Simple Lipoamino Acids
While simple fatty acyl amides such as anandamide have been attracting enormous interest, it has become apparent that many different simple fatty acyl-amino acids or ‘lipoamino acids’ are also present in animal tissues. Lipoamino acids and lipopeptides are also well-known constituents of bacterial lipids. As these are different in structure and function, they are discussed separately in this document. In comparison to most other lipids, the biological functions of lipoamino acids have barely been explored. Fatty acid conjugates with short peptides, such as glutathione, are discussed here also, but proteolipids in which fatty acids are conjugated to proteins have their own web page. Similarly, the cysteinyl leukotrienes could be considered to be lipoamino acids and these also are discussed elsewhere on this site. Although not discussed here, synthetic lipoamino acids are being used increasingly in pharmaceutical applications, for example in lipid emulsions for peptide delivery. Complex lipoamino acids are discussed elsewhere on this site.
1. Simple Lipoamino Acids from Animal Cells
N-Acetyl derivatives of amino acids are minor but ubiquitous components of animal tissues, and may simply be a means of excreting or detoxifying excesses of particular amino acids acids or fatty acid metabolites. In addition, more than 50 different long-chain fatty acid conjugates with amino acids have been found in rat brain, some of which have important biological properties while those of most have yet to be determined.
N-Acylglycine derivatives of short-chain fatty acids (C2 to C12) have long been recognized as minor constituents of urine and blood, and their compositions in the former especially may have some relation to metabolic diseases. They are formed in the liver as a detoxification mechanism for removal of excess acyl-coenzyme A esters. It is possible that they function as intercellular messengers via cell surface receptors.
N-Palmitoylglycine is produced in most tissues, but especially skin and spinal cord, and it has a role in sensory neuronal signalling. N-Oleoylglycine was first detected in mouse neuroblastoma cells, and it is now known to have a regulatory effect on body temperature and locomotion. As in the biosynthesis of oleamide, cytochrome C catalyses the formation of oleoylglycine from oleoyl-CoA and glycine, in the presence of hydrogen peroxide. However, other biosynthetic routes are possible.
N-Arachidonoylglycine is present in bovine and rat brain as well as other tissues at low levels. It is synthesised from anandamide by two pathways in mammalian cells. One route involves oxidation of anandamide via an alcohol dehydrogenase, proceeding via an aldehyde intermediate.
In the second pathway, hydrolysis of anandamide by the fatty acid amide hydrolase is followed by conjugation of the released arachidonic acid with glycine by the pathway proposed for oleamide and N-oleoylglycine.
N-Arachidonylglycine has been shown to suppress inflammatory pain. It does not bind to the CB1 receptor for endocannabinoids, but it is a ligand for other receptors and may have a role in regulating tissue levels of anandamide by inhibiting the fatty acid amide hydrolase. It is also a substrate for cyclooxygenase-2, producing glycine conjugates of prostaglandins. N-Arachidonylglycine may also divert the biosynthetic pathway from the pro-inflammatory PGE2 towards the anti-inflammatory J prostaglandins
It has been suggested that simple lipoamino acids or fatty acid-amino acid conjugates of which N-arachidonylglycine is the considered to be the prototype should be termed 'elmiric' acids, although the name does not appear to have caught on.
N-Arachidonylserine has been detected at trace levels in bovine brain. It does not bind strongly to cannabinoid receptors, but it does have a potent vasodilatory effect on rat arteries in vitro amongst other biological effects. At least three other arachidonyl amino acids, i.e. of γ-aminobutyric acid, alanine and asparagine, occur naturally and also inhibit pain, suggesting that such biomolecules may be integral to pain regulation and perhaps have other functions in mammals. They can be converted to primary fatty amides in vitro, and it is possible that this also occurs in vivo.
The N-arachidonoyl amino acid and vanilloid derivatives are minimally oxidized by cyclooxygenase-2 (COX-2), but are good substrates for the 12S- and 15S-lipoxygenases. It is not yet clear whether this leads to inactivation of these lipids or rather converts them to new bioactive compounds. While the fatty acid amide hydrolase will cleave the N-acyltaurines and N-arachidonoylglycine to the corresponding fatty acid and amino acid, the other N-acyl amino acids are not affected and their catabolic fate is uncertain.
N-(17-Hydroxy)-linolenoyl-L-glutamine (volicitin), N-(17-hydroxy)-linoleoyl-L-glutamic acid and related lipoamino acids have been found in insect larvae. Their presence in oral secretions elicits a defense response in plants.
A range of fatty N-acyltaurines have been isolated from both the central nervous system and peripheral tissues (see also our web page dealing with other taurolipids). In the former, the fatty acyl groups are largely long-chain saturated, but in liver and kidney, arachidonoyl and docosahexaenoyl species predominate.
In kidney, N-acyltaurines have been shown to activate receptors that control calcium channels. Like the other biologically active amides in animals, the levels of these metabolites are controlled by the activity of the fatty acid amide hydrolase. Arachidonoyltaurine has been shown to be an excellent substrate for lipoxygenases, but the functions of the resulting hydroxyeicosatetraenoyltaurines have yet to be determined. N-Arachidonoyl and N-oleoyl taurine have been shown to induce a significant reduction in vitro in a cancer cell line. An unusual N-acyltaurine, linked to a dihydroxy acid, has been found in a sea urchin.
Glutathione is the tripeptide, γ-L-glutamyl-L-cysteinylglycine, and is most abundant thiol-containing small molecule (3 to 4 mM) in animal cells, where it is located mainly in the cytosol. It has a major defensive role in combating oxidative stress, for example by undergoing oxidation to glutathione disulfide while reducing lipid (and other) hydroperoxides to hydroxides. It also reacts with lipid oxidation products, including peroxy-fatty acids and unsaturated aldehydes, to produce lipid-glutathione adducts, facilitated by the action of glutathione S-transferases. For example, bioactive eicosanoids and α,β-unsaturated aldehydes (e.g. trans,trans-2,4-decadienal) and malondialdehyde can be deactivated or detoxified by conversion to inactive glutathione conjugates.
In contrast, the eicosanoid 5-hydroperoxyeicosapentaenoic acid (5-HPETE) is converted to the glutathione adduct as an intermediate in the biosynthesis of the cysteinyl leukotrienes, as described elsewhere on this web site.
4. Simple Lipoamino Acids from Bacteria
A variety of lipoamino acids have been isolated from bacterial species, of which the best know is probably the zwitterionic N-acyl-ornithine derivative illustrated, which is widely distributed among prokaryotes, but especially gram-negative bacteria and other eubacteria, where it is located predominantly on the outer membrane. It is normally a minor component, but can assume major proportions when phosphate is limiting or in response to stress in some species. Ornithine lipids contain a nonhydroxy fatty acid with an estolide linkage to a 3-hydroxy acid (often but not always C16 or C18) and thence via an amide bond to the α-amino group of ornithine. It may be relevant that such fatty acid linkages are also seen in the bacterial endotoxin lipid A.
Biosynthesis of ornithine lipids occurs in two steps via acyl-ACP-dependent acylation of ornithine by two different acyltransferases. In some bacterial species, the ester-linked fatty acid has a hydroxyl group in position 2 and analogous lipids in which the ornithine moiety is hydroxylated are known, with both modifications inserted post synthesis of the basic lipid.
Rhodobacter sphaeroides contains both ornithine and related glutamine lipids, while cerilipin characterized from Gluconobacter cerinus has an ornithine-containing lipid core and an additional amide-linked taurine moiety. Structurally related lipids with lysine, serine, glycine, glutamine and taurine residues occur in microorganisms such as the gliding bacterium Cytophaga johnsonae and the gram-negative marine species Cyclobacterium marinus. They include one containing a glycine-serine dipeptide linked to branched chain acids and termed 'flavolipin'. Some of these lipoamino acids have interesting and potentially useful pharmacological properties.
Simple N-acyl derivatives (without a secondary fatty acid constituent) also occur in bacteria, including N-acyl leucine (or isoleucine) derivatives in Deleya marina, N-acyl D-asparagine in Bacillus pumilus and N-acylserine in Serratia sp. On the other hand, other lipoamino acid forms of increasing complexity have been characterized, including molecules with a long-chain alcohol moiety linked to the carboxyl group of the amino acid (such as siolipin A from Streptomyces species). The more complex microbial lipopeptides have their own web page.
It appears that such lipoamino acids have a variety of different functions in bacteria depending upon species, and it does not seem possible to generalise. For example, they have been implicated in temperature and stress tolerance in some species, while in others they may be recognised by plant defense systems or be essential for symbiotic relationships. In gram-negative bacteria, they may stabilise the outer membrane by counteracting the negative charge of the lipopolysaccharides.
N-Acyl-L-homoserine lactones are produced by a number of gram-negative bacteria. They are used for a form of intercellular signalling termed 'quorum sensing’, which controls gene expression in response to population density. They have many different functions some of which are associated with their pathogenic properties.
The fatty acid components can vary in chain length from C4 to C18, sometimes with one or two double bonds (in position 2 and/or more central positions), with hydroxyl or keto groups in position 3, and/or with methyl branches. The photosynthetic bacterium Rhodopseudomonas palustris contains p-coumaroyl-homoserine lactone.
The main problems in the analysis of simple lipoamino acids relate to the low levels at which they occur naturally. There is a concern that artefactually high results can be obtained because of the physiological effects of sampling methods. Until recently, high-performance liquid chromatography with fluorescent detection or gas chromatography-mass spectrometry with selected ion monitoring were most used for the purpose. Liquid chromatography allied to tandem mass spectrometry with electrospray ionization would probably be the preferred method now.
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Updated April 28, 2014