Phosphonolipids

Phosphono derivatives : After the discovery of 2-aminoethylphosphonic acid in lipid extracts of the sea anemone (Kittredge JS et al., Biochemistry 1962, 1, 624), unusual phosphatidylethanolamine analogues containing a carbon-phosphorus bond instead of the classical carbon-oxygen-phosphorus bond have been described in marine invertebrates and protozoa (Tetrahymena pyriformis). They were first reported in 1966 in Tetrahymena as glycerophosphonolipids (Liang CR et al., Biochim Biophys Acta 1966, 125, 548) but, later, they were identified in that species as derivative of chimyl alcohol (Thomson GA, Biochemistry 1967, 6, 2015) : an alcohol is linked with an ether bond in sn-1 of the glycerol molecule. They were also identified in fish, some mammals, egg yolk, insects, several invertebrates and even in some plants (Mukhamedova KS et al., Chem Nat Compounds 2000, 36, 329). Thus, egg yolk contains about 1% of phosphonolipids, mainly all as phosphatidylethanolamine.
These phosphonolipids, often termed phosphonylethanolamine, are extremely resistant to acid or enzymatic hydrolysis. They were proposed as a possible artificial pulmonary surfactant.

The alk-1′-enyl, acyl derivative (ethanolamine plasmalogen) occurs widely in nature, mainly present in high concentration in the white matter (myelin) of the nervous system, in the heart and the kidney. It is practically absent in fungi, plants and bacteria (except in obligate anaerobes).

The fatty acid acylated at the 2-position of glycerol is most frequently polyunsaturated (arachidonic or docosahexaenoic acid). Ethanolamine plasmalogens constitute more than 80 mol% of the ethanolamine phospholipid pool in non-neuronal brain membranes and more than 60 mol% in neurons and synaptosomes (Han X et al., J Neurochem 2001, 77, 1168). Those found in white matter contain predominantly 18:1, 20:1, and 22:4 fatty acids at the sn-2 position, whereas in gray matter, 22:6, 20:4, and 22:4 are found in the highest concentrations (Horrocks LA et al., In Phospholipids. J. N. Hawthorne and G. B. Ansell, editors. Elsevier, Amsterdam, 1982, 51–93). 

The function of ethanolamine plasmalogen remains obscure, hypotheses concerning their role in arachidonic acid reservoir and protection of other lipids against oxidation (through their vinyl group) were suggested (Leray C et al., Lipids 2002, 37, 285). The antioxidative properties of plasmalogens are based on two electron-free oxidants reacting with the vinyl ether bond, leading to the production of stable products. Thus, the reaction of plasmalogen with HOCl results in the formation of a lysophospholipid and a a-chlorofatty aldehyde. That a-chlorofatty aldehyde can be further oxidized to a a–chlorofatty acid or reduced to a a-chlorofatty alcohol (Wang WY et al., Anal Biochem 2013, 443, 148). Myeloperoxidase, found in neutrophils, monocytes, and macrophages, converts during activation of these cells hydrogen peroxide into hypochlorous acid (HOCl). HOCl contributes to the antimicrobial and cytotoxic properties of these leukocytes, but may be also involved in the pathogenesis of several diseases.

The alkyl-, acyl-derivative (1-alkyl-, 2-acyl-sn-glycero-3-phosphorylethanolamine) is present in small amount in all phospholipid extracts.

It was first isolated from bovine erythrocytes but is present also in reasonably amount in human lens (Deeley JM et al., Anal Chem 2009, 81, 1920), bone marrow, platelets, certain protozoa and molluscs. Ethanolamine plasmalogens are about 5 times and 4 times more abundant than the diacyl forms in oyster (Crassostrea gigas) and in mussel (Mytilus edulis), respectively (Kraffe E et al., Lipids 2004, 39, 59). These plasmalogens were found to be enriched with non-methylene-interrupted fatty acids (mainly 7,17-22:2).

N-methyl and N,N-dimethyl phosphatidylethanolamine

These groups have either N-methylethanolamine or N,N-dimethylethanolamine substituted for ethanolamine in their structure. Bremer (Biochim Biophys Acta 1959, 35, 287) proposed a possible role of these compounds in interconversions from phosphatidylethanolamine to phosphatidylcholine in rat liver. This was confirmed later and extensively studied in nervous tissues.
N-Methyl phosphatidylethanolamine has been detected in a Gram-positive bacteria Clostridium acetobutylicum (Lepage C et al., J Gen Microbiol 1987, 133, 103).
These phospholipids may be separated from phosphatidylcholine and phosphatidylethanolamine by mass spectrometry (Bilgin M et al., Biochim Biophys Acta 2011, 1811, 1081).

Phosphatidylethanolamine-hydroxy-alkenals

Phosphatidylethanolamine can be covalently modified in vitro and in cellular systems by hydroxy-alkenals, such as 4-hydroxy-2-nonenal (4-HNE; from n-6 fatty acids), 4-hydroxy-2-hexenal (4-HHE; from n-3 fatty acids), and 4-hydroxy-dodecadienal (4-HDDE; from the 12-lipoxygenase product of arachidonic acid), to form PE-alkenal Michael adducts (Guichardant M et al., Free Rad Biol Med 1998, 25, 1049; Bacot S et al., J Lipid Res 2007, 48, 816). Several other bioactive aldehyde-modified phosphatidylethanolamines have been described (Guo L et al., Free Rad Biol Med 2012, 53, 1226). Below, the adduct formed between PE and 4-HHE is shown.

Several experimental results suggest that these adducts could be used as specific markers of membrane disorders occurring in pathophysiological states with associated oxidative stress and might affect cell function.

A2-PE is formed as a byproduct of the visual cycle in the photoreceptor outer segment, its synthesis occurring when molecules of all-trans– retinal, rather than undergoing reduction to retinol, react with phosphatidylethanolamine (PE) (Liu J et al., J Biol Chem 2000, 275, 29354). That compound was tentatively identified by Adams RG in 1967 as a Schiff base of retinal and phosphatidylethanolamine in bovine rod outer segments (Adams RG et al., J Lipid Res 1967, 8, 245).
It has been proved that exposure to bright light favors A2-PE formation in the retina. Furthermore, a close relationship between lipofuscin accumulation and retinal degeneration and juvenile onset recessive Stargardt disease is now evident. The fluorophore A2E is generated by phosphate hydrolysis of its precursor, A2-PE.

Phosphatidylethanolamine-bisretinoid
R1 and R2 are the fatty acid chains

Isoketal phosphatidylethanolamine adducts

Isoketals (or isolevuglandins) form adduct to phosphatidylethanolamine in vitro (Bernoud-Hubac N et al., Free Radic Biol Med 2004, 37, 1604), whether PE adducts are formed in cells is unknown.

Isoketal phosphotidylethanolamine adduct

It has been demonstrated that these PE adducts dose dependently induced cytotoxicity (LC 50 2.2 mM) in human umbilical vein endothelial cells (Sullivan CB et al., J Lipid Res 2010, 51, 999). Furthermore, isoketal dose-dependently increased PE adduct concentrations to a greater extent than protein adduct. These results indicate that cellular PE is a significant target of isoketals, and that formation of PE adducts may mediate some of the biological effects of isoketal relevant to diseases. Whether these adducts contribute to pathogenesis of diseases associated with inflammation and oxidative stress (atherosclerosis, end stage kidney disease, myocardial infarction, Alzheimer’s Disease, glaucoma) or are simply byproducts of the disease remains an active area of investigation. It has been proposed that isolevuglandin-PE adducts provide a dosimeter of oxidative injury, being potential biomarkers for assessing risk for oxidative stress-stimulated diseases (Li W et al., Free Rad Biol Med 2009, 47, 1539).

That unusual phosphatidylethanolamine derivative was described in a filamentous fungus, Absidia corymbifera, and formed about 6% of total membrane lipids (Batrakov SG et al., Biochim Biophys Acta 2001, 1531, 169).

The main fatty acids (R and R’) are 16:0, 18:0 and 18:2

Another unusual PE derivative was, as the previous one, isolated from Absidia corymbifera, N-ethoxycarbonyl phosphatidylethanolamine and formed about 9% of total membrane lipids (Batrakov SG et al., Biochim Biophys Acta 2001, 1531, 169).

The fatty acid pattern is similar to N-acetyl PE but also with a prominent proportion of 18:3. 

N-acyl phosphatidylethanolamine

The mechanism proposed for the synthesis of this complex phospholipid includes the action of an N-acyl transferase catalyzing an exchange reaction between the sn-1 position of phospholipids (probably phosphatidylcholine) and the primary amine of phosphatidylethanolamine.

Several reports indicate the importance of N-acyl phosphatidylethanolamine as precursors for N-acylethanolamines, which in turn play a physiological role during germination of seeds (Chapman KD et al., Plant Physiol 1999, 120, 1157) and in defense systems in plants (Tripathy S et al., Plant Physiol 1999, 121, 1299). The biosynthesis and functions of N-acyl phosphatidylethanolamine in plants has been reviewed (Coulon D et al., Biochimie 2012, 94, 75).
The biological functions of the N-acylation of phosphatidylethanolamine in mammals have been reviewed (Wellner N et al., Biochim Biophys Acta 2013, 1831, 652).
In addition to be a source of acylethanolamines, N-acyl phosphatidylethanolamine was shown to be secreted in the rat into circulation from the small intestine in response to ingested fat and that systemic administration of the most abundant circulating N-acyl phosphatidylethanolamine (C18 acylated), at physiologic doses, decreases food intake (Gillum MP et al., Cell 2008, 135, 813). Metabolic studies have shown that its effects may be mediated through direct interactions with the central nervous system. These acylated phospholipids may be the base of future therapeutic agents for the treatment of obesity.

N-Acylated ethanolamines were first isolated from preparations of egg yolk, peanut oils, and soybeans due to their anti-inflammatory and anti-allergienic properties (Kuehl FA et al., J Am Chem Soc 1957, 79, 5577). Interestingly, a unique N-acyl phosphatidylethanolamine was proposed in the brain as a source of N-arachidonoyl ethanolamine (named also anandamide, from the Sanskrit "ananda" that means bliss), this metabolite appears to be formed (with a molecule of phosphatidic acid) by phospholipase D action. Further investigations provide evidence for a multistep pathway by the sequential actions of hydrolases and glycerophosphodiesterase (Simon GM et al., J Biol Chem 2008, 283, 9341).

Anandamide

This component (known as endocannabinoid), identified in the porcine brain, behaves like an endogenous cannabinoid (Devane et al., Science 1992, 258, 1946) and is now considered as the main ligand of the cannabinoid receptor discovered in 1988 in the rat brain (Devane WA et al., Mol Pharmacol 1988, 34, 605) and thereafter in various types of cells. It was determined that it has essential properties to be classified as a cannabinoid/anandamide receptor agonist (Felder CC et al., PNAS 1993, 90, 7656). Intense research are made at present in this field (Review in: Hillard CJ et al., J Lipid Res 1997, 38, 2383; Bezuglov VV et al., Biochemistry, Moscow 1998, 63, 22). Numerous roles have been proposed for the involvement of anandamide in fertilization, blastocyst development, implantation, pregnancy maintenance, and parturition. Many observations support the hypothesis that low systemic levels of anandamide are required for normal pregnancy progression (Maccarrone M et al., Mol Hum Reprod 2002, 8, 188). Anandamide was reproducibly quantified in human placenta, cord blood and fetal membranes using solid-phase extraction combined with liquid chromatography tandem mass spectrometry (Marczylo TH et al., Anal Biochem 2010, 400, 155).
Recent insights on intracellular stores and anandamide binding proteins call for a reconsideration of the dogma that endocannabinoids are exclusively synthesized and released "on demand", and suggest that their metabolic control is complemented by intracellular transport and storage in specific reservoirs (adiposomes) (Maccarrone M et al., Tr Biochem Sci 2010, 35, 601).
Vaccenic acid has been shown to attenuate complications observed in the metabolic syndrome, including dyslipidemia, fatty liver disease, and low-grade inflammation. An explanation of that property could be that vaccenic acid lowers intestinal inflammation by increasing the production of anandamide and related N-acylethanolamines (Jacome-Sosa M et al., J Lipid Res 2016, 57, 638).
Two new ethanolamides were isolated from porcine brain and shown to be ligands of the cannabinoid receptors CB1 : dihomo-g-linolenoylethanolamine and docosatetraenoylethanolamine ( Hanus L et al., J Med Chem 1993, 36, 3032; Pertwee R et al., Eur J Pharmacol 1994, 259, 11). It is also possible that a palmitoylethanolamine exists in brain tissue. 
Because neural tissue is rich in n-3 polyunsaturated fatty acids, especially docosahexaenoic acid (DHA, 22:6 n-3), it was suggested that they could be present in acylethanolamides in the brain (Hanus L et al., J Med Chem 1993, 36, 3032). It was later shown that this DHA-derived structural analog of an anandamide has a number of bioactive functions such as neuritogenesis and synaptogenesis (Kim HY et al., Biochem J 2011, 435, 327). These properties led to the proposition of the term "synaptamide" to indicate its synaptogenic properties and chemical nature (Kim HY et al., Prostagl Other Lipid Mediators 2011, 96, 114). The brain synaptamide content is dependent on the dietary DHA intake, suggesting an endogenous mechanism whereby diets containing adequate amounts of omega-3 fatty acids improve synaptogenesis in addition to well-recognized anti-inflammatory effects (Kim HY et al., Prost Leukotr Essential Fatty acids 2013, 88, 121).
It has been clearly determined that endocannabinoids are involved in the regulation of food intake and body energy reserves (control of energy homeostasis) and they participate to pathological mechanisms leading to various metabolic dysfunctions (Review: Kunos G et al., J Biol Chem 2008, 283, 33021). A review on the biological activities, pharmacology, and signal transduction mechanisms for the cannabinoid receptors has been released (Howlett AC, Prost Lipid Med 2002, 68-69, 619). Palmitoylethanolamine and stearoylethanolamine are known for their anti-inflammatory properties (Costa B et al., Pain, 2008, 139, 541; Dalle Carbonare M et al., J Neuroendocrinol 2008, 20, 26). Oleoylethanolamide has been described to have anorexogenic properties (Schwartz GH et al., Cell Metab 2008, 8, 281). Furthermore, all N-acylethanolamines activate PPAR-a (Sun Y et al., Biochem Soc Trans 2006, 34, 1095). Short-term feeding experiments have shown that the levels of N-acylethanolamines could be affected (Artmann A et al., Biochim Biophys Acta 2008, 1781, 200).