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POLYENOIC FATTY ACIDS

These fatty acids (also called polyunsaturated fatty acids, PUFA) have 2 or more cis double bonds which are the most frequently separated from each other by a single methylene group (methylene-interrupted polyenes). Linoleic acid is a typical member of this group. Some rare polyenoic fatty acids may have also a trans double bond. A graphical chart of the oxidation of polyunsaturated fatty acids by free radicals may be found on Wikipedia web site.

-C-C=C-C-C=C-
methylene-interrupted double bonds

Some other polyunsaturated fatty acids undergo a migration of one of their double bonds which are not again methylene-interrupted and are known as conjugated fatty acids.

-C-C=C-C=C-C-
conjugated double bonds

Some unusual fatty acids have not the regular structure with a methylene group between two double bonds but are polymethylene-interrupted polyenes (known also as non-methylene-interrupted fatty acids). They are found in certain classes of bacteria, plants, marine invertebrates and insects.

-C=C-C-C-C-C=C-
polymethylene-interrupted double bonds

Rare fatty acids have allenic double bonds. They are found in some higher plants.

-C=C=C-
allenic double bonds

Very rare fatty acids have cumulenic double bonds. They are present in some higher plants.

-HC=C=C=CH-
cumulenic double bonds



METHYLENE-INTERRUPTED POLYENES



The most important fatty acids can be grouped into 2 series with a common structural feature: CH
3(CH2)xCH=R . x=4 for the (n-6) series and x=1 for the (n-3) series and x=7 for the (n-9) series.
Some rare fatty acids have other structural features.
Below, as an example, we give the structure  of a common polyene of the (n-3) series having the double bonds in the 5, 8, 11, 14, and 17 positions (eicosapentaenoic acid or osbond acid).


pict132.JPG (6555 octets)

The commonest polyenoic fatty acids are listed below:

 

Systematic name

Trivial name

Shorthand designation

Molecular wt.

MP
°C

9,12-octadecadienoic linoleic acid

18:2(n-6)

280.4

-5

6,9,12-octadecatrienoic g-linolenic acid

18:3(n-6)

278.4

 
8,11,14-eicosatrienoic dihomo-g-linolenic acid

20:3(n-6)

306.5

 
5,8,11,14-eicosatetraenoic arachidonic acid

20:4(n-6)

304.5

-50

7,10,13,16-docosatetraenoic

22:4(n-6)

332.6

 
4,7,10,13,16-docosapentaenoic

osbond acid

22:5(n-6)

330.6

 
9,12,15-octadecatrienoic a-linolenic acid

18:3(n-3)

278.4

-11

6,9,12,15-octadecatetraenoic stearidonic acid

18:4(n-3)

276.4

-57

8,11,14,17-eicosatetraenoic

20:4(n-3)

304.5

 
5,8,11,14,17-eicosapentaenoic EPA

20:5(n-3)

302.5

-54

7,10,13,16,19-docosapentaenoic DPA ou clupanodonic acid

22:5(n-3)

330.6

 
4,7,10,13,16,19-docosahexaenoic DHA

22:6(n-3)

328.6

-44

6,9,12,15,18,21-tetracosenoic  nisinic acid 24:6(n-3) 356.6  
5,8,11-eicosatrienoic Mead acid

20:3(n-9)

306.5

 

 



N-6 FATTY ACIDS

  • Linoleic acid (18:2 n-6) is the most common polyunsaturated fatty acids, in plants and animal tissues. The major sources for human food are soybean, sunflower, palm, canola, and cotton.

linoleic acid

It was isolated in 1844 by Sacc (Ann 1844, 51, 213), and after a long controversy its exact structure was clarified in 1939 (Hilditch TP et al., J Soc Chem Ind 1939, 58, 233) and it was synthesized only in 1950 (Raphael RA et al., Nature, 1950, 165, 235).
It cannot be synthesized by animals which must find it in plant foodstuff. It is said an essential fatty acid for animals. Walnut, peanut, seeds of sunflower, grape, corn, sesame and soya contain large amounts of that fatty acid. Linoleic acid is the precursor of all the (n-6) series formed by desaturation and elongation.
Two trans isomers of linoleic acid have been detected in seed oils. The 9c,12t isomer (M.P. = -5°C) was found in Crepis rubra and the 9t,12t isomer (M.P. = 29°C) was found in Chilopsis linearis.

It has been reported that linoleic acid is the most abundant polyunsaturated fatty acid (0.45-2.7g/100g fresh insect, 9-21% of total FA) in insect body (Yang LF et al., J Food Lipids 2006, 13, 277-285).

  • gamma-Linolenic (18:3 n-6) is the first intermediate formed and therapeutic properties have been claimed for it (antihypercholesterolaemic). This fatty acid was first noted in evening primrose in 1919 (Heiduschka A et al., Arch Pharm 1919, 257, 33) and its structure elucidated in 1927 (Eibner A et al., Chem Umshau 1927, 34, 312).
    This fatty acid is available from some seed oils from several plant families. Thus, Boraginaceae with borage (Borago officinalis) (10-25%), Echium spp (5.5-11.7%), and Myosotis spp (4.4-20.2),  Onagraceae with evening primerose (Oenothera biennis) (7-10%), Cannabaceae with Cannabis sativa (3-6%), Loasaceae with Nasa (3.5-10%), Caryophyllaceae with Minuartia laricifolia (15.6%), and Saxifragaceae with black currant (Ribes nigrum)
    4-20% and Grossularia burejensis (12%) are the main groups which are sources of g-linolenic. A review on the distribution, properties, and extraction may be consulted for further information (Clough PM, Structured and modified lipids, Gunstone FD Ed, M Dekker, NY 2001, pp.75-117). A comprehensive review of 45 plant species that are potential sources of g-linolenic may be consulted (Guil-Guerrero JL et al., JAOCS 2001, 78, 677). Seeds from 50 species of Caryophyllaceae were also surveyed (Guil-Guerrero JL et al., JAOCS 2004, 81, 659).
    There are reports of the development of a genetically modified rapeseed containing this fatty acid (Lassner M, Lipid Technol 1997, 9, 5). 
    In surveying the fatty acid composition of eukaryotic microorganisms, it was found that
    g-linolenic was present in lower fungi (Shaw R, Adv Lipid Res 1966, 4, 107). Several Mucor and Mortierella species are potential sources of g-linoleic (8-18% of neutral lipids). Attempts have been made to produce materials from these sources on a commercial basis. A detailed review on this topic may be consulted (Ratledge C, Structured and modified lipids, Gunstone FD Ed, M Dekker, NY 2001, p. 351).
  • Dihomo-gamma-linolenic (20:3 n-6) : This fatty acid was found in small amounts (up to 5%) in some fungi such as Mortierella or Condiobolus. Higher levels (up to 18%) could be produced under particular fermentation conditions (Ratledge C, Structured and modified lipids, Gunstone FD Ed, M Dekker, NY 2001, p. 351).
  • Arachidonic acid (20:4 n-6) is the most important of this series: it is a major constituent of membrane phospholipids and is the principal precursor by enzymatic action of hormone-like compounds known as eicosanoids including the prostaglandins (prostanoids), isoprostanes, and isofurans

arachidonic acid

It was first isolated in 1940 from phospholipids from beef suprarenal glands by Shinowara GY et al. (J Biol Chem 1940, 134, 331) and its structure was elucidated three years later by Arens CL et al. (Biochem J 1943, 37, 1). The first total synthesis of arachidonic acid was made in 1961 (Osbond JM et al., J Chem Soc 1961, p.2779). 
Rare in the plant kingdom, it can be found in some fungi, mosses and ferns but is a major component of several microalgae and some marine brown algae. It was shown to be abundant in a green alga, Parietochloris incisa, where it reaches up to 47% of the triglyceride pool (Bigogno C et al. Phytochemistry 2002, 60, 497). Unlike higher plants, mosses contain substantial levels of arachidonic acid, protonema cells containing   20-40% of this compound (Gellerman JL et al., Biochim Biophys Acta 1975, 388, 277). The production of arachidonic acid by microorganisms (fungi, microalgae) has been reviewed by Ratledge C (Structured and modified lipids, Gunstone FD Ed, M Dekker, NY 2001, p. 351). In the 1960s certain fungi were found to have lipids with a high content of arachidonic acid (more than 50% of the total fatty acids). Mortierella alpina (Zygomycetes) was selected for the development of an industrial fermentation process (Suntory in Japan, Martek in USA, and DSM in The Netherlands). An extensive overview of process investigations related to microbial and microalgae productions of arachidonic acid (and
g-linolenic acid) may bee consulted (Owen PW et al., Proc Biochem 2005, 40, 3627).
The production of arachidonic acid in transgenic plants which might lead to a sustained source of that fatty acid for use in human and animal food was reviewed by Domergue F et al. (Trends Plant Sci 2005, 10, 113).
Its current commercial application is the supplementation of infant formula.

Oxidations of arachidonic acid by reactive oxygen radicals generate several oxidized lipids known as iso-eicosanoids (isoprostanoids and isoleukotrienes). A unique family of free radical-generated derivatives generated by NO2-mediated isomerization of arachidonic acid  were described (Jiang H et al., J Biol Chem 1999, 274, 16235). Several isomers (named trans-arachidonic acid) were observed and appeared to have one trans-bond and three cis-bonds. Thus, four such isomers of arachidonic acid can potentially be generated. 

trans-arachidonic acid

The detection and quantification of trans-arachidonic acids in vivo may be used as a specific index to assess the degree of cellular injury mediated by NO2 since these isomers were shown to be produced in human blood plasma (Zghibeh CM et al., Anal Biochem 2004, 332, 137).

The increased concentration with age of pentaenoic acid (22:5n-6) observed for the first time in rat testes suggests that lipids may have an essential role in the maturation of the testis (Kirschman JC et al., Arch Biochem Biophys 1961, 93, 297). Later, the biosynthesis of 24:4n-6 and 24:5n-6 were described in rat testes (Bridges RB et al., J Biol Chem 1970, 245, 46). Whereas human spermatozoa contain predominantly di-, tri- and tetraenoic fatty acids with up to 32 carbon atoms, boar, ram and bull spermatozoa contain pentaenoic and/or hexaenoic acids with up to 34 carbon atoms (Poulos A et al., Biochem J 1986, 240, 891).
Several other fatty acids were described in mammals (Poulos A, Lipids 1995, 30, 1) and n-6 fatty acids with up to 6 double bonds and 34 carbon atoms have been determined in sphingomyelin and ceramides extracted from the head of mammalian spermatozoa (Bull, ram) (Furland NE et al., J Biol Chem 2007, 282, 18141 and 18151). Most of these very-long chain fatty acids have an even number of carbon atoms but odd-chains also occurred in lower amounts. The mostly represented fatty acids are 27:4 and 29:4n-6.
Polyenoic n-6 fatty acids with carbon chain lengths from 26 to 38 have been detected in abnormal amounts in brain of patients with the rare inherited disorder, Zellweger syndrome (Sharp P et al., Biochem J 1987, 248, 61). They probably derived by chain elongation of shorter-chain n-6 fatty acids and accumulate because of a lack of a specific coenzyme A synthetase, the first enzyme in the beta-oxidation pathway.

An uncommon (n-6) fatty acid was discovered in retina, c14:2 (n-6), acylating a NH2 terminus of a retinal protein, recoverin, involved in the regulation of the photoreception mechanism (Dizhoor AM et al., J Biol Chem 1992, 267, 16033).

The very long-chain (n-6) fatty acid 34:9 (n-6) has been identified in the freshwater crustacean species Bathynella natans living in caves of central Europe (Rezanka T et al., Tetrahedron 2004, 60, 4261). To date, this compound may be considered as the most unsaturated fatty acid discovered in a living structure.

The 28:7n-6 fatty acid and other very long-chain polyunsaturated fatty acids had been found in fish oil, and these had probably been derived from the diet (Rezanka T, J Chromatogr 1990, 513, 344). The identification of 28:7n-6 in several marine dinoflagellates support that hypothesis (Mansour MP et al., Phytochemistry 1999, 50, 541). That very long-chain highly unsaturated fatty acid was shown to be associated with phospholipids, and not with glycolipids (Leblond JD et al., J Phycol 2000, 36, 1103).
Several very long-chain n-6 fatty acids have been isolated from a dinoflagellate Amphidinium carterae, they had 22 to 36 carbon atoms and 3 to 7 double bonds (Rezanka T et al., Phytochemistry 2008, 69, 2391).



N-3 FATTY ACIDS

  • Linolenic acid (18:3 n-3) is found only in plants (it comes mainly from the oil obtained from Linus usitatissimum which is used in industry as drying oil, ink, or linoleum component). 

linoleic acid

It was recognized as a separate fatty acid in 1887 (Hazura K, Monatsh 1887, 8, 158) and its structure was elucidated in 1909 (Erdmann E et al., Ber 1909, 42, 1334) while it was synthesized only forty years later (Raphael RA et al.,J Chem Soc 1950, 2100). Linolenic acid is the major fatty acid of plant leaves, stems and roots and is the precursor of the (n-3) series which is essential in fish and probably in other animals. The major sources for human food are soybean and canola. The question of the possibility of these plant fatty acids to be the precursors of the long-chain n-3 compounds (EPA and DHA) in transgenic plants has been examined (Napier JA et al., Biochimie 2004, 86, 785, Williams CM et al., Proc Nutr Soc 2006, 65, 42). Recent progress demonstrates the feasibility of using transgenic plants to synthesize long-cain polyunsaturated fatty acids. 
The main source of n-3 fatty acids is fish oil but the market prices of that product are increasing significantly. This has prompted a significant amount of research on the use of single-cell oils as a source of n-3 fatty acids. Some of the microorganisms (phototrophic or heterotrophic) reported to produce edible oil that contains omega-3 fatty acids are from the genus Schizochytrium, Thraustochytrium and Ulkenia. An overview of advances in the production of single cell oils rich in n-3 fatty acids may be consulted (Armanta RE et al., JAOCS 2013, 90, 167).
Besides Linus, the plant Lallemantia iberica (Lamiaceae), originated from the Caucasus and Middle East regions produces a seed oil rich in linolenic acid (67-74%). This potential source of n-3 fatty acid is successfully cultivated in some central and southern European countries (Zlatanov M et al., JAOCS 2012, 89, 1393). Chia seeds are tiny black or white seeds which are formed by a plant of the mint family (Salvia Hispanica, Lamiaceae) cultivated in Central America. They are one of the highest plant based sources of linolenic acid (approximately 25 to 40 percent of the chia seed is composed of oil and 60 percent of the oil is made of linolenic acid, n-6:n-3 ratio of 1:3).

Stearidonic acid (18:4 n-3) is produced in vivo by desaturation of a-linolenic acid and is the precursor of EPA. The stearidonic acid supplementation was claimed to be beneficial in term of skin moisturization, thrombosis, inflammation, and cancer (Guil-Guerrero JL et al., In: Omega-3 Fatty Acids: New Research. Ed. F