AMINO COMPOUND-CONTAINING LIPIDS

Amino acid-containing lipids

Acyl carnitines

Dopamine-containing lipids

5-HT containing lipids

Aminoalcohol-containing lipids

A - AMINO ACID-CONTAINING LIPIDS (Simple Lipoamino acids and lipopeptides)

Some bacteria species are known to contain in their inner and outer membranes amphipatic lipids based on one or two amino acids linked to a fatty acid through an amide bond and sometimes another through an ester bond. Thus, they have a structural similarity to ceramides or lipopolysaccharides.
Some of these conjugates of lipids and amino acids were also found in mammalian tissues.

New forms of these compounds are frequently described in marine organisms. Among them, unusual structures formed of fatty acids and amino acid derivatives (bromotyrosine) are isolated from sponges, they were named mololipids.

Surface-active molecules containing several amino acids and one or two fatty acids are produced by bacteria, but also in mammals, and are known as lipopeptides. 

Several types of lipoamino acids have been identified :

1 - Lipids containing serine

The best known is serratamic acid or hydroxydecanoyl serine.

This compound was detected in Serratia species of bacteria (Cartwright NJ, Biochem J 1957, 67, 663).
It was suggested that this compound may contribute to the virulence of the bacteria (inhibition of phagocytosis, hemolytic activity). A hydroxydodecanoyl derivative is also known.

Another form was described in an opportunistic pathogen Flavobacterium (Kawai Y et al., Eur J Biochem 1988, 171, 73). In this compound (named "flavolipin"), serine is amide-linked to 3-hydroxyisoheptadecanoic acid which is esterified to isopentadecanoic acid. It has high hemaglutinating activity.

N-arachidonoyl serine was isolated from bovine brain (Milman G et al., PNAS 2006, 103, 2428). This compound binds very weakly to cannabinoid and vanilloid receptors but it was identified as an endocannabinoid-like constituent with vasodilatory properties.

A N-acyl homoserine lactone (with an oxohexanoyl acyl group) was first described in Photobacterium fischeri and identified as an autoinducer of luciferase synthesis when excreted into the medium (Eberhard A et al., Biochemistry 1981, 20, 2444).
Later, a family of N-acyl homoserine lactone (AHLs), differing in either the presence or absence of an acyl chain C-3 substituent (oxo or hydroxy) or the length of the N-acyl side chain, has been described. One of the best studied among these molecules is N-(3-oxododecanoyl) homoserine lactone produced by Pseudomonas aeruginosa (Pearson JP et al., Proc Natl Acad Sci USA 1994, 91, 197) :  

This molecule was shown to activate several virulence genes and to be a component of the "quorum-sensing" system involved in gram-negative bacteria for controlling gene expression in response to population density. It was also shown to have an important immune modulatory activity (Telford G et al., Infect Immun 1998, 66, 36 and Chhabra SR et al., J Med Chem 2003, 46, 97).
On another hand, there is now a considerable interest in the ability of specific enzymes to inactivate these molecules, thus decreasing the virulence of a number of pathogenic bacteria (Roche DM et al., Microbiology 2004, 150, 2023).  

2 - Lipids containing ornithine

The most common structure of lipoamino acids shown below has a 3-hydroxy amide-linked acid which is itself esterified by a second fatty acid, generating an acyloxyacyl function. That structure is reminiscent of that found in lipid A of Gram-negative bacteria. They are found among others in several species of Bordetella, Pseudomonas, Flavobacterium and Achromobacter.

This ornithine-containing lipid was recently found to be a strong stimulant for macrophages and, taking in account its biological activities, is expected to be used as a potent and nontoxic adjuvant in immunology studies (Kawai Y et al., FEMS Immunol Med Microbiol 1999, 23, 67). Similar molecules have been shown to protect animals from lethal endotoxemia (Kawai Y et al., Infect Immun 1991, 59, 2560).

Other forms were found in various Flavobacterium, opportunistic pathogens which cause infantile meningits (Kawai Y et al., Eur J Biochem 1988, 171, 73-80). In these compounds, ornithine is amide-linked to a hydroxylated fatty acid (3-hydroxyisoheptadecanoic acid) which is itself esterified to isopentadecanoic acid or 2-hydroxyisopentadecanoic acid. They were shown to exhibit high hemagglutinating activity.

Another form of ornithine-containing lipid is found in photosynthetic bacteria and contains amide and ester linkages between the amino acid and the fatty acids (one being normal, the other being hydrolylated).

One form synthesized by several bacteria species (Bordetella, Pseudomonas, Achromobavter) was found to be a strong stimulant for macrophages (Kawai Y et al., FEMS Immunol Med Microbiol 1999, 23, 67).

More complex forms containing ornithine linked to a fatty acid and a long-chain fatty alcohol were also described.
An interesting feature of these molecules appears to be in the membranes functionally interchangeable with phosphatidylethanolamine. Thus, in a culture of  Pseudomonas, phosphorus deficiency increases the acylornithine production to the exclusion of phospholipids (Minnikin DE et al., FEBS Lett 1974, 43, 257).

The bacterial Cerilipin described in Gluconobacter cerinus has an ornithine-containing lipid core but with an additional amide-linked taurine (Tahara Y. et al., Agric Biol Chem 1976, 40, 243). It was shown to exhibit excellent biosurfactant activity.

3 - Lipids containing tyrosine

A new series of lipophilic bromotyrosine derivatives was isolated as a white waxy extract from a Verongid sponge from Hawaii. Named mololipids, the extract contained a group of molecules consisting of bis-amides between a bromotyrosine derivative (formerly known as moloka'iamine) and long chain fatty acids ranging in size from C14  to C20 (Ross SA et al. J Nat Prod 2000, 63, 501). These compounds were shown to display anti-HIV activity.

Pure samples of mololipids with defined fatty acid composition have been prepared by total synthesis for pharmacological investigations (Schoenfeld RC et al., Bioorg Med Chem Lett 2000, 10, 2679).

4 - Lipids containing glycine

A glycine-containing lipoamino acid was described for the first time in a gliding bacteria, Cytophaga johnsonae. 

Its structure showed an iso-3-hydroxy heptadecanoic acid, amide linked to glycine and esterified to isopentadecanoic acid, it formed about 6% of the total bacteria lipids (Kawazoe R et al., J Bacteriol 1991, 173, 5470).

 Another glycine-containing lipoamino acid, a conjugate of arachidonic acid and glycine (N-arachidonoylglycine), was shown to be present in bovine and rat brain as well as in other tissues, Its biosynthesis and degradation being further observed to occur in the rat brain (Huang SM et al., J Biol Chem 2001, 276, 42639). Consistent with its high levels in skin and neural tissues, N-arachidonoylglycine is capable of suppressing pain via a peripheral action, suggesting that it may serve endogenously to regulate pain.

This lipoamino acid was first synthesized (Sheskin T et al., J Med Chem 1997, 40, 659) as a structural analog of the endogenous cannabinoid anandamide and later was shown to cause hot plate analgesia in mice (Burstein SH et al., Prost Lipid Mediat 1999, 61, 29). Thus, one possible physiological function of N-arachidonoylglycine is pain suppression (Huang SM et al., J Biol Chem 2001, 276, 42639). 
It was reported that cyclooxygenase-2 selectively metabolized N-arachidonoylglycine to PGH(2) glycine and hydroxyeicosatetraenoic glycine (Prusakiewicz JJ et al., Biochem Biophys Res Com 2002, 296, 612). These results suggest a possible role for cyclooxygenase-2 in the regulation of N-arachidonoylglycine levels and the formation of a novel class of eicosanoids.

Two other arachidonyl amino acids were shown to be natural constituents in mammalian brain: N-arachidonyl g-aminobutyric acid and N-arachidonoylalanine (Huang SM et al., J Biol Chem 2001, 276, 42639).

5 - Lipids containing Leucine

Lipstatin, a new and very potent inhibitor of pancreatic lipase (the key enzyme of intestinal fat digestion) was isolated from Streptomyces toxytricini  (Weibel EK et al., J Antibiot 1987, 1081). Lipstatin contains a beta-lactone ring, probably accounting for the irreversible lipase inhibition, which carries two aliphatic residues with chain lengths of 6 and 13 carbon atoms. One of the side chains contains two isolated double bonds and a hydroxy group esterified to N-formyl-leucine.

Several research were focused on the biosynthesis of that potent molecule (Eisenreich W et al., J Biol Chem 1997, 272, 867 and J Med Chem 2003, 46, 4209). Data indicate that the lipstatin molecule can be assembled by Claisen condensation of octanoyl-CoA with 3-hydroxy-tetradecanoyl-CoA formed from linoleic acid.
The tetrahydro derivative of lipstatin, Orlistat (Xenical), is used for the treatment of severe obesity in forming a covalent adduct with a serine moiety of human pancreatic lipase by transesterification.

6 - Lipides containing proline

In search of surfactants, it was shown that among several lipoaminoacids synthesized by coupling stearic acid with the a-amino group of an amino acid, N-stearoyl proline had the most efficient surface-active properties and is highly biodegradable. It has antimicrobial activity against several Gram-positive (Staphylococcus aureus, Micrococcus luteus, and Bacillus cerceus), and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria, and against the yeast Candida albicans. Thus, it has a potential utility as biostatic additive in commercial products (Sivasamy A et al., JAOCS 2001, 78, 897). 

7 - Lipopeptides

A large number of cyclic lipopeptides including decapeptide antibiotics (gramicidins) and lipopeptide antibiotics (polymyxins), produced by Bacillus brevis (Marahiel M et al., Eur J Biochem 1977, 99, 49), respectively, possess remarkable surface-active properties.
As an example, the cyclic lipopeptide surfactin, produced by B. subtilis, is one of the most powerful biosurfactant known today. It lowers the surface tension from 72 to 27.9 mN/m at concentrations as low as 0.005% (Arima K et al., Biochem Biophys Res Commun 1968, 31, 488). Surfactin is composed of a seven-amino-acid ring structure coupled to one molecule of 3-hydroxy-13-methyl-tetradecanoic acid.

A similar biosurfactant is produced by Bacillus licheniformis, with a lipophilic fatty acid moiety (4 different chain lengths) joined via a lactone linkage to a hydrophilic peptide ring structure (Jenny K et al., J Appl Microbiol Biotechnol 1991, 36, 5).

Cyclic lipopeptides are mainly described in Bacillus species where several groups are found : the iturin group including iturin, mycosubtilisin, bacillomycin, the octapeptide group and the polymyxin group (polymyxins). 
Polymixins has a peptide ring attached to a peptide chain ending with a branched fatty acid (6-methyloctanoic acid in polymyxin B). Polymyxin B binds to the lipid A portion of lipopolysaccharide and also to phospholipids, thus it is an antibiotic used to treat urinary, meninges and bloodstream infections caused by Gram-negative bacteria (the cell membrane is not exposed in this group).

Polymyxins

Biosurfactants in these groups have been noted for their bactericidal or fungicidal activity, their surfactant properties being of only minor interest.
A lipopeptide antibiotic, tauramamide, was isolated and characterized from culture of Brevibacillus laterosporus (Desjardine K et al., J Nat Prod 2007, 70, 1850). Its structure was described to be:

7-methyloctanoyl-Tyr-Ser-Leu-Trp-Arg

This lipopeptide has shown potent and selective inhibition of pathogenic  sp.

B - ACYL CARNITINES

These molecules are formed in the cytoplasm by transfer of the acyl moiety of acyl-CoA to carnitine (carnitine palmitoyltransferase in the mitochondrial membrane). Acyl carnitines are then transferred into mitochondria and hydrolyzed to release acyl-CoA into the matrix, where they are subject to beta oxidation.

C - DOPAMINE-CONTAINING LIPIDS

If the existence of acylated dopamine was previously hypothesized, N-arachidonoyldopamine was the first compound in this group of lipid mediators to be determined in mammalian nervous tissue (Huang SM et al., PNAS 2002, 99, 8400).

This lipid appears similar to capsaicin (the pungent ingredient of "hot" chili peppers) not only with respect to its chemical structure (a vanillylamine moiety linked to an unsaturated acyl chain via an amide bond) but also to its potency at vanilloid receptors of type 1 present in peripheral nerves and in several brain areas (striatum, hippocampus, cerebellum). It was also shown to activate cannabinoid CB1 receptors (Bisogno T et al., Biochem J 2000, 351, 817). Furthermore, it has been shown that this lipid has a potent anti-HIV activity (Sancho R et al., J Immunol 2005, 175, 3990). 
Further analysis of bovine brain indicated the existence of another potent capsaicin-like lipid, N-oleoyldopamine, which is similar to N-arachidonoyldopamine in its chemical structure and activity on vanilloid receptors (Chu CJ et al., J Biol Chem 2003, 278, 13633).

D - 5-HT CONTAINING LIPIDS

Carboxylic acid-5-hydroxytryptamides were described as the main constituents of the waxy layer at the surface of green coffee beans (0.5-2.4 g/kg).  They are formed by amides of serotonine (5-hydroxytryptamine, 5-HT) and fatty acids with different chain lengths. 

The first amides to be described had fatty acids with 20, 22, and 24 carbon atoms (Harms U et al., Z Lebensm Unters Forsch 1968, 138, 75). Later, several other species were described with a fatty acid from C14 to C22. Technological treatments were initiated to removed that waxy layer, thus preventing  potential irritating effects on certain coffee drinkers.

E - AMINOALCOHOL-CONTAINING LIPIDS

The aminoalcohol may be linked to the fatty acid through :

- an amide bond
or
- an ester bond

1 - Aminoalcohol linked as an amide to the fatty acid

N-Acylethanolamines (NAEs) were first reported as constituents of soy lecithin and peanut meal and as a anti-inflammatory agent (Kuehl FF et al., J Am Chem Soc 1957, 79, 5577).
Long-chain N-acylethanolamines are ubiquitous trace constituents of animal and human cells, tissues and body fluids, Their cellular levels appear to be tightly regulated and they accumulate as the result of injury (Berdyshev EV et al., Biochem J 2000, 346, 369). They are found at only very low concentrations. Thus, in rat plasma, the concentrations of palmityl- and oleylethanolamide and arachidonoyl were found to be 17, 8 and 5 pmol/ml, respectively. Somewhat higher concentrations are reported in brain and other tissues. Similar lipids have also been found in fish, molluscs, slime moulds, and certain bacteria.
Saturated and monoenoic N-acylethanolamines may also function as intracellular messengers by activating specific kinases and interacting with the signalling pathways mediated by ceramides. As an example,  oleoylethanolamide may be a lipolysis stimulator (via activation of the nuclear peroxisome proliferator-activated receptor) and an endogenous regulator of food intake, and could have some potential as an anti-obesity drug (Guzman M et al., J Biol Chem 2004,279, 27849).

N-arachidonoyl ethanolamine (named also anandamide) is the first endogenous ligand to be reported at the end of 1992 (Devane WA et al., Science 1992, 258, 1946). It acts as a partial CB1 agonist  and only as a weak CB2 agonist. and is formed in the brain from a molecule of N-acyl phosphatidylethanolamine (Bisogno T et al., Pharmacol Biochem Behav 2005, 81, 224). Anandamide is now considered as the main ligand of the cannabinoid receptor (see the ethanolamine glycerolipid chapter)

Additional endogenous lipids discovered that show affinity for cannabinoid receptors include dihomo-g-linolenoyl ethanolamide and docosatetraenoyl ethanolamide (Hanus L et al., J Med Chem 1993, 36, 3032).
It has been shown that C18-N-acylethanolamine (based on 18:1, 18:2 and 18:3) are present in ganglia and vascular tissue and, while poor ligands at cannabinoid receptors, they activate the vanilloid 1 receptor (Movahed P et al., J Biol Chem 2005, 280, 38496).   
N-Acylethanolamines are also minor but ubiquitous components of plant tissues, but they are especially abundant in some desiccated seeds (Venables BJ et al., Phytochemistry 2005, 66, 1913). The fatty acids have up to three double bonds and 12 to 18 carbon atoms, linoleic acid being the predominant species (Chapman KD et al., Plant Physiol 1998, 116, 1163). Their total contents in seeds range from about 0.2 to 40 mg/g fresh weight with no relationship to phylogeny. Some species (Medicago truncatula, Glycine max) could be good candidates for natural sources of these compounds. It has been demonstrated that N-acylethanolamines may be metabolized by lipoxygenase into oxylipins which could play a role in seed germination (Shrestha R et al., Plant Physiol 2002, 130, 391). It appears that these compounds have a variety of biological functions in plants such as defense signaling, but research is still at a relatively early stage. 

2 - Aminoalcohol linked as an ester to the fatty acid

O-arachidonoyl ethanolamine has its ethanolamine moiety linked as an ester instead of an amide to arachidonic acid, as in anandamide. It is known also as virodhamine.

It was isolated from human and rat brain and it was shown to have mixed agonist/antagonist activity at the cannabinoid receptor (CB1) (Porter AC et al., J Pharmacol Exp Ther 2002, 301, 1020). In rat brain the concentration of that compound was found to be similar to that of anandamide. Despite these properties, this lipid is probably not the native endogenous agonist of the CB1 receptor.