer segment membrane of the eye to form N‑retinylidene-phosphatidylethanolamine as part of a transport mechanism and a means of preventing non-specific aldehyde reactions. Normally, the trans-retinal is regenerated for reuse, but the conjugate can sometimes react further to generate a stable bis-retinoid condensation product (illustrated).

bis-Retinoid phosphatidylethanolamine condensation product

This lipid conjugate, together with a 1-alkyl-lysophosphatidylethanolamine analogue and hydrolysis products formed by cleavage of the ethanolamine-phosphate bond by phospholipase D, can accumulate in retinal pigment epithelial cells with age, where it can be involved in the pathogenesis of retinal disorders such as Stargardt disease.

8. Phosphatidylethanol

Formula of phosphatidylethanol Phosphatidylethanol has little in common with phosphatidylethanolamine other than the obvious structural similarity. It is formed slowly in cell membranes, notably those of erythrocytes, by a transphosphatidylation reaction from phosphatidylcholine in the presence of ethanol and catalysed by the enzyme phospholipase D. As it has a long half-life in serum relative to alcohol per se, potentially for 2-3 weeks, it is a useful biochemical and forensic marker for alcohol abuse; chronic alcoholics have much higher levels in the blood than healthy subjects who consume alcohol in moderation. While this lipid is usually considered to be physiologically inert, there is evidence with Drosophila that it affects lipid-regulated potassium channels. There are suggestions that it may be beneficial towards ethanol tolerance but harmful for colon cancer.

9. Analysis

Analysis of phosphatidylethanolamine and related lipids is straightforward, as they are readily isolated by thin-layer or high-performance liquid chromatography methods for further analysis, for example by modern mass spectrometric methods. Analysis of phosphatidylethanolamine adducts is much more challenging.