CLASSIFICATION OF LIPIDS

CLASSIFICATION OF LIPIDS

The lipids are a group of substances found in plant and animal tissues.They are insoluble in water but soluble in common organic solvents such as benzene, ether and chloroform. They act as electron carriers, as substrate carriers in enzymic reactions, as components of biological membranes, and as sources and stores of energy. In the proximate analysis of foods they are included in the ether extract fraction.

Plant lipids are of two main types: structural and storage. The structural lipids are present as constituents of various membranes and protective surface layers and make up about 7 per cent of the leaves of higher plants.The surface lipids are mainly waxes, with relatively minor contributions from long-chain hydrocarbons, fatty acids and cutin. The membrane lipids, present in mitochondria, the endoplasmic reticulum and the plasma membranes, are mainly glycolipids (40–50 per cent) and phosphoglycerides. Plant storage lipids occur in fruits and seeds and are, predominantly, triacylglycerols. Over 300 different fatty acids have been isolated from plant tissues, but only about seven are of common occurrence. The most abundant is alfa-linolenic acid; the most common saturated acid is palmitic acid and the most common monounsaturated acid is oleic acid.

In animals, lipids are the major form of energy storage, mainly as fat, which may constitute up to 97 per cent of the adipose tissue of obese animals. The yield of energy from the complete oxidation of fat is about 39 MJ/kg DM compared with about 17 MJ/kg DM from glycogen, the major carbohydrate form of stored energy. In



addition, stored fat is almost anhydrous, whereas stored glycogen is highly hydrated. Weight for weight, fat is, therefore, about six times as effective as glycogen as a stored energy source.

The structural lipids of animal tissues, mainly phosphoglycerides, constitute 0.5–1 per cent of muscle and adipose tissue; the concentration in the liver is usually 2–3 per cent.The most important non-glyceride neutral lipid fraction of animal tissue is made up of cholesterol and its esters, which together make up 0.06–0.09 per cent of muscle and adipose tissue.

 

FATS
Fats and oils are constituents of both plants and animals and are important sources of stored energy. Both have the same general structure but have different physical and chemical properties. The melting points of the oils are such that at ordinary
room temperatures they are liquid and they tend to be more chemically reactive than the more solid fats.The term ‘fat’ is frequently used in a general sense to include both groups.As well as its major function of supplying energy, stored fat is important as a thermal insulator and, in some warm-blooded animals, as a source of heat for maintaining body temperature. This is especially important in animals that are born hairless, those that hibernate and those that are cold-adapted. Such animals have special deposits of ‘brown fat’ in which oxidation is uncoupled from adenosine
triphosphate (ATP) production (see Chapter 14) and all the energy is liberated as heat. Palmitate oxidised to produce ATP would yield about 13 MJ/kg as heat, compared with the uncoupled yield of 39 MJ/kg. In these tissues, the mitochondria are
liberally supplied with respiratory electron carriers, particularly cytochromes, which accounts for their brown colour.

Structure of fats
Fats are esters of fatty acids with the trihydric alcohol glycerol; they are also referred to as glycerides or acylglycerols.When all three alcohol groups are esterified by fatty acids, the compound is a triacylglycerol (triglyceride):



It is important to appreciate that, in stereochemical terms, the positions occupied by the acid chains are not identical. Under the stereospecific numbering system the positions are designated sn-1, sn-2 and sn-3, as shown.They are readily    distinguished  by enzymes and this may lead to preferential reactivity at one or more of the positions. Phosphorylation, for example, always takes place at carbon atom sn-3 rather than at carbon atom sn-1. Although triacylglycerols are  predominant, mono- and diacylglycerols do occur naturally, but in much smaller amounts.

Triacylglycerols differ in type according to the nature and position of the fatty acid residues. Those with three residues of the same fatty acid are termed simple triacylglycerols, as illustrated above.When more than one fatty acid is concerned in the esterification, a mixed triacylglycerol results:



R1, R2 and R3 represent the chains of different fatty acids. Naturally occurring fats and oils are mixtures of such mixed triacylglycerols. Soya bean oil has been estimated to contain about 79 per cent of mixed triacylglycerols compared with about 21 per cent of the simple type. Comparable figures for linseed oil are 75 and 25 per cent, respectively. Triacylglycerols with residues of one fatty acid only do occur naturally; laurel oil, for example, contains about 31 per cent of the triacylglycerol of lauric acid.

Most of the naturally occurring fatty acids have an even number of carbon atoms, which is to be expected in view of their mode of formation (see Chapter 9).The majority contain a single carboxyl group and an unbranched carbon chain, which may be saturated or unsaturated. The unsaturated acids contain one (monoenoic), two (dienoic), three (trienoic) or many (polyenoic) double bonds. Fatty acids with more than one double bond are frequently referred to as polyunsaturated fatty acids (PUFA). The unsaturated acids possess different physical and chemical properties from the saturated acids: they have lower melting points and are more chemically reactive.
The presence of a double bond in a fatty acid molecule means that the acid can exist in two forms, depending upon the spatial arrangement of the hydrogen atoms attached to the carbon atoms of the double bond. When the hydrogen atoms lie on the same side of the double bond, the acid is said to be in the cis form, whereas it is said to be in the trans form when the atoms lie on opposite sides, as shown here:



Most naturally occurring fatty acids have the cis configuration.

The fatty acids are named by replacing the final -e of the name of the parent hydrocarbon by the suffix -oic. Thus, a saturated 18-carbon acid would be named octadecanoic after the parent octadecane.An 18-carbon acid with one double bond would be octadecenoic after octadecene. The position of the double bond is indicated by reference to the carboxyl carbon atom (carbon atom 1). Thus, 9-octadecenoic acid would have 18 carbon atoms and a double bond between carbon atoms 9 and 10. Similarily, 9,12,15-octadecatrienoic acid would have 18 carbon atoms and double bonds between carbon atoms 9 and 10, 12 and 13, and 15 and 16. The names may be abbreviated by stating the number of carbon atoms followed by a colon, followed by the number of double bonds (⌬), the positions of which are stated as a superscript. Thus, octadeca-
trienoic acid would be designated 18:3⌬9,12,15. Alternatively it may be written 9,12, 15-18:3. Carbon atoms 2 and 3 are designated alpha (␣) and beta (␤), respectively, and the methyl carbon at the distal end of the chain as the omega (␻) carbon atom. In nutritional work, the unsaturated acids are frequently named in relation to the terminal methyl as carbon atom 1. Under this system 9,12,15-octadecatrienoic acid would become ␻-3,6,9-octadecatrienoic acid, since carbon atoms 3, 6 and 9 correspond to carbon atoms 16, 13 and 10 under the former system.The abbreviated designation would be ␻-3,6,9-18:3. It has become common practice to use n instead of ␻ and we then have n-3,6,9-18:3 and frequently 18:3(n-3). In addition the configuration of the double bonds is indicated by the use of the prefixes cis and trans.Thus, ␣-linolenic acid would be all cis-9,12,15-octadecatrienoic, or more simply all cis 9,12,15-18:3.

For certain purposes the PUFA are grouped into families, based on oleic (n-9-18:1), linoleic (n-6,9-18:2) and ␣-linolenic (n-3,6,9-18:3) as precursors.The families are called omega-9 (␻-9), omega-6 (␻-6) and omega-3 (␻-3), referring to the positions of the double bonds nearest to the omega carbon atom in these acids. Again, n is frequently sub-
stituted for ␻.

Two low-molecular-weight saturated fatty acids, namely butyric (C3H7.COOH) and caproic (C4H10.COOH), are found in significant amounts in the milk fats of ruminants, and caproic along with caprylic acid is present in a few oils such as palm kernel and coconut. Other fatty acids containing two carboxyl groups, odd numbers of carbon atoms and branched chains have been isolated from natural fats, but they are not considered to be of great importance.

Triacylglycerols are named according to the fatty acids they contain, e.g:



The fatty acid residues are not distributed randomly between the alcohol groups of the parent glycerol. Thus, in cow’s milk fat, for example, the short-chain acids are concentrated at position 3. In human milk fat, the unsaturated acids are predomi-
nantly at position 1 and the saturated acids at position 2. Animal depot fats tend to have saturated acids at position 1 and unsaturated and short-chain acids at position 2; PUFA tend to accumulate at position 3.

There is evidence that the configuration of the constituent triacylglycerols of fats can influence the extent to which they are digested.Thus, palmitate (hexadecanoate) distributed randomly throughout the 1, 2 and 3 positions was found to be less
digestible than that which occupied position 2, the favoured position for attack by pancreatic lipase.

The fatty acid composition of the triacylglycerols determines their physical nature. Those with a high proportion of low-molecular-weight (short-chain) and unsaturated acids have low melting points. Thus, tristearin is solid at body temperature whereas triolein is liquid.

Composition of fats
It is frequently important in nutritional investigations to assess the quality of the fat being produced under a certain treatment.When the effect of the diet is considerable, the results may be obvious in a softening or hardening of the fat. Less obvious changes may occur, and for these a more objective assessment is necessary. Differences between fats are a function of their fatty acid composition since glycerol is common to all fats. The logical method of following changes in fats is, therefore, to measure their fatty acid constitution. Analysis of fats for individual fatty acids has

presented great problems in the past, but the introduction of techniques such as gas chromatography has allowed determinations to be made more easily and accurately. As well as its major role as an energy source, fat has a vital role in providing individual fatty acids with specific nutritional roles within the animal body. Information on fatty acid composition is, therefore, a prerequisite in the evaluation of fats in this context.

Some typical values for a number of important fats and oils are given in Table 3.2. In general, plant and marine oils, especially those of fish, are more highly unsaturated than those of mammalian origin. This is because of the presence of varying amounts of linoleic and linolenic acids in addition to the monounsaturated oleic (cis9-octadecenoic) acid, which is quantitatively the major fatty acid in most natural fats. In addition, the fish oils have significant concentrations of highly unsaturated C20 and C22 acids. In mammalian depot fat, the proportion of the more unsaturated acids is lower and there is a higher proportion of high-molecular-weight saturated acids such as palmitic and stearic acids, with smaller but significant contributions from lauric (dodecanoic) and myristic (tetradecanoic) acids. For this reason, fats such as pig lard, and beef and mutton tallow are firm and hard, whereas fish and plant oils are softer and frequently are oils in the true sense.
Within individual animals, subcutaneous fats contain a higher proportion of unsaturated acids and are thus softer than deep-body fat. The physical nature of fat varies between animals, marine mammals having softer body fat than land mammals. The reason in both cases is that animal fat has to maintain a degree of malleability at the temperature of the tissue, which is influenced by ambient temperatures. Thus, the fats of the feet and ears, which are inclined to be colder than the interior of the body, tend to be unsaturated.

Ruminant milk fats are characterised by their high content of low-molecularweight fatty acids, these sometimes forming as much as 20 per cent of the total acids present. As a result they are softer than the depot fats of the respective animals but
not as soft as fats of vegetable and marine origin, being semi-solid at ordinary temperatures. Milk fats of non-ruminants resemble the depot fat of the particular animal.

In most commercially important edible plant oils, the dominant fatty acids are oleic, linoleic and linolenic acids. Coconut oil is an exception in having the saturated 12:0 lauric acid as its major acid. Families of plants tend to produce characteristic
oils that frequently contain unusual fatty acids. Examples are the erucic acid of rapeseed; ricinoleic acid, the 18-carbon, monoenoic, hydroxy acid of the castor bean; and vernolic acid, the 18-carbon, trienoic, epoxy acid of the Compositae.

Essential fatty acids
In 1930, linoleic (cis, cis-9,12-octadecadienoic) acid was shown to be effective in preventing the development of certain conditions in rats given diets almost devoid of fat. These animals showed a scaly appearance of the skin and suboptimal performance in growth, reproduction and lactation; eventually they died as a result of the deficient diet. More recent work has demonstrated a wide range of symptoms in a variety of animals, including some in human beings under certain circumstances (Table 3.3).

Arachidonic (all cis 5,8,11,14-eicosatetraenoic) acid has been shown to have equivalent or even greater activity than linoleic acid, and linolenic (all cis 9,12,15octadecadienoic) acid is about 1.5 times as effective as linoleic acid. Mammals cannot
synthesise fatty acids with double bonds closer than carbon atom 9 from the terminal methyl group. Such acids have to be supplied in the diet. Linoleic acid (18:2n-6) and ␣-linolenic acid (18:3 n-3) are thus dietary essentials. Arachidonic acid is synthesised in the body from linoleic acid. However, one of the steps in the synthesis, a ⌬-6 desaturation, is rate-limiting and production may be slow and an exogenous supply advantageous (see Box 3.1). Linoleic and ␣-linolenic acids are referred to as the essential fatty acids (EFA). Like other polyunsaturated acids, they form part of various membranes and play a part in lipid transport and certain lipoprotein enzymes. In addition, they are the source materials for the synthesis of the eicosanoids. These include the prostaglandins, thromboxanes and leukotrienes, hormone-like substances that regulate

Symptoms associated with essential fatty acid deficiencies

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Growth retardation
Increased permeability to water and increased water consumption
Increased susceptibility to bacterial infections
Sterility
Less stable biomembranes
Capillary fragility
Kidney damage, haematuria and hypertension
Decreased visual acuity
Decreased myocardial contractility
Decreased ATP synthesis in liver and heart
Decreased nitrogen retention
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Relationship between the essential fatty acids and the eicosanoids.


many functions, including blood clotting, blood pressure, smooth muscle contraction and the immune response. They are also the source of other important C20 acids in the form of eicosapentaenoic (EPA), hydroxy-eicosatrienoic (HETrR) and docosahexaenoic (DHA) acids. All are involved in maintaining the fluidity of mammalian cell membranes. EPA is the precursor of the 3-series of prostaglandins and thromboxanes and the 5-series of leukotrienes. DHA is thought to play an important role in brain and retinal function, and EPA and HETrR have a modulating effect on the production of  eicosanoids from arachidonic acid.

The 1- and 3-series prostaglandins are anti-inflammatory and inhibit platelet aggregation, whereas the 2-series are pro-inflammatory and pro-aggregatory.The 1- and 3-series thromboxanes mildly stimulate platelet aggregation and stimulate the contraction of respiratory, intestinal and vascular smooth muscle, as do the leukotrienes. The 2-series thromboxanes have a much more powerful action in this respect.

As a general rule, mammals are considered to have an EFA requirement of 3 per cent of the energy requirement (3en%) as linoleic acid, although estimates have ranged from 1 per cent to 15 per cent. Estimates for individual species have been
more specific. Thus, The Nutrient Requirements of Pigs (see Further reading, Chapter 12) gives the requirements of pigs under 30 kg liveweight as 3en% as linoleic acid or 2en% as arachidonic acid. For pigs of 30–90 kg, the figures are 1.5en% as
linoleic acid and 1en% as arachidonic acid.

The oilseeds are generally rich sources of linoleic acid, and linseed is a particularly good source of ␣-linolenic acid. Pigs and poultry, which normally have considerable quantities of oilseed residues in their diets, will, therefore, receive an adequate supply of the essential fatty acids.

Ruminant animals are largely dependent on grasses and forages for their nutritional needs and are thereby supplied with liberal quantities of linoleic and ␣-linolenic acids. Although considerable hydrogenation of unsaturated acids to saturated takes place in the rumen, with consequent overall reduction of EFA supply (on average 85–95 per cent is lost
between the mouth and the small intestine), the possibility of ruminants having a deficiency is remote. A certain proportion of dietary EFA escapes hydrogenation (approximately 5–15 per cent of dietary intake) and this, allied to very efficient utilisation and conservation of EFA by ruminants, is enough to ensure adequacy under normal conditions. EFA deficiency is rare in human beings although, under certain conditions, it does occur in infants, elderly people and people taking drugs that inhibit lipid absorption.

Properties of fats


Hydrolysis

Fats may be hydrolysed by boiling with alkalis to give glycerol and soaps:


Such a hydrolysis is termed saponification since it produces soaps, which are sodium and potassium salts of the fatty acids.The process of fat breakdown may take place naturally under the influence of enzymes, collectively known as lipases, when it is termed lipolysis.The enzymes may have a certain specificity and preferentially catalyse hydrolysis at particular positions in the molecule. Removal of the fatty acid residue attached to carbon atom 2 of an acylglycerol is more difficult than those at positions 1 and 3. Under natural conditions, the products of lipolysis are usually mixtures of mono- and
diacylglycerols with free fatty acids. Most of these acids are odourless and tasteless, but some of the lower ones, particularly butyric and caproic, have extremely powerful tastes and smells; when such a breakdown takes place in an edible fat, it may frequently be rendered completely unacceptable to the consumer. The lipases are mostly derived from bacteria and moulds, which are chiefly responsible for this type of spoilage, commonly referred to as rancidity. Extensive lipolysis of dietary fats takes place in the duodenum and during their absorption from the small intestine. Lipolysis also precedes the hydrogenation of fats in the rumen, and the oxidation of fats in the body.

Oxidation
The unsaturated fatty acids readily undergo oxidation at the carbon atom adjacent to the double bond to form hydroperoxides:


These break down to give shorter-chain products, including free radicals, which then attack other fatty acids much more readily than does the original oxygen. More free radicals are produced, with the result that the speed of the oxidation increases exponentially. Eventually the concentration of free radicals becomes such that they react with each other and the reaction is terminated. Such a reaction, in which the products catalyse the reaction, is described as autocatalytic. This particular reaction is an autoxidation. The formation of the free radicals is catalysed by ultraviolet light and certain metal ions, particularly copper, and the presence of either increases the rate of oxidation dramatically.

The products of oxidation include shorter-chain fatty acids, fatty acid polymers, aldehydes (alkanals), ketones (alkanones), epoxides and hydrocarbons.The acids and alkanals are major contributors to the smells and flavours associated with oxidised fat, and they significantly reduce its palatability. The potency of these compounds is typified by deca-2,4 dienal, which is detectable in water at concentrations of as little as 1 in 10 000 million.

Oxidation of saturated fatty acids results in the development of a sweet, heavy taste and smell commonly known as ketonic rancidity. This is due to the presence of the methyl ketones resulting from the oxidation, which may be represented as
follows:



Similar reactions following mould-induced lipolysis are responsible for the characteristic flavours of various soft and blue cheeses.

Antioxidants
Natural fats possess a certain degree of resistance to oxidation, owing to the presence of compounds termed antioxidants. These prevent the oxidation of unsaturated fats until they themselves have been transformed into inert products. A
number of compounds have this antioxidant property, including phenols, quinones, tocopherols, gallic acid and gallates. In the European Union, propyl, octyl or dodecyl-gallate, butylated hydroxyanisole, butylated hydroxytoluene and
ethoxyquin may be added to edible oils as antioxidants in amounts specified in the EC Community Register of Feed Additives 2009. Other substances such as synthetic ␣-, ␥- and ␦-tocopherols and various derivatives of ascorbic acid may be used without limit.

The most important naturally occurring antioxidant is vitamin E, which protects fat by preferential acceptance of free radicals.The possible effects of fat oxidation in diets in which vitamin E levels are marginal are of considerable importance.

Hydrogenation
This is the process whereby hydrogen is added to the double bonds of the unsaturated acids of a fat, thereby converting them to their saturated analogues. Oleic acid, for example, yields stearic acid:


The process (hardening) is important commercially for producing firm hard fats from vegetable and fish oils in the manufacture of margarine. The hardening results from the higher melting point of the saturated acids. For the rate of reaction to be practicable, a catalyst has to be used, usually finely divided nickel. Hardening has the added advantage of improving the keeping quality of the fat, since removal of the double bonds eliminates the chief centres of reactivity in the material.

Dietary fats consumed by ruminants first undergo hydrolysis in the rumen and this is followed by progressive hydrogenation of the unsaturated free fatty acids (mainly 18:2 and 18:3 acids) to stearic acid.This helps to explain the apparent anomaly that, whereas their dietary fats are highly unsaturated, the body fats of ruminants are highly saturated.

Hydrogenation results in the production not only of saturated acids but also of trans acids. In addition a redistribution of double bonds within the fatty acid chain takes place, accounting for the presence in ruminant fats of vaccenic (trans-11,18:1)
and elaidic (trans-9,18:1) acids. A similar transformation occurs in the industrial hydrogenation of plant and fish oils. Partially hydrogenated vegetable oils, for example, commonly contain 3–5 g trans acids/100 g of the total fatty acids, and partially hydrogenated fish oils about 20 g.

Digestion, absorption and metabolism of the trans acids is comparable with that of their counterparts. They have higher melting points than their cis analogues and their incorporation into ruminant body fats contributes to the hardness of the latter. Trans acids do not possess essential fatty acid activity, but there is evidence that some may enter pathways leading to eicosanoid formation and give rise to substances of unknown physiological effects. There is evidence, too, that they decrease
the activity of the desaturases involved in EFA metabolism. However, it would appear that, as long as EFA intake is adequate and trans acids intake is not excessive, they do not have any significant effect on EFA status. Trans fatty acids, particularly
those produced from the partial hydrogenation of vegetable oils (PHVO), have also been associated with an increased risk of cardiovascular disease, cancer, inflammation and type II diabetes. This has led in the USA to the requirement for the trans
fatty acid content of food to be included on the labelling, with a view to eliminating trans fatty acids from the human diet. The profile of trans fatty acids in ruminant products is, however, quite different from that of PHVO, and there is evidence that some of these, such as trans-11, 18:1 (vaccenic acid) and cis-9,trans-11 conjugated linoleic acid (rumenic acid), which are found in ruminant milk and meat, have beneficial effects on reducing diseases such as cancer and atherosclerosis.

 

GLYCOLIPIDS
In these compounds two of the alcohol groups of the glycerol are esterified by fatty acids and the other is linked to a sugar residue. The lipids of grasses and clovers, which form the major part of the dietary fat of ruminants, are predominantly (about 60 per cent) galactolipids. Here the sugar is galactose and we have:


Galactolipids
The galactolipids of grasses are mainly of the monogalactosyl type illustrated above, but smaller quantities of the digalactosyl compounds are also present.These have two galactose residues at the first carbon atom.The fatty acids of the galactosides of grasses and clovers consist largely of linoleic and ␣-linolenic acids, as shown in Table 3.4.

Rumen microorganisms are able to break down the galactolipids to give galactose, fatty acids and glycerol. Preliminary lipolysis appears to be a prerequisite for the galactosyl glycerides to be hydrolysed by the microbial galactosidases.

In animal tissues, glycolipids are present mainly in the brain and nerve fibres.The glycerol of the plant glycolipids is here replaced as the basic unit by the nitrogenous base sphingosine:


 

Fatty acid composition of some forage lipids (g/100 g)


In their simplest form, the cerebrosides, the glycolipids have the amino group of the sphingosine linked to the carboxyl group of a long-chain fatty acid and the terminal alcohol group to a sugar residue, usually galactose. The typical structure is:


More complex substances, the gangliosides, are found in the brain. They have the terminal alcohol group linked to a branched chain of sugars with sialic acid as the terminal residue of at least one of the chains.

PHOSPHOLIPIDS
The role of the phospholipids is primarily as constituents of the lipoprotein complexes of biological membranes. They are widely distributed, being particularly abundant in the heart, kidneys and nervous tissues. Myelin of the nerve axons, for
example, contains up to 55 per cent of phospholipid. Eggs are one of the best animal sources and, among the plants, soya beans contain relatively large amounts. The phospholipids contain phosphorus in addition to carbon, hydrogen and oxygen.

Phosphoglycerides
These are esters of glycerol in which only two of the alcohol groups are esterified by fatty acids, with the third esterified by phosphoric acid.The parent compound of the phosphoglycerides is, thus, phosphatidic acid, which may be regarded as the simplest phosphoglyceride.


Phosphoglycerides are commonly referred to as phosphatides. In the major biologically important compounds, the phosphate group is esterified by one of several alcohols, the commonest of which are serine, choline, glycerol, inositol and ethanolamine. The chief fatty acids present are the 16-carbon saturated and the 18-carbon saturated and monoenoic, although others with 14–24 carbon atoms do occur. The most commonly occurring phosphoglycerides in higher plants and animals are the lecithins and the cephalins.

Lecithins
Lecithins have the phosphoric acid esterified by the nitrogenous base choline and are more correctly termed  phosphatidylcholines. A typical example would have the formula:


The fatty acid residues at sn-1 are mostly palmitic (16:0) or stearic (18:0) acid. At sn-2 they are primarily oleic (18:1), linoleic (18:2) or ␣-linolenic (18:3) acid.

Cephalins
Cephalins differ from the lecithins in having ethanolamine instead of choline and are correctly termed phosphatidylethanolamines. Ethanolamine has the following formula:


The fatty acids at sn-1 are the same as in lecithin, but those at sn-2 are unsaturated, mainly linoleic, eicosatetraenoic and docosahexaenoic acid.

Phosphoglycerides are white waxy solids that turn brown when exposed to the air, owing to oxidation followed by polymerisation.When placed in water, the phosphoglycerides appear to dissolve. However, the true solubility is very low, the apparent solubility being due to the formation of micelles.

Phosphoglycerides are hydrolysed by naturally occurring enzymes, the phospholipases, which specifically cleave certain bonds within the molecule to release fatty acids, the phosphate ester, the alcohol and glycerol.The release of choline, when followed by further oxidative breakdown, has been considered to be responsible for the development of fishy taints by the release of the trimethyl amine group or its oxide; currently these taints are considered to be the result of fat oxidation and not of lecithin breakdown.

The phosphoglycerides combine within the same molecule both the hydrophilic (water-loving) phosphate ester groups and the hydrophobic fatty acid chains. They are therefore surface-active and play a role as emulsifying agents in biological systems, for example in the duodenum. Their surface-active nature also explains their function as constituents of various biological membranes.

Sphingomyelins
Sphingomyelins belong to a large group, the sphingolipids, which have sphingosine instead of glycerol as the parent  material.They differ from the cerebrosides in having the terminal hydroxyl group linked to phosphoric acid instead of a sugar residue.The phosphoric acid is esterified by either choline or ethanolamine. The sphingomyeli also have the amino group linked to the carboxyl group of a long-chain fatty acid by means of a peptide linkage:


Like the lecithins and cephalins, the sphingomyelins are surface-active and are important as components of membranes, particularly in nervous tissue. They may constitute up to 25 per cent of the total lipid in the myelin sheath that protects the
nerve cells, but they are absent from, or present only in very low concentrations in, energy-generating tissue.

Ether phospholipids
Ether phospholipids are glycerol-based but have an alkyl rather than an acyl group at carbon atom 1, as is the case in the glycerides.Typical are the plasmologens, which have a vinyl ether grouping as shown here:


Such compounds may form up to 50 per cent of the phospholipids of heart tissue, but their function is unclear.An ether phospholipid called platelet activating factor is a highly potent aggregator of blood platelets.

WAXES
Waxes are simple, relatively non-polar lipids consisting of a long-chain fatty acid combined with a monohydric alcohol of high molecular weight. They are usually solid at ordinary temperatures. The fatty acids present in waxes are those found in
fats, although acids lower than lauric acid are very rare; higher acids such as carnaubic (C23H47.COOH) and mellissic (C30H61.COOH) acid may also be present.The most common alcohols found in waxes are carnaubyl (C24H49.OH) and cetyl (C16H33OH) alcohol.

Natural waxes are usually mixtures of a number of esters. Beeswax is known to consist of at least five esters, the main one being myricyl palmitate:


Waxes are widely distributed in plants and animals, where they often have a protective function. The hydrophobic nature of the wax coating reduces water losses caused by transpiration in plants, and provides wool and feathers with waterproofing in Steroids

animals. Among better-known animal waxes are lanolin, obtained from wool, and spermaceti, a product of marine animals. In plants, waxes are usually included in the cuticular fraction, where they form a matrix in which cutin and suberin are embedded. The term wax is used here in the collective sense and, although true waxes are always present, the major part is made up of a complex mixture of substances.Alkanes (from C21 to C37) make up a large proportion of the whole, with odd-chain compounds predominating. Branched-chain hydrocarbons, aldehydes, free fatty acids (from C12 to C36) and various ketols are commonly occurring though minor constituents. Free alcohols are usually of minor importance but may form up to half of some waxes.

Cutin is a mixture of polymers of C16 and C18 monomers, commonly 16-hydroxypalmitic and 10,16-dihydroxypalmitic acids. Phenolic constituents such as paracoumaric and ferulic acids are usually present, but in small amounts only. Suberin is found in the surfaces of the underground parts of plants and on healed wound surfaces.The major aliphatic constituents are ␻-hydroxy acids, the corresponding dicarboxylic acids and very-long-chain acids and alcohols. There are also substantial
amounts of phenolic substances, mainly p-coumaric acid, which form a phenolic core to which the acids are attached. Both cutin and suberin are highly resistant to breakdown and are not of any significant nutritional value.The waxes, too, are resistant to breakdown and are poorly utilised by animals. Their presence in foods in large amounts leads to high ether extract figures and may result in the nutritive value being overestimated.

STEROIDS
The steroids include such biologically important compounds as the sterols, the bile acids, the adrenal hormones and the sex hormones. They have a common structural unit of a phenanthrene nucleus linked to a cyclopentane ring (Fig. 3.3).

The individual compounds differ in the number and positions of their double bonds and in the nature of the side chain at carbon atom 17.

Sterols
These have eight to ten carbon atoms in the side chain, an alcohol group at carbon atom 3, but no carbonyl or carboxyl groups. They may be classified into:


 Basic steroid structural unit.

The phytosterols and the mycosterols are not absorbed from the gut and are not found in animal tissues.

Cholesterol
Cholesterol is a zoosterol that is present in all animal cells. It has a low solubility in water, about 0.2 mg/100 ml. It is the major sterol in human beings and is important as a constituent of various biological membranes. It is particularly important in the myelinated structures of the brain and central nervous system and may constitute up to 170 g/kg. It is the precursor of the steroid hormones. It is also the precursor of the bile acids.

Normal concentrations in the blood plasma are in the range 1200–2200 mg/l. Some 30 per cent of this is in the free state, the remainder being bound to lipoproteins. These are complexes of proteins and lipids held together by non-covalent bonds. Each has a characteristic size, molecular weight, chemical composition and density. They are classified on the basis of their density. The five classes, of which one, the chylomicrons, occurs only in the post-absorptive state, are shown in Table 3.5.

In the plasma, the lipoproteins exist as spherical structures with a core of triacylglycerols and cholesterol esters. This is surrounded by a shell, about 20 Å thick, containing proteins, unesterified cholesterol and phosphatidylcholines. Since they have a greater surface to volume ratio, the smaller particles have a higher protein to lipid ratio and are more dense.Thus, the HDLP fraction has about 45 per cent protein and 55 per cent lipid, whereas the VLDLP fraction has about 10 per cent protein and 90 per cent lipid. Cholesterol is very insoluble and prolonged high levels in blood result in its deposition on the walls of the blood vessels. These deposits eventually harden to atherosclerotic plaque. This narrows the blood vessel and serves as a site for clot formation and may precipitate myocardial infarction or heart attack.

There is strong evidence that the risk of coronary heart disease is directly related to the plasma concentration of  LDL-cholesterol and inversely related to that of HDL-cholesterol, and that the risk is reduced significantly by lowering elevated serum cholesterol levels. It has been known for many years that one of the most important dietary factors regulating serum cholesterol levels is the ratio of polyunsaturated fatty acids (PUFA) to saturated fatty acids (SFA). The SFA increase and the PUFA decrease cholesterol levels, except for the trans PUFA, which have a similar effect to the SFA.A ratio of 0.5–0.9 SFA : PUFA is considered to be satisfactory. It is

 Density-based classes of lipoproteins


important to appreciate that the different families of PUFA affect lipid metabolism in different ways.Thus, the ␻-6 acids significantly decrease serum cholesterol levels and have a minor effect only on triacylglycerol levels, whereas the ␻-3 acids have a minor effect on serum cholesterol but significantly lower triacylglycerol levels.This is important in the light of recent evidence that high serum triacylglycerol level per se is an important risk factor in coronary heart disease.The ␻-3 acids are the precursors of the 3-series of prostaglandins and thromboxanes. The former strongly inhibit platelet aggregation and the latter are weakly pro-aggregating.The ␻-6 acids are precursors of the 2-series of prostaglandins and thromboxanes, the former being strongly pro-aggregating and the latter weakly anti-aggregating. On balance, from this point of view, the ␻-3 acids may be regarded as having a more beneficial effect than the ␻-6 acids.They have a further beneficial effect in that they inhibit the transformation of the ␻-6 acids to their eicosanoid products.

 

Dehydrocholesterol
This substance, which is derived from cholesterol, is important as the precursor of vitamin D3, which is produced when the sterol is exposed to ultraviolet light

 

 Formation of vitamin D3.




This is a good illustration of how relatively small changes in chemical structure may bring about radical changes in physiological activity.

Ergosterol
This phytosterol is widely distributed in brown algae, bacteria and higher plants. It is important as the precursor of ergocalciferol or vitamin D2, into which it is converted by ultraviolet irradiation.The change is the same as that which takes place in the formation of vitamin D3 from 7-dehydrocholesterol and involves opening of the second phenanthrene ring.

Bile acids
The bile acids have a five-carbon side chain at carbon atom 17 which terminates in a carboxyl group bound by an amide linkage to glycine or taurine

 Glycocholic acid



The bile acids are synthesised from cholesterol and this constitutes the major end point of cholesterol metabolism. Under physiological conditions the acids exist as salts. They are produced in the liver, stored in the gall bladder and secreted into the upper small intestine. They are important in several ways:

Steroid hormones
These include the female sex hormones (oestrogens), the male sex hormones (androgens) and progesterone, as well as cortisol, aldosterone and corticosterone, which are produced in the adrenal cortex.The adrenal hormones have an important role in the control of glucose and fat metabolism.

TERPENES
Terpenes are made up of a number of isoprene units linked together to form chains or cyclic structures. Isoprene is a five-carbon compound with the following structure:


Many terpenes found in plants have strong characteristic odours and flavours and are components of essential oils such as lemon or camphor oil. The word ‘essential’ is used to indicate the occurrence of the oils in essences and not to imply that they are required by animals. Among the more important plant terpenes are the phytol moiety of chlorophyll, the carotenoid pigments, plant hormones such as giberellic acid and vitamins A, E and K. In animals, some of the coenzymes, including those of the coenzyme Q group, are terpenes.