Unsaturated Fatty Acids

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Animal fats contain a high proportion of glyc-erides of saturated fatty acids and tend to be solids, whilst those from plants and fish contain predominantly unsaturated fatty acid esters and tend to be liquids. Some of the common naturally occurring unsaturated fatty acids are also included in Table 3.1. A convenient shorthand representation for fatty acids indicating chain length with number, position and stereochemistry of double bonds is also presented in Table 3.1. A less systematic numbering starting from the methyl (the m end) may also be encountered. Major groups of fatty acids are designated m-3 (omega-3), m-6 (omega-6), m-9 (omega-9), etc (or sometimes n-3, n-6, n-9), if there is a double bond that number of carbons from the methyl terminus. This has some value in relating structures when an unsaturated fatty acid is biosynthetically elongated from the carboxyl end as during prostaglandin biosynthesis (see page 45). Double bonds at position 9 are common, but unsaturation can occur at other positions in the chain. Polyunsaturated fatty acids tend to have their double bonds in a non-conjugated array as a repeating unit —(CH=CHCH2)„—. In virtually all cases, the stereochemistry of the double bond is Z (cis), introducing a 'bend' into the alkyl chain. This interferes with the close association and aggregation of molecules that is possible in saturated structures, and helps to maintain the fluidity in oils and cellular membranes.

Fats and oils represent long term stores of energy for most organisms, being subjected to oxidative metabolism as required. Major oils which are produced commercially for use as foods, toiletries, medicinals, or pharmaceutical formulation aids are listed in Table 3.2. Typical fatty acid analyses are shown, though it must be appreciated that these figures can vary quite widely. For instance, plant oils show significant variation according to the climatic conditions under which the plant was grown. In colder climates, a higher proportion of polyunsaturated fatty acids is produced, so that the plant can maintain the fluidity of its storage fats and membranes. The melting points of these materials depend on the relative proportions of the various fatty acids, reflecting primarily the chain length and the amount of unsaturation in the chain. Saturation, and increasing chain length in the fatty acids gives a more solid fat at room temperature. Thus, butterfat and cocoa butter (theobroma oil) contain a relatively high proportion of saturated fatty acids and are solids. Palm kernel and coconut oils are both semi-solids having a high concentration of the saturated Ci2 acid lauric acid. A characteristic feature of olive oil is its very high oleic acid (18:1) content, whilst rapeseed oil possesses high concentrations of long chain C20 and C22 fatty acids, e.g. erucic acid (22:1). Typical fatty acids in fish oils have high unsaturation and also long chain lengths, e.g. eicosapentaenoic acid (EPA) (20:5) and docosahexaenoic acid (DHA) (22:6) in cod liver oil.

Unsaturated fatty acids can arise by more than one biosynthetic route, but in most organisms the common mechanism is by desaturation of the corresponding alkanoic acid, with further desaturation in subsequent steps. Most eukaryotic organisms possess a A9-desaturase that introduces a cis double bond into a saturated fatty acid, requiring O2 and NADPH or NADH cofactors. The mechanism of desaturation does not involve any intermediates hydroxylated at C-9 or C-10, and the requirement for O2 is as an acceptor at the end of an electron transport chain. A stearoyl (C18) thioester is the usual substrate giving an oleoyl derivative (Figure 3.7), coenzyme A esters being utilized by animal and fungal enzymes, and ACP esters by plant systems. The position of further desaturation then depends very much on the organism. Non-mammalian enzymes tend to introduce additional double bonds between the existing double bond and the methyl terminus, e.g. oleate ^ linoleate ^ a-linolenate. Animals always introduce new double bonds towards the carboxyl group. Thus oleate is desaturated to

The term fat or oil has no precise significance, and merely describes whether the material is a solid (fat) or liquid (oil) at room temperature. Most commercial oils are obtained from plant sources, particularly seeds and fruits, and the oil is extracted by cold or hot expression, or less commonly by solvent extraction with hexane. The crude oil is then refined by filtration, steaming, neutralization to remove free acids, washing, and bleaching as appropriate. Many food oils are then partially hydrogenated to produce semi-solid fats. Animal fats and fish oils are usually extracted by steaming, the higher temperature deactivating enzymes that would otherwise begin to hydrolyse the glycerides.

Oils and fats feature as important food components and cooking oils, some 80% of commercial production being used as human food, whilst animal feeds account for another 6%. Most of the remaining production is used as the basis of soaps, detergents, and pharmaceutical creams and ointments. A number of oils are used as diluents (carrier or base oils) for the volatile oils employed in aromatherapy.

Oil Source Part used Oil content1' Typical fatty acid Uses, notes

Almond

Prunus amygdalus

var. dulcís, or var.

amara (Rosaceae)

Arachis

Arachis hypogaea

(groundnut,

(Leguminosae/

peanut)

Fabaceae)

Borage

Borago officinalis

(Boraginaceae)

Butterfat

cow

Bos taurus

(Bovidae)

Castor

Ricinus communis

(Euphorbiaceae)

seed seed seed milk seed seed seed seed milk seed

40-55 oleic (62-86), linoleic (7-30), palmitic (4-9), stearic (1-2)

45-55 oleic (35-72), linoleic (13-43), palmitic (7-16), stearic (1-7), behenic (1-5), arachidic (1-3)

28-35 linoleic (38), y-linolenic (23-26), oleic (16), palmitic (11)

2-5 palmitic (29), oleic (28), stearic (13), myristic (12), butyric (4), lauric (3), caproic (2), capric (2), palmitoleic (2)

35-55 ricinoleic (80-90), oleic (4-9), linoleic (2-7), palmitic (2-3), stearic (2-3)

emollient base, toiletries, carrier oil (aromatherapy)

food oil, emollient base dietary supplement for y-linolenic acid content (see page 46)

food emollient base, purgative, soap manufacture

Castor seeds contain the highly toxic, but heat-labile protein ricin (see page 434)

{Continued overleaf )

Table 3.2

Oil Source Part used Oil content1' _(%)

Coconut Cocos nucifera seed kernel 65-68

(Palmae/ Arecaceae)

Cod-liver cod fresh liver 50

Gadus morrhua (Gadidae)

Cottonseed Gossypium seed 15-36

hirsutum (Malvaceae)

Evening primrose

Honesty

Oenothera biennis seed (Onagraceae)

Lunaria annua seed

(Cruciferae/

Brassicaceae)

30-40

('Continued)

Typical fatty acid composition1' (%)

lauric (43-53), myristic (15-21), palmitic (7-11), caprylic (5-10), capric (5-10), oleic (6-8), stearic (2-4)

oleic (24), DHA (14), palmitic (11), EPA (6), palmitoleic (7), stearic (4), myristic (3)

linoleic (33-58), palmitic (17-29), oleic (13-44), stearic (1-4)

linoleic (65-80), y-linolenic (7-14), oleic (9), palmitic (7)

Uses, notes soaps, shampoos

Fractionated coconut oil containing only short to medium length fatty acids (mainly caprylic and capric) is a dietary supplement dietary supplement due to presence of EPA and DHA, plus vitamins A (see page 230) and D (see page 259); halibut-liver oil from halibut Hippoglossus vulgaris (Pleurnectideae) has similar properties and is used in the same way solvent for injections, soaps Cotton seeds also contain 1.1-1.3% gossypol (see page 200) and small amounts of cyclopropenoid fatty acids, e.g. sterculic and malvalic acids (see page 50)

dietary supplement for y-linolenic acid content (see page 46)

nervonic acid is being investigated for the treatment of multiple sclerosis; the disease is characterized by a deficiency in nervonic acid

Lard

Sus scrofa (Suidae)

abdominal fat

Maize (corn) Olive

Palm kernel

Linum seed 35-44

usitatissimum

(Linaceae)

Zea mays embryo 33-39

(Graminae/

Poaceae)

Olea europaea fruits 15-40

(Oleaceae)

Elaeis guineensis kernel 45-50

(Palmae/

Arecaceae)

Rapeseed

Brassica napus seed 40-50

(Cruciferae/

Brassicaceae)

oleic (45), palmitic (25), stearic (12), linoleic (10), palmitoleic (3)

foods a-linolenic (30-60), oleic (39), linoleic (15), palmitic (7), stearic (4)

liniments, dietary supplement for a-linolenic acid content linoleic (34-62), oleic (19-50), palmitic (8-19), stearic (0-4)

Formerly the basis of paints, reacting with oxygen, polymerizing, and drying to a hard film food oil, dietary supplement, solvent for injections oleic (56-85), palmitic (8-20), linoleic (4-20), stearic (1-4)

lauric (40-52), myristic (14-18), oleic (9-16), palmitoleic (6-10), caprylic (3-6), capric (3-5), stearic (1-4), linoleic (1-3)

erucic (30-60), oleic (9-25), linoleic (11-25), gadoleic (5-15), a-linolenic (5-12), palmitic (0-5)

food oil, emollient base soaps

Fractionated palm oil is a solid obtained by fractionation and hydrogénation and is used as a suppository base food oil, using varieties producing lower levels of erucic acid where the main components are now oleic (48-60%), linoleic (18-30%), a-linolenic (6-14%), and palmitic (3-6%) acids

Erucic acid is used as a plasticizer in PVC clingfilm

{Continued overleaf )

Table 3.2 (Continued)

Oil Source Part used Oil content1' Typical fatty acid Uses, notes

Table 3.2 (Continued)

Oil Source Part used Oil content1' Typical fatty acid Uses, notes

Sesame

Sesamum indicum (Pedaliaceae)

seed

44-

-54

oleic (35-50), linoleic (35-50), palmitic (7-12), stearic (4-6)

food oil, soaps, solvent for injections, carrier oil (aromatherapy)

(Leguminosae/

Fabaceae)

seed

18-

-20

linoleic (44-62%), oleic (19-30), palmitic (7-14), a-linolenic (4-11), stearic (1-5)

food oil, dietary supplement, carrier oil (aromatherapy)

Soya oil contains substantial amounts of the sterols sitosterol and stigmasterol (see page 256)

Suet (mutton tallow)

sheep Ovis aries (Bovidae)

abdominal fat

stearic (32), oleic (31), palmitic (27), myristic (6)

foods

Suet (beef tallow)

Bos taurus (Bovidae)

abdominal fat

oleic (48), palmitic (27), palmitoleic (11), stearic (7), myristic (3)

foods

Sunflower

Helianthus annuus

(Compositae/

Asteraceae)

seed

22-

-36

linoleic (50-70), oleic (20-40), palmitic (3-10), stearic (1-10)

food oil, carrier oil (aromatherapy)

Theobroma

Theobroma cacao (Sterculiaceae)

kernel

35-

-50

oleic (35), stearic (35), palmitic (26), linoleic (3)

suppository base, chocolate manufacture

Theobroma oil (cocoa butter) is a solid

tThe oil yields and fatty acid compositions given in the table are typical values, and can vary widely. The quality of an oil is determined principally by its fatty acid analysis. Structures of the fatty acids are shown in Table 3.1 (see page 38).

tThe oil yields and fatty acid compositions given in the table are typical values, and can vary widely. The quality of an oil is determined principally by its fatty acid analysis. Structures of the fatty acids are shown in Table 3.1 (see page 38).

CO-SR

CO-SR

R = SCoA in animals/fungi R = ACP in plants

R = SCoA in animals/fungi R = ACP in plants stearic 18:0

desaturation towards methyl terminus stearic 18:0

desaturation towards methyl terminus

CO-SR

plants fungi

CO-SR

animals linoleic 18:2 (9c,12c)

desaturation towards carboxyl terminus

CO-SR

animals

CO-SR

CO-SR

y-linolenic 18:3 (6c,9c,12c)

chain extension by Claisen reaction with malonate; chain length increased by two carbons plants fungi animals

CO-SR

CO-SR

prostaglandins 1-series

CO-SR

dihomo-y-linolenic 20:3 (8c,11c,14c)

CO-SR

CO-SR

a-linolenic 18:3 (9c,12c,15c)

animals

CO-SR

animals

CO-SR

stearidonic

CO-SR

CO-SR

eicosatetraenoic 20:4 (8c,11c,14c,17c)

prostaglandins 2-series

CO-SR

arachidonic 20:4 (5c,8c,11c,14c)

CO-SR

CO-SR

eicosapentaenoic (EPA) t^ 20:5 (5c,8c,11c,14c,17c)

Note: the names given are for the appropriate fatty acid; the structures shown are actually the thioesters involved in the conversions prostaglandins 3-series

CO-SR

CO-SR

CO-SR

docosahexaenoic (DHA) 22:6 (4c,7c,10c,13c,16c,19c)

docosapentaenoic (DPA) 22:5 (7c,10c,13c,16c,19c)

Figure 3.7

A69-octadecadienoate rather than linoleate. However, animals need linoleate for the biosynthesis of dihomo-y-linolenate (A81114-eicosatrienoate) and arachidonate (A5'81114-eicosatetraenoate), C20 polyunsaturated fatty acid precursors of prostaglandins in the 'one' and 'two' series respectively (see page 52). Accordingly, linoleic acid must be obtained from plant material in the diet, and it is desaturated towards the carboxyl to yield y-linolenate, which is then used as the substrate for further chain extension, adding a C2 unit from malonate, and producing dihomo-y-linolenate. Arachidonate derives from this by additional desaturation, again towards the carboxyl end of the chain (Figure 3.7). a-Linolenate is similarly a precursor on the way to a5,8,11,14,17-eicosapentaenoate (EPA), required for the synthesis of prostaglandins of the 'three' series, and it is also obtained from the diet. A similar chain extension process using further molecules of mal-onate is encountered in the sequence from a-linolenate in animal systems (Figure 3.7). Chain extension/dehydrogenations lead to formation of eicosapentaenoate (EPA) with further elaborations producing docosapentaenoate (DPA) and then docosahexaenoate (DHA). DHA is a component of lipids in sperm, the retina, and the brain. It is thought to be important for brain development, and deficiency is associated with abnormalities in brain function. Linoleate and a-linolenate are referred to as 'essential fatty acids' (EFAs) since they and their metabolites are required in the diet for normal good health. Some food sources such as the oils present in fish are rich in the later metabolites derived from a-linolenic acid, e.g. EPA and DHA, and are also beneficial to health. Since these fatty acids all have a double bond three carbons from the methyl end of the chain, they are grouped together under the term rn-3 fatty acids (omega-3 fatty acids). Regular consumption of fish oils is claimed to reduce the risk of heart attacks and atherosclerosis.

Although most plant-derived oils contain high amounts of unsaturated fatty acid glycerides, including those of linoleic and a-linolenic acids, the conversion of linoleate into y-linolenate can be blocked or inhibited in certain conditions in humans. This restricts synthesis of prostaglandins. In such cases, the use of food supplements, e.g. evening primrose oil* from Oenothera biennis (Onagraceae), which are rich in y-linolenic esters, can be valuable and help in the disorder. Many plants in the Boraginaceae, e.g. borage (Borago afficinalis), also accumulate significant amounts of y-linolenic acid glycerides, as does evening primrose, indicating their unusual ability

Evening Primrose Oil

Evening primrose oil is extracted from the seeds of selected strains of the evening primrose (oOenothera biennis; Onagraceae), a biennial plant native to North America, which is now widely cultivated in temperate countries. The seeds contain about 24% fixed oil, which has a high content of glycerides of the unsaturated fatty acids linoleic acid (65-80%) and y-linolenic acid (gamolenic acid) (7-14%). Because of this high y-linolenic acid content, evening primrose oil is widely used as a dietary supplement, providing additional quantities of this essential fatty acid, which is a precursor in the biosynthesis of prostaglandins, which regulate many bodily functions (see page 54). Genetic and a number of other factors may inhibit the desaturation of linoleic acid into y-linolenic acid. Ageing, diabetes, excessive alcohol intake, catecholamines, and zinc deficiency have all been linked to inhibition of the desaturase enzyme. The conversion may also be inhibited if there is a high proportion of fatty acids in the diet, which compete for the desaturase enzyme, including saturated and trans-unsaturated fatty acids. The latter group may be formed during the partial hydrogenation of polyunsaturated fatty acids which is commonly practised during food oil processing to produce semi-solid fats. Evening primrose oil appears to be valuable in the treatment of premenstrual tension, multiple sclerosis, breast pain (mastalgia), and perhaps also in eczema. There is potential for further applications, e.g. in diabetes, alcoholism, and cardiovascular disease. In evening primrose, y-linolenic acid is usually present in the form of a dilinoleoylmono-y-linolenylglycerol. This triglyceride is also being explored as a drug material for the treatment of diabetes-related neuropathy and retinopathy. y-Linolenic acid is also found in the fixed oil of other plants, e.g. blackcurrant, comfrey, and borage, and in human milk. Borage oil (starflower oil) from the seeds of Borago officinalis (Boraginaceae) is used in the same way as evening primrose oil. It contains higher concentrations of y-linolenic acid (23-26%), but rather less linoleic acid.

oleic acid

ricinoleic acid

CO2H

pyrolysis

CO2H

undecenoic acid

Figure 3.8

to desaturate linoleic esters towards the carboxyl terminus, rather than towards the methyl terminus as is more common in plants. Arachidonic acid itself has not been found in higher plants, but does occur in some algae, mosses, and ferns.

Ricinoleic acid (Figure 3.8) is the major fatty acid found in castor oil from seeds of the castor oil plant (Ricinus communis; Euphorbiaceae), and is the 12-hydroxy derivative of oleic acid. It is formed by direct hydroxylation of oleic acid (usually esterified as part of a phospholipid) by the action of an O2- and NADPH-dependent mixed function oxidase, but this is not of the cytochrome P-450 type. Castor oil has a long history of use as a domestic purgative, but it is now mainly employed as a cream base. Undecenoic acid (A9-undecenoic acid) can be obtained from ricinoleic acid by thermal degradation, and as the zinc salt or in ester form is used in fungistatic preparations.

Primary amides of unsaturated fatty acids have been characterized in humans and other mammals, and although their biological role is not fully understood, they may represent a group of important signalling molecules. Oleamide, the simple amide of oleic acid, has been shown to be a sleep-inducing lipid, and the amide of erucic acid, eru-camide, stimulates the growth of blood vessels.

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