Fatty Acids: Hydroxy and Other Oxygenated
The following is a guide to some of the more important naturally occurring fatty acids with hydroxy, epoxy (furanoid), methoxy and oxo substituents, but it is not intended to be a comprehensive list. As with more conventional fatty acids, oxygenated fatty acids exist in nature mainly in lipid-bound form as esters or amides. However, hydroxy fatty acids can also self esterify, i.e. form lactones, which are usually relatively volatile and can serve in communication. Alternatively, they can exist in ester linkage to other fatty acids via their hydroxyl groups, and di-, tri- or polyesters of this type are usually termed estolides. In addition to the examples listed below, estolides are found in ceramides, lipid A and rhamnolipids.
The prostaglandins and other eicosanoids and the plant oxylipins are in essence fatty acids with additional oxygenated functional groups, but these are of such importance metabolically or as signalling molecules that they require their own documents in this website. While the intention here is to identify those oxygenated fatty acids where the physical properties of the oxygen functionality may be of primary importance, there are times when it can be difficult to identify whether or not an oxygenated fatty acid is a lipid mediator. Hydroxy, oxo and epoxy fatty acids may also arise adventitiously from hydroperoxides, and these are not discussed here. Nor are the many oxygenated fatty acids that are produced by synthetic means for industrial applications.
1. Hydroxy Fatty Acids in Animals
2-(R)-Hydroxy fatty acids are conventional lipid components and are important constituents of animal sphingolipids (see our Introduction to sphingolipids, for example). The chain-lengths vary from about C16 to C26, and they are normally saturated, although monoenoic components are also known. Sphingomyelin containing 2‑hydroxylated polyenoic very-long-chain fatty acids has been found in mammalian testes and spermatozoa.
The hydroxyl group is believed to add to the hydrogen-bonding capacity of the sphingolipids, helping to stabilize membrane structures and strengthen the interactions with membrane proteins. It is introduced during the biosynthesis of the ceramide component of sphingolipids by a fatty acid 2‑hydroxylase, which is essential for the normal functioning of the nervous system. In addition, the enzyme also regulates differentiation of various cell types, while mutations to it in humans and mice give rise to a demyelination disorder. It is evident that sphingolipids containing 2‑hydroxy acids have unique functions in membranes that cannot be substituted by non-hydroxy analogues. 2-Hydroxyoleic acid, in particular, has remarkable biological activities in that it suppresses the growth and induces autophagy in certain cancer cells. It therefore has appreciable potential as a non-toxic anticancer drug, and the European Medicines Agency has designated it as an orphan drug for the treatment of glioma. The mechanism for its action is uncertain but may involve effects upon phosphatidylcholine metabolism, but not to activation of sphingomyelin synthesis as originally thought. Intriguingly, it is the unnatural S-enantiomer that produces this effect.
Such fatty acids, together with isomers containing iso/anteiso-methyl branches, have been found in the glycerophospholipids, especially phosphatidylethanolamine of sponges. They also occur in ester linkage in wool wax and Harderian gland secretions. 2,3-Dihydroxy-long-chain fatty acids are occasionally reported from the glycosphingolipids of marine invertebrates, and similar fatty acids have been found in the uropygial gland secretions of storks, together with 2- and 3-mono-hydroxy components.
2-Hydroxy-phytanic acid is formed during alpha-oxidation of phytanic acid (see the web page on branched-chain fatty acids) by liver mitochondria and peroxisomes, but it is detected in tissues only in patients with peroxisomal disorders.
3-Hydroxy-fatty acids (C6 to C16) are formed during beta-oxidation of fatty acids in mammalian tissues (see our web page on carnitines), and increased concentrations of the free acids or acyl-carnitines in blood and urine are indicative of disorders of fatty acid oxidation. Similarly, 3‑hydroxy-dicarboxylic acids, produced by omega-oxidation of 3-hydroxy fatty acids, are present in urine, while 3‑hydroxy-pristanic acid has been found in the plasma of patients with defects of beta-oxidation. C8 to C12 3-Hydroxy acids are normal components of the wax secretions from the uropygial glands of certain bird species, however.
Cow's milk contains small amounts of a range of saturated and monoenoic hydroxy- and keto-fatty acids with the hydroxyl group in positions 5 to 16, but their biosynthetic origin is not known.
A family of enzymes, cytochromes P450s, in liver microsomes are involved in fatty acid oxidation with the 12-carbon lauric acid as the primary substrate, although other fatty acids can also be utilized. The enzymes are unusual in that they hydroxylate with high specificity at the energetically unfavourable terminal (ω) or ω-1 carbons, but the products do not appear to accumulate in significant amounts in tissues. These enzymes are discussed in greater detail in our web page on hydroxyeicosatetraenoic acids.
Lactones: Sebum lipids from animals of the horse family contain appreciable amounts of very-long-chain ω-hydroxy fatty acids (C33, C35, C37 mainly) that have cyclized to form lactones. Those from the horse are monoenes and have a methyl branch, while those from the donkey are mainly saturated and unbranched. 4- and 5-Hydroxy fatty acids form lactones that contribute to the flavour of cow's milk and dairy products. Certain species of frogs from Madagascar possess femoral glands on their hind legs that disseminate volatile compounds including macrocyclic lactones derived from fatty acids oxidized in the ω-1 position.
Estolides - FAHFA: 9-Hydroxy-octadecanoic acid, presumably derived from a hydroperoxide, is a minor component of animal tissues, and induces apoptosis in vitro in cell lines, including cancers. This fatty acid and its isomers with a free carboxyl group and a centrally located hydroxyl group to which a further fatty acid is linked as an estolide or 'FAHFA' (Fatty Acid ester of Hydroxy Fatty Acid), such as the palmitoyl ester of 9‑hydroxy-stearic acid illustrated, have been found in the adipose tissue (white and brown, 100 to 150 ng/g), serum, milk and many other tissues of mice and humans, and they have been detected in many common foods. The palmitoleic acid ester of 9‑hydroxystearic acid and the oleic acid ester of 9‑hydroxystearic acid are reported to be the main forms in the circulation of healthy humans. At least 51 families or 300 molecular species of such lipids have been detected in rat adipose tissue, including 8 or more regioisomers of the hydroxy component (5, 7 to 13), and these vary in concentration with age; these numbers will no doubt increase as the methodology improves. Short-chain fatty acids (acetic and propionic) linked to long-chain (>C20) 2‑hydroxy acids have been detected in plasma and in tissues and contents of the gastrointestinal tract. As more is learned of their biochemistry and biological functions, FAHFA are increasingly being classified among the oxylipins.
Relatively little is known of the biosynthesis of these lipids, but it has been established that they are produced endogenously with defined stereochemistry, i.e. the hydroxyl group has mainly the R-configuration, and several candidate genes for a mono-oxygenation reaction have been identified. Peroxisomal acyltransferases are believed to be involved in the esterification step. In white adipose tissue, biosynthesis of saturated FAHFAs is positively regulated by carbohydrate responsive element binding protein (ChREBP), while the transcription factor nuclear factor erythroid-2-related factor 2 (Nrf2), which regulates the expression of antioxidant genes, may contribute to this. Two atypical integral membrane hydrolases, designated AIG1 and ADTRP, and carboxyl ester lipase degrade bioactive FAHFAs, and they presumably have a regulatory role; they are especially active towards isomers with the ester bond at carbon 9. Rather than having catalytic serine residues, the activities of these enzymes depend upon threonine nucleophiles.
FAHFA-containing triacylglycerols are present in adipose tissue at concentrations more than 100-fold greater than that of non-esterified FAHFAs. As they can be released by lipase activity, it is suggested that this tissue may be a major reservoir for such compounds. Adipose triglyceride lipase not only releases FAHFA from estolide-triacylglycerols, but also is involved in transesterification and remodelling reactions that can change the acyl compositions, while hormone-sensitive lipase is a more potent estolide bond hydrolase for both triacylglycerol-bound and free FAHFAs.
Having entered the circulation, FAHFA function as lipokines, i.e. lipid molecules derived from adipose tissue that can act as hormonal regulators and coordinate a wide array of cellular processes that are beneficial in general. For example, they have anti-diabetic and anti-inflammatory effects, even when administered orally, and they protect against colitis by regulating gut innate and adaptive immune responses. In addition, they may have an important role in maintaining normal blood sugar levels and insulin sensitivity possibly by acting as selective agonists for the GPR40 and GPR120 receptors. In obese mice, palmitic acid esters of hydroxy stearic acids (PAHSAs) in particular enhance hepatic insulin sensitivity directly and indirectly by mechanisms involving inter-tissue communication between adipose tissue and liver. Obese human patients were found to have lower FAHFA levels than non-obese controls. In contrast, there is a suggestion that esterification of 9-hydroxy-octadecanoic acid in tumors may be an escape mechanism to minimize the risk of apoptosis. The linoleic acid ester of 13-hydroxy linoleic acid (13‑LAHLA) derived from oat oil has been shown to be an anti-inflammatory lipid, which suppresses lipopolysaccharide-stimulated secretion of cytokines and the expression of pro-inflammatory genes.
While most of these are not derived from the essential fatty acids, others have been found in adipose tissue with docosahexaenoic acid (DHA) esterified to 9- and 13‑hydroxyoctadecadienoic and 14-hydroxydocosahexaenoic acids; they are present at concentrations similar to those of the specialized pro-resolving mediators and have profound anti-inflammatory effects. For example, the estolide of DHA and 12-hydroxystearate is a potent stimulator of the nuclear factor erythroid 2-related factor 2 (NRF2), which is involved in upregulation of antioxidant enzymes and has a protective cellular function.
Estolides - others: Fatty acids with esterified hydroxyl groups in either the 2- or terminal position are present in meibomian secretions and tears of some animals; more than 70 different molecular species of this type (ω-O-acyl) have been found in humans, for example. Triacylglycerol estolides are present in the paracloacal gland of the brushtail possum with an 18:1 or 18:2 hydroxy fatty acid component.
The skin from animals, but studied especially in pigs and humans, contains O-acyl ceramides in which a long-chain fatty acid component (up to C36) has a terminal hydroxyl group, which may be in the free form or esterified with linoleic acid (see the web page on Ceramides for a detailed discussion of the structures, biosynthesis and functions of these lipids). These ceramides eventually lose the estolide fatty acid and link directly via the terminal hydroxyl group to structural proteins, as vital components of the epidermal permeability barrier. Similar (O-acyl)-ω-hydroxy-fatty acids, i.e. with a terminal rather than a centrally located hydroxyl group, have now been identified in sperm and amniotic fluid. For example, Vernix caseosa, the biofilm that coats the skin of the fetus towards the end of gestation, contains cholesterol esters of ω-(O-acyl)-hydroxy fatty acids, i.e. with very-long chain ω-hydroxy acids (e.g. 32:1) linked to more conventional fatty acids (14:0 to 18:1), i.e. 50 to 52 carbons in total. These also exist in free form together with analogous compounds derived from 2-hydroxy fatty acids. Estolides have long been known as components of seed oils and fungi, and some examples are described below.
Insects: 15-Hydroxy-hexadecanoic and 17-hydroxy-octadecanoic acids are important constituents of beeswax (together with small amounts of other homologues and (ω-2)-hydroxy-isomers). Royal jelly, a secretion produced by worker honey bees (Apis mellifera), contains a number of mono- and dihydroxy fatty acids (C8 to C14), but especially 10‑hydroxy,trans-2-decenoic acid, while the 9-hydroxy-isomer is a queen retinue pheromone. These are believed to provide a variety of health benefits in humans, and in particular they are reported to protect against bone loss. Bees and many other insect species produce lactonized hydroxy fatty acids as pheromones or simply for recognition by their kin. Many other insect waxes and secretions contain hydroxy fatty acids, and perhaps the best known of these is the insect Tachardia (Laccifer) lacca, which produces the polymeric material shellac. This contains substantial amounts of 9,10,16‑trihydroxy-hexadecanoic (aleuritic) and 6-hydroxy-tetradecanoic acids, among others.
Note that the stereochemistry of adjacent hydroxyl groups in the figure is defined from that of the sugars threose and erythrose with threo-dihydroxyls on opposite sides of the alkyl-chain, while erythro groups are on the same side.
The larvae of the European cabbage butterfly, Pieris rapae, secret droplets of an oily liquid consisting of 11R-hydroxy-α-linolenic acid to which straight-chain saturated fatty acids are linked as estolides, and are termed 'mayolenes'. These act as potent chemical deterrents to larval predators such as ants.
2. Hydroxy Fatty Acids in Higher Plants
Many unusual fatty acids are produced by plants of particular families, and these are found most often in seed oils but only rarely in membrane lipids. However, hydroxy fatty acids are key components of plant polymers, such as cutin (see below). As in animal tissues, 2‑(R)‑hydroxy fatty acids occur in appreciable amounts in the sphingolipids of plants. Other 2-hydroxy acids may be encountered in seed oils. For example, 2‑hydroxy-octadeca-9,12,15-trienoate is a minor component of the seed oil of Thymus vulgaris, 2-hydroxy-oleic and linoleic acids are found in Salvia nilotica, and 2‑hydroxy-sterculic acid is occasionally encountered in seed oils of the Malvales.
The seed oils of higher plants contain a number of hydroxy acids, some of which are important agricultural commodities. The best known of these is ricinoleic acid (D‑(-)12‑hydroxy-octadec-cis-9-enoic acid), which comprises up to 90% of castor oil (from Ricinus communis). Biosynthesis occurs via the action of an oleate 12‑hydroxylase, an enzyme closely related structurally to a fatty acyl desaturase. In this instance, the substrate oleate is attached to position sn-2 of phosphatidylcholine and the reaction requires molecular oxygen and NADPH or NADH.
The homologous 14-hydroxy-eicos-cis-11-enoic (lesquerolic) acid is a component of seed oils from the genus Lesquerella, while the isomeric 9-hydroxy-octadeca-cis-12-enoic (isoricinoleic) acid is found in the genus Strophanthus. Other non-conjugated dienoic fatty acids, which appear to be related biosynthetically to these monoenes have been reported in seed oils, and include 12-hydroxy-octadeca-cis-9,cis-15-dienoic (densipolic) and 14-hydroxy-eicosa-cis-11,cis-17-dienoic (auricolic) acids. Among many others, 7,18‑dihydroxy-tetracos-15-enoic (nebraskanic) and an isomer with a further double bond in position 21 (wuhanic acid) acid have been characterized from the seed oil of the Chinese violet cress (Orychophragmus violaceus). The last are of special interest in that the sequence of chain-length elongation reactions in their biosynthesis is interrupted in a manner usually seen only with bacterial fatty acids; they are present in the oil as estolides in addition to the link to glycerol.
Similar lipids to the FAHFA found in animal tissues described above have been found in a variety of plant species, with stearic acid linked to hydroxy stearic acid often the most abundant form. 15‑Hydroxylinoleate and an estolide of this acid with linoleate are components of galactolipids in the seeds of oats (Avena sativa). After ingestion of oat oil by humans, the free estolide been detected in plasma, and it has been shown to have anti-inflammatory activity in an animal system in vitro. The linoleic acid ester of 13-hydroxy linoleic acid has also been detected in plants and has anti-inflammatory properties in an animal system, although the functions of such estolides in the parent plants are not known. Many other lipid-linked estolides have been described from seed oils, including castor oil and the Chinese tallow tree (Sapium sebiferum), which contains trans-2,cis-4-decadienoic acid in estolide linkage to 8-hydroxy-5,6-octadienoic acid.
Conjugated dienoic fatty acids with a hydroxyl group are also known from a number of seed oils, and some may have commercial importance. Almost all are C18 in chain-length. These include 9-hydroxy-octadeca-trans-10,trans-12-dienoic (dimorphecolic) acid from the seed oil of Dimorphotheca sinuata, while the geometrical isomer with a cis-double bond in position 12 is present in the seed oil of Calendula officinalis among others. 13‑Hydroxy-octadeca-cis-9,trans-11-dienoic (coriolic) acid has been reported from Coriaria nepalensis seed oil. Further conjugated hydroxy-fatty acids have acetylenic bonds, including ximenynolic (8-hydroxy,9a,11t) and isanolic (8‑hydroxy,9a,11a,17e) acids.
Dihydroxy acids, such as 9,10-dihydroxy-octadecanoic acid and higher homologues, can be minor components of several seed oils, including castor oil, but they occur in significant concentrations in the seed oil of Cardamine impatiens. Trihydroxy acids (+)-threo-9,10,18-trihydroxyoctadecanoic (phloionolic) acid and (+)‑threo-9,10,18-trihydroxy-cis-12-octadecenoic acid occur in the seed oil of Chamaepeuce afra.
Cutin and suberin: The aerial surfaces of higher plants are covered with a continuous extracellular layer, termed the cuticle that contains cutin as the major structural component. Suberin is a related but distinct material that forms near the plasma membrane in the plant cells of the periderm, i.e. the tissue that surrounds secondary stems in woody plants as part of the bark, and it comprises both aliphatic and aromatic polymers and glycerol; it is also an important component at root surfaces. The function of these layers is to act as barriers to control the movement of gases, water and solutes, and to impart resistance to pathogens or herbivores. They are distinct from the waxy coating that covers the external surface (see our web page on waxes) and indeed act as a support for it. Both cutin and suberin contain polyesters with linear and branched chains that consist mainly of mono-, di- and trihydroxy fatty acids together with α,ω-dicarboxylic fatty acids; the last may be linked via glycerol moieties. In cutin, these fatty acids have chain lengths of 16 and 18 carbons, while suberin is much more complex as it contains wax components and the range of chain-lengths can go up to C28.
The fatty acids of cutins include ω-monohydroxy acids (saturated and monoenoic), 9(or 10),16-dihydroxy-hexadecanoic acid (and analogous C18 acids), 9,10,18‑trihydroxy-octadecanoic acid, and occasionally related fatty acids with epoxyl or keto groups in central positions, e.g. 9,10-epoxy,18-hydroxy-octadecanoate. Suberins contain similar fatty acids but with a wider range of chain-lengths and varying degrees of oxygenation. For example, oleic acid is believed to be the biosynthetic precursor of 18-hydroxy-oleic, 9,10,18‑trihydroxy-octadecanoic, 1,18-octadec-9-enedioic and 9,10‑dihydroxy-1,18-octadecanedioic acids, together with 9,10-epoxy analogues. Phenolic acids, such as ferulic, are also incorporated into the polymer. In the 'model' plant Arabidopsis thaliana, leaf cutin is enriched in 1,18-octadec-6,9-dienedioic acid, while flower cutin contains predominantly 10,16‑dihydroxypalmitic acid. The best-known and characterized source of suberin is cork, derived from the bark of the cork oak (Quercus suber).
The lipid precursors of both cutin and suberin are 16:0, 18:0, 18:1 and 18:2 fatty acids, synthesised in plastids and trafficked to the endoplasmic reticulum, where they can undergo chain elongation and reduction to alcohols. For biosynthesis of oxygenated fatty acids, a hydroxyl group is introduced first at the terminal carbon atom of an existing fatty acid by means of cytochrome P450-dependent fatty acid monooxygenases, and this can then be converted to a dicarboxylic acid by a fatty acid oxygenase. Epoxidation by a cytochrome P450 enzyme followed by the action of an epoxide hydrolase is required for the introduction of mid-chain hydroxyl groups. Long-chain acyl-coenzyme A synthetases, a family of lipase/esterases and acyltransferases (glycerol-3-phosphate acyltransferase (GPAT)) are then required for the synthesis of the complex polymer. Esterification to glycerol differs from that for bulk membrane lipid synthesis in that the relevant GPAT is specific for the sn-2 rather than the sn-1 position, and 2‑monoacylglycerols are the main cutin precursors.
Although much remains to be learned of how the various monomers are transported to the plant surface prior to polymer formation, it is likely that ABC transporters, exocytosis or lipid transfer proteins are involved possibly via microtubules. The final step is polymerization by cutin a synthase, although it has been proposed that some cutin formation may occur by non-enzymatic polymerization of aggregates of the precursors in "cutinsomes". A. thaliana has been widely used for biosynthetic studies of cutin lipids, although many appear to regard this species as atypical.
Sporopollenin is the structured outer pollen wall or exine surrounding pollen grains in flowering plants, and its chemical resilience is evident from its preservation in fossil spores from 450 million years ago that provide the earliest record of plant life on land. This chemical stability has meant that analysis has proved to be technically very difficult as there is no easy way to solubilize and break down such a complex biopolymer for analysis. While it appears that there is still much to be learned, it is now evident that it consists of hydroxylated aliphatic units linked to polyhydroxylated alpha-pyrone subunits with extensive cross-linking; phenylpropanoid units are incorporated also. Through a gene-targeted approach, it has been determined that polyketide and fatty acid synthases, together with cytochrome P450 oxygenases, produce the backbone of polyhydroxylated subunits, with remarkable conservation of biochemical pathways across the plant kingdom.
3. Hydroxy Fatty Acids in Bacteria, Yeasts and Fungi
Ester- and/or amide-bound 2-hydroxy fatty acids, with both straight and branched chains, are common constituents of bacterial lipids. In bacterial lipopolysaccharides, the 2-hydroxyl group has the S-configuration in contrast to that in animals and plants. As such they are often found in environmental samples, like soils, biofilms or sediments. Fatty acids of this kind are usually saturated, in some instances with iso- or anteiso-methyl branches, but monoenes are occasionally found. Estolides with a fatty acid linked to a 2-hydroxy fatty acid have been found in intestinal bacteria.
(R)3(or β)-Hydroxy long-chain fatty acids usually with a saturated fatty acid attached via an estolide linkage are important constituents of lipopolysaccharides and lipid A, from Gram-negative bacteria especially, and they are essential to the endotoxin activity. The fatty acids can be linked by both ester and amide bonds to glucosamine or other amino-sugars in these lipids. Endotoxin levels in samples are often determined by analysis of the content of 3‑hydroxy acids, and they may be assayed in environmental samples, such as dust, aerosols, soils and sewage, for this purpose. In plants, their immune defense responses are activated by the 3-hydroxy acids derived from lipopolysaccharides rather than by lipid A or the lipopolysaccharide per se. 3‑Hydroxy acids are the main fatty acid constituents of rhamnolipids produced by pseudomonads again in part estolide linked. Most fatty acids of this type tend to be fully saturated, sometimes with iso- or anteiso-methyl branches. Mycolic acids, discussed elsewhere in the pages, also contain a hydroxyl group in position 3, and can have keto and methoxyl groups elsewhere in the chain. 3-Hydroxy acids are produced in yeasts and bacteria by direct hydroxylation, mainly of saturated fatty acids, but as the ubiquitous beta-oxidation system can also be involved in their synthesis, fatty acid of many different kinds can be 3-hydroxylated. For example, Candida albicans produces 3‑hydroxy-14:2 as a signalling molecule, and it can also produce a 3-hydroxy oxylipin from arachidonic acid in host cells. Many bacterial species produce fatty acid lactones as signalling molecules, and for example, Streptomycetes species use a family of lactones termed butanolides to regulate their production of secondary metabolites.
The short-chain hydroxy acid, β-hydroxybutyrate, occurs in the form of a polyester in intracellular inclusions in many bacterial species, where it functions as a reserve of carbon and of energy. Currently, these are of considerable interest to industry as a source of biodegradable polymers, substituting for petroleum-derived plastics. Copolymers with 3‑hydroxy-valerate or with C5 to C14 3‑hydroxyalkanoate units are also known from various bacterial species.
10-Hydroxyoctadecanoate and 10-hydroxyhexadecanoate, together with the corresponding oxo fatty acids, are major components of adipocere, the waxy material produced by microbial decay in corpses. The former can be produced by a number of bacterial species, including lactic acid bacteria, from oleic acid added to the growth medium, and it is a major component of the lipids of the protozoon Cryptosporidium parvum. In these bacteria, an oleate hydratase requiring FAD as a cofactor catalyses the addition of water to the double bond of oleic acid to produce (R)-10-hydroxystearic acid with high stereospecificity. A number of microbial enzymes of this type that produce saturated and unsaturated hydroxy and keto acids are being studied as the products have potential industrial value. One suggested explanation of this activity is that it is a mechanism to remove potentially toxic unsaturated fatty acids from a growth medium. Some bacterial species are capable of hydroxylating fatty acids close to the terminal part of the molecule. For example, preparations from Bacillus megaterium are able to convert palmitic acid to 14-hydroxypalmitate mainly, although 15- and 13-hydroxypalmitate are also produced. 27‑Hydroxy-octacosanoic acid is a characteristic component of the lipopolysaccharide Lipid A from all soil bacteria of the family Rhizobiaceae.
Hydroxy-acids are also produced by bacteria in the gut, such as Lactobacillus plantarum, that affect the host metabolism. For example, 10‑hydroxy-cis-12-octadecenoic acid is synthesised by the action of a hydratase produced by this organism on dietary linoleate, and this has anti-inflammatory protective effects on the intestinal cell walls. Oxo-metabolites of linoleate and of conjugated linoleate isomers, which can be incorporated into host tissues, are also formed and for example affect adipose tissue metabolism and the response of macrophages by activating various receptors.
In yeasts and fungi, 2-hydroxy fatty acids are major constituents of sphingolipids (amide linked) as in plants and animals. In some species of fungi, there is an additional trans double bond in position 3, while 2,3-dihydroxy fatty acids have been reported from yeasts. Unusual fatty acids with hydroxyl groups in central positions, together with methyl groups and double bonds, occur in species of fungi, lichens and slime moulds. For example, 7-hydroxy-8,14-dimethyl-9-hexadecenoic acid, and structurally related fatty acids, were found in fungal species belonging to the Ascomycetes and Basidiomycetes. 9,10,13- and 9,12,13‑Trihydroxyoctadecenoic acids found in beer are derived from microbial fermentation of linoleic acid.
Ricinoleic acid, the main component of castor oil, is also a major component (44%) of the triacylglycerols of the fungus ergot (Claviceps purpurea), but there are no free hydroxyl groups as these are acylated with normal long-chain fatty acids, i.e. as estolides, to form tetra, penta- and hexa-acid molecules. The biosynthetic pathway differs from that in the castor oil plant in that linoleic acid is the substrate and hydroxylation occurs under anaerobic conditions, i.e. the hydroxyl group comes from water and not molecular oxygen. Some yeasts produce extra-cellular glycosides containing ω-1 hydroxy fatty acids (C16 and C18), while certain filamentous fungi of the genus, Ustilago, contain ustilagic acids, i.e. 15,16-dihydroxy- and 2,15,16-trihydroxyhexadecanoic acids linked to β-cellobiose.
4. Epoxy Fatty Acids
Epoxy fatty Acids: cis-9,10-Epoxy-octadecanoic acid has been detected and quantified in plasma of humans, and it is believed to be formed by epoxidation of the double bond of oleic acid by a cytochrome P450 enzyme. Its physiological function and fate are not known. Similar mono-epoxy fatty acids are formed in lung and other tissues from linoleate and termed leukotoxins (the term includes a range of diverse compounds), because they produce their primary toxic effects against leukocytes. Together with dihydroxy metabolites formed on ring opening, they are believed to have unpleasant cardiovascular effects and to be involved in acute respiratory distress syndrome, especially in burn patients. Epoxidized polyunsaturated fatty acids are important oxylipins, and it is more appropriate to consider them in the context of eicosanoid metabolism.
Frank Gunstone found and characterized the first natural epoxy fatty acid, i.e. (+)-vernolic acid or cis-12,13-epoxy-octadec-cis-9-enoic acid, from the seed oil of Vernonia anthelmintica. In this instance, the stereochemistry of the epoxyl group was subsequently shown to be 12S,13R, but the optical antipode, (‑)‑vernolic acid, has been isolated from certain seed oils of the Malvaceae.
Subsequently, the isomeric cis-9,10-epoxy-octadec-cis-12-enoic ('coronaric') acid, cis-15,16-epoxy-octadeca-cis-9,12-dienoic acid, and cis‑9,10‑epoxy-octadecanoic were found in seed oils. Although they are usually minor components, vernolic acid can amount to 60% of the total fatty acids in the seed oil of Vernonia galamensis. Biosynthesis involves an epoxygenase enzyme, closely related in structure to desaturases. On prolonged storage, small amounts of epoxy (or hydroxy) acids of defined stereochemistry are generated by enzymic processes in many different seed oils. Vernolic and coronaric acids have an obvious biosynthetic relationship to linoleic acid. Saturated epoxy fatty acids, such as 9,10‑epoxy-octadecanoic and 9,10-epoxy,18-hydroxy-octadecanoate acids, are found in plant cutins (see above).
5. Furanoid Fatty Acids
Furanoid fatty acids are heterocyclic lipids with the general structural formula illustrated, and they are minor but widespread components of algae and plant lipids. However, interest in these compounds was stimulated by the discovery that they were present at low levels in fish, especially in their reproductive tissues, although they are presumed to originate in the diet. Subsequently, they were found in other animal products, especially dairy products where they presumably come from the diet of the herbivores, and even in human plasma and erythrocytes. They have also been detected in some species of bacteria.
The first natural acid of this type (m = 6 and n = 7, and with no methyl substituents) was found in the seed oil of Exocarpus cupressiformis, although there has been a suggestion that it may have been an artefact of the isolation method. On the other hand, it is much more usual to encounter furanoid fatty acids with a methyl substituent in position 3 of the ring (R = H), or with two methyl substituents in the ring, the latter being more common. Most natural fatty acids of this type found to date have 16 to 22 carbon atoms, and they occur in series with mainly terminal propyl (m = 2) or pentyl (m = 4) groups. At least 23 different furanoid fatty acids have now been detected in fish lipids, including some with one or two additional double bonds in positions outwith the ring. They can amount to 0.1 to 1% of the total fatty acids, and the most abundant is often 12,15-epoxy-13,14-dimethyleicosa-12,14-dienoic acid, together with its homologues and smaller relative proportions of monomethyl acids, such as 12,15-epoxy-13-methyleicosa-12,14-dienoic acid. In fish, furanoid fatty acids occur in triacylglycerols and cholesterol esters, especially the latter where they can occur in higher concentrations than the conventional fatty acids. Other marine sources include sponges and soft corals. In human and bovine plasma and in bacteria, they are found mainly in phospholipids.
Similar fatty acids have been found in a variety of plant sources. For example, in the latex of the rubber tree Hevea brasiliensis, 10,13-epoxy-11-methyloctadeca-10,12-dienoic acid has been identified as the main component of the lipid fraction, and this and other furanoid fatty acids have been detected as minor components in numerous other plants (seeds, leaves and fruit) in amounts comparable to the tocopherols, i.e. 15 to 2650 µg/100 g fresh weight), as well as in yeasts, algae and marine bacteria. Indeed, it is suggested that plants, algae and bacteria via the food chain are the main source of furanoid fatty acids in animals and fish, especially for the pentyl-substituted fatty acids and those with double bonds in the terminal part of the chain. Linoleic acid is reported to be the biosynthetic precursor of the more abundant furanoid fatty acids, but this is no longer certain. While the origin of the propyl-substituted compounds is less debated, they may be derived from metabolism of 9,12-hexadecadienoic acid in algae such as Phaeodactylum tricornutum.
Recently, furanoid fatty acids were detected in bacterial species for the first time (from Dehalococcoides sp.); 9-(5-pentyl-2-furyl)-nonanoate, 9-(5-butyl-2-furyl)-nonanoate, and 8-(5-pentyl-2-furyl)-octanoate are present in the phospholipids. The biosynthetic mechanism has been studied in two α-proteobacteria (Rhodobacter sphaeroides and Rhodopseudomonas palustris) in which 11-cis-octadecenoic acid is the primary precursor; all steps occur on pre-existing phospholipid fatty acid chains. The first step in biosynthesis in these species is now known to involve methylation of carbon 11 with S‑adenosylmethionine (SAM) as the methyl donor, and this is accompanied by migration of the double bond with a change in conformation to produce 11M-12t-18:1. This is followed by a desaturation step to produce 11M-10t,12t-18:2, and then oxygenation by a novel monooxygenase and cyclization to form the furan ring (9M5-FuFA); molecular oxygen (O2) is the source of the oxygen atom. A further methylation reaction can introduce a second methyl into the ring to form 9D5-FuFA. Although an alternative pathway has been suggested for algae, it is possible that biosynthesis of furanoid fatty acids proceeds via analogous steps and related enzymes in other organisms.
A number of short-chain dibasic furanoid fatty acids have been isolated from human blood and plasma, and have been termed urofuranic acids. They are believed to be beta-oxidation metabolites of the longer-chain furanoid fatty acids from fish in the diet. When kidney function is impaired, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) in particular can accumulate and may be a significant uremic toxin. Also, the concentration of this metabolite increases in plasma of patients who progress from pre-diabetes to type 2 diabetes, and it may be a marker for this disease; it reportedly increases oxidative stress and impairs insulin secretion.
Furanoid fatty acids can arise spontaneously from autoxidation of fatty acids with conjugated double bonds. They are scavengers of hydroxyl and hydroperoxyl radicals and this may be the function of natural furanoid fatty acids in membranes, i.e. as antioxidants. For this reason, it has been suggested that they may play a part in the cardio-protective effects of dietary fish oils. It would not be surprising if such distinctive oxylipins were found to have other signalling functions in vivo.
A fatty acid containing a tetrahydrofuran ring, i.e. (+)-(2S, 3S, 5R)-tetrahydro-3-hydroxy-5-[(1R)-1-hydroxyhexyl]-2-furanoctanoic acid, is a secreted pheromone that controls the migratory behaviour of a fish species, the sea lamprey. A tetrahydropyran ring-containing fatty acid, i.e. 2,3-dihydroxy-9,13-oxy-7-trans-octadecenoic acid, occurs linked to taurine in the protozoon Tetrahymena thermophila. Isofurans with a tetrahydrofuran ring are formed adventitiously together with isoprostanes by autoxidative processes in animal tissues.
6. Methoxy Fatty Acids
The mycolic acids, described in our web page on branched-chain fatty acids, are unusual in many ways not least in that they contain methoxy substituents. Otherwise, methoxy fatty acids are not common in nature, although a number of 2-methoxy substituted fatty acids, all with the R‑configuration at the chiral centre, have been isolated from sponges, especially those of Caribbean origin. They include saturated, monoenoic and very-long chain dienoic acids. In addition, some mid-chain methoxylated fatty acids have been isolated from marine cyanobacteria, especially Lyngbya species, where they exist as N‑substituted amides termed ‘malyngamides’. One of these unusual fatty acids, which also contains a double bond of the trans configuration, is illustrated below. Other methoxy fatty acids are occasionally reported from algae, fungi and bacteria. Methoxy fatty acids are currently of some pharmaceutical interest as they display antibacterial, antifungal, anticancer and antiviral activities.
Unfortunately, methoxyl groups can also be introduced to fatty acyl chains artefactually during analysis if cyclopropane or brominated fatty acids are methylated by inappropriate methods.
7. Keto (Oxo) Fatty Acids
Keto fatty acids are often reported as artefacts of oxidation of fatty acids but relatively rarely as natural fatty acids in their own right. However, analogues of some of the common plant hydroxy fatty acids are occasionally found. For example, 13-oxo-trans-9, trans-11-octadecadienoic acid, an analogue of coriolic acid, is a major component of Monnina emarginata seed oil, where it is accompanied by a small amount of 13-oxo-trans-9-octadecenoic acid. Similarly, 9-oxo-trans-10,trans-12-octadecadienoic acid accompanies dimorphecolic acid in Dimorphotheca seed oils. Oxo-monoenoic fatty acids have been found in a number of seed oils including Cuspidaria pterocarpa and Mappia foetida. The distinctive keto acid, licanic or 4-keto-9-cis,11-trans,13-trans-octadecatrienoic or 4-keto-α-eleostearic acid, amounts to 60% of the seed oil of Licania rigida, while a 4‑keto analogue of α‑parinaric acid was found in Chrysobalanus icacao. The traumatin family of plant oxylipins contains keto- or terminal aldehyde-moieties.
5-Oxo-6t,8c,11c,14c-eicosatetraenoic acid (5-Oxo-ETE) is an important oxylipin and is discussed best in that context. 3-Keto fatty acids found as minor components of animal tissues are generally intermediates in β-oxidation, but the origin of a range of minor keto fatty acids, differing in chain length and the positions of double bonds and oxo groups, in cow's milk is not known (51 isomers characterized). Small amounts of saturated oxo-fatty acids have recently been detected in human plasma and may be beneficial to health. 9-Keto,2-trans-decenoic acid is a pheromone found in the royal jelly produced by queen honey bees to control the activities of workers bees.
There are appear to be no general reviews on this topic currently available, but the following are useful guides to various aspects of the chemistry, biochemistry and analysis of naturally occurring oxygenated fatty acids.
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- Brechany, E.Y. and Christie, W.W. Identification of the saturated oxo fatty acids in cheese. J. Dairy Res., 59, 57-64 (1992); DOI - also unsaturated at - DOI.
- Brejchova, K., Balas, L., Paluchova, V., Brezinova, M., Durand, T. and Ondrej, K. Understanding FAHFAs: From structure to metabolic regulation. Prog. Lipid Res., 79, 101053 (2020); DOI.
- Cahoon, E.B. and Li-Beisson, Y. Plant unusual fatty acids: learning from the less common. Curr. Opinion Plant Biol., 55, 66-73 (2020); DOI.
- Carballeira, N.M. New advances in the chemistry of methoxylated lipids. Prog. Lipid Res., 41, 437-456 (2002); DOI.
- Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010) - see Science Direct.
- Grienenberger, E. and Quilichini, T.D. The toughest material in the plant kingdom: an update on sporopollenin. Front. Plant Sci., 12, 703864 (2021); DOI
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- Schulz, S. and Hötling, S. The use of the lactone motif in chemical communication. Nat. Prod. Rep., 32, 1042-1066 (2015); DOI.
- Smith, C.R. Occurrence of unusual fatty acids in plants. Prog. Chem. Fats other Lipids, 11, 137-177 (1971); DOI.
- Xu, L., Sinclair, A.J., Faiza, M., Li, D., Han, X., Yin, H. and Wang, Y. Furan fatty acids – Beneficial or harmful to health? Prog. Lipid Res., 68, 119-137 (2017); DOI.
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