Phosphonolipids

Phosphonolipids consist of 2-aminoethylphosphonic acid (ciliatine) residues, i.e. with a phosphorus-carbon bond, attached to a lipid backbone, which can be either a ceramide or diacylglycerol. Lipid-bound aminoethylphosphonic acid was first detected in the single-celled microorganism Tetrahymena pyriformis, and then in protozoa. However, it appears that the first of the phosphonolipids to be definitively characterized was ceramide 2-aminoethylphosphonate from sea anemones. Subsequently, this lipid was detected in a wide variety of marine invertebrates, especially ciliated protozoa, coelenerates, gastropods and bivalves, sometimes accompanied by small amounts of N-methylaminoethyl, N,N-dimethylaminoethyl and choline analogues, and it appears to be the most widespread phosphonolipid in nature. Phosphonolipids have also been observed in plants, bacteria and several vertebrates, including humans, although with the last they almost certainly originate from dietary sources, and are not synthesised de novo.

Formulae of ceramide 2-aminoethylphosphonate and 6-O-(aminoethylphosphono)galactosyl ceramide

Formula of phosphonolipid containing 1-hydroxy-2-aminoethaneMore recently, phosphonolipids with 1-hydroxy-2-aminoethane attached to the phosphorus moiety have been found in some bacterial species. For example, Bacteriovorax stolpii strain UKi2, a facultative predator-parasite of larger gram-negative bacteria, synthesises sphingophosphonolipids with this novel head group. The long-chain base components in this instance are mainly C17 iso-methyl-branched phytosphingosine and iso-branched dihydrosphingosine, while the N-linked fatty acids are iso-methyl branched usually with a 2-hydroxyl group. This organism also contains sphingolipids with a 2-amino-3-phosphonopropanate head group.

In addition, further phosphonoceramides and phosphonoglycosphingolipids, such as 6-O-(aminoethylphosphono)galactosyl ceramide and its N-methylethane analogue, related oligoglycosphingolipids, and a triphosphonoglycosphingolipid have been found in marine invertebrates. Similar glucose-containing phosphonolipids and others with the aminoethylphosphonate group on position 4 of the glucose unit have also been characterized. In marine invertebrates, in addition to sphingosine, sphingadiene and other dihydroxy bases, the ceramide component can contain appreciable amounts of trihydroxy bases, together with both 2-hydroxy and nonhydroxy fatty acids, depending on the species and tissue. For example, in the phosphonolipids of the marine invertebrate Anthopleura elegantissima, palmitic acid comprises 80% of the total, while 2-hydroxy fatty acids only are found in these lipids of Pinctada martensii. Similarly, insects, such as Manduca sexta, contain phosphonoethanolamine linked to complex oligosaccharides (arthrosides).

A phosphono analogue of phosphatidylethanolamine, i.e. 1,2-diacyl-sn-glycerol-3-(2'-aminoethyl)phosphonate (phosphonylethanolamine) is the main phosphonolipid in Tetrahymena pyriformis. It has also been found in several species of protozoa, and at low levels in some plant species, various bovine tissues and even in human aorta. It can exist in diacyl, alkylacyl and alkenylacyl forms. In addition, related N-methyl- and N,N-dimethyl phosphonolipids have been detected, together with phosphono analogues of phosphatidylcholine, phosphatidylglycerol and phosphatidylserine.

Formula of 1,2-diacyl-sn-glycerol-3-(2'-aminoethyl)phosphonate

In plants (Kenaf and cotton seeds), the main fatty acids in the phosphonylethanolamine are saturated and monoenoic, largely 16:0 and 18:1. In the phosphonylethanolamine of Tetrahymena pyriformis, which exists only in the diacyl form, C16 and C18 fatty acids with one to three double bonds predominate with the unsaturated fatty acids concentrated in position sn-2, as listed in Table 1.

Table 1. Positional distributions of fatty acids (mol %) in the phosphonylethanolamine of T. pyriformis grown at 39.5°C
 Fatty acid
 14:016:09-16:117:09-18:16,11-18:29,12-18:26,9,12-18:3
 sn-1 26 39 8 3 1 1 1 4
 sn-2 2 3 12 5 7 8 12 45
 
Watanabe, T., Fukushima, H. and Nozawa, Y. Biochim. Biophys. Acta, 620, 133-140 (1980).

 

An unusual biosurfactant, 2-acyloxyethylphosphonate, has been isolated from waterblooms of the cyanobacterium Aphanizomenon flos-aquae. Palmitic acid comprised 80% of the fatty acid components of the lipid, and it was accompanied by some trienoic acids.

Formula of 2-acyloxyethylphosphonate

More than one pathway for the biosynthesis of 2-aminoethylphosphonate is known, but the simplest requires three enzymes and utilizes phosphoenolpyruvate as the key precursor. Little appears to be known of how it is then incorporated into lipids, although in rat hepatocytes, it is incorporated into phosphonolipid by a similar pathway to that for phosphatidylethanolamine biosynthesis.

While there has been some speculation, little is known of the function of phosphonolipids. They are presumed to be membrane constituents, and they are known to be resistant towards the action of phospholipases. For example, in Tetrahymena, phosphatidylethanolamine turns over much more rapidly than the phosphono analogue. Therefore, they may have a role in protecting organisms from attack by enzymes or from harsh environmental conditions.

Analysis of phosphonolipids presents no particular problems, except when they co-exist with the conventional phosphate forms of the lipids, which have very similar chromatographic properties. However, methods of isolation have been devised even in these difficult circumstances (see the references cited below). The carbon-phosphorus bond is not hydrolysed by such harsh chemical treatments as boiling in strong acid or base. 31P-Nuclear magnetic resonance spectroscopy is invaluable for detecting the presence of phosphonolipids in lipid extracts, while electrospray-ionization tandem mass spectrometry now appears to hold particular promise for structural analyses.

 

Recommended Reading

  • Jayasimhulu, K., Hunt, S.M., Kaneshiro, E.S., Watanabe, Y. and Giner, J.L. Detection and identification of Bacteriovorax stolpii UKi2 sphingophosphonolipid molecular species. J. Am. Soc. Mass Spectrom., 18, 394-403 (2007) (DOI: 10.1016/j.jasms.2006.10.014).
  • Metcalf, W.W. and van der Donk, W.A. Biosynthesis of phosphonic and phosphinic acid natural products. Annu. Rev. Biochem., 78, 65–94 (2009) (DOI: 10.1146/annurev.biochem.78.091707.100215).
  • Moschidis, M.C. Phosphonolipids. Prog. Lipid Res., 23, 223-246 (1985) (DOI: 10.1016/0163-7827(84)90012-2).
  • Mukhamedova, K.S. and Glushenkova, A.I. Natural phosphonolipids. Chem. Nat. Compounds, 36, 329-341 (2000).

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Updated December 12, 2011