Unsaturated fatty acids are susceptible to oxidation and peroxidation reactions. They can occur when the oil or fat is exposed to atmospheric oxygen or is inside the body and the fats are exposed to respiratory oxygen. These reactions are extremely complex and lead to the formation of various products, the first of these being free radicals which are unstable and cause a breakdown of the fatty acid molecule. Various rearrangements occur within the molecule and various new substances are formed, such as alcohols, ketones and aldehydes. These substances are responsible for the unpleasant odours and taste associated with oxidised fats and oils.
Autoxidation of fatty acids increases with the number of double bonds present in the molecule. The presence of natural or synthetic antioxidants is essential for the prevention of autoxidation of foods containing substantial amounts of polyunsaturated fatty acids. The polyunsaturated fatty acids present in the living cells are exposed to many peroxidation reactions. The free radicals formed during these reactions are similar to those formed during exposure to radioactivity (Fig. 4). The danger of autoxidation is present in human cells and must be constantly protected by natural antioxidant systems consisting of vitamins E and C, and carotene and selenium. Certain other substances act as pro-oxidants. These include copper, iron or their compounds and increase the autoxidation of fatty acids.
Peroxidation reactions associated with polyunsaturated fatty acids, therefore, pose a constant hazard to the food industry as well as to the living cells. Natural and synthetic antioxidants protect against these undesirable reactions. Hydrogenation of vegetable oils and fish oils diminish the risk of peroxidation of foods containing these oils but may increase other dangers.
Fig. 4 Sequence of events associated with the peroxidation of polyunsaturated fatty acids
Fats and oils do not contain any appreciable amount of free fatty acids. They exist in the form of triglycerides consisting of molecules of glycerol and three of fatty acids. Glycerol molecules contain three (3) carbon atoms and each of them is attached to a hydroxyl (-OH) group. These hydroxyl groups react with the carboxyl (-COOH) of three fatty acids to form a triacyl ester molecule. An example of this is given in Fig. 5.
Fig. 5 - Schematic illustration of triglyceride
It can be seen that the three positions (, and ) on the glycerol molecule are occupied by three fatty acids (R1, R2 and R3). If two fatty acids form the esters the molecule is a diglyceride and if one fatty acid forms an ester it is called a monoglyceride. In diglyceride and monoglyceride molecules the unoccupied (-OH) groups retain their original properties.
Dietary oils and fats and depot fats in humans and animals consist of mixed triglycerides, meaning that the positions on the glycerol are occupied by different fatty acids. There are normally twenty fatty acids which form the natural fats and oils; this means there are a great number of chemically different triglycerides that can be found in edible fats and oils. For example, one could formulate at least 15 different triglycerides when three fatty acids such as palmitic, stearic and linoleic are used. Each of these 15 triglycerides would have different physical and biological properties.
The fatty acid distribution on the glycerol molecules in nature does not happen by chance. It is now known that the allocation of fatty acids to positions , or follows a preference. For example, in milk fat, long chain saturated fatty acids tend to be located in the a position whereas the short chain fatty acids are found in the position. Intermediate chain fatty acids are found mainly in the position. Thus butyric acid (C4:0) in cow's milk fat is found mainly in the position and stearic acid (C18:2) in the a position.
In human milk, unsaturated acids occupy mainly the and positions (Table 7).
In fats of animal origin the saturated fatty acids palmitic (C16:0) and stearic (C18:0) are found mainly in the and positions. However, the notable exception is pork fat where palmitic acid (C16:0) is primarily located in the position. Unsaturated fatty acids in the depot fat of man and cattle are preferentially present in the position while in pork fat they are found in high proportions in the and positions.
In vegetable oils, about 70% of linoleic acid (C18:26) content occupies the position while the saturated fatty acids occupy the and positions. In coconut oil, which contains a high proportion of short and medium chain fatty acids, the distribution of different fatty acids resembles that of human milk fat.
The position of the particular fatty acids in the triglyceride gains significance during the digestion and absorption of oils and fats when mono-glycerides and diglycerides are formed. The short and medium chain fatty acids are hydrolysed from and positions and are absorbed into the blood and carried straight to the liver where they are oxidised. The long chain fatty acids, however, are incorporated into triglycerides after absorption and form chylomicrons. The position of the fatty acids is also of significance during the formation of phospholipids in the body, where the enzyme activity is specific for the different positions on the glycerol molecule (3). The polyunsaturated fatty acids are predominantly located on the position of the phospholipid molecule. Similarly the polyunsaturated fatty acids are located mainly at the position of the vegetable oils. They have a significant influence on the metabolism of trans fatty acids produced in hydrogenated vegetable oils.
The phosphoglycerides are found in animals and plants and their nomenclature is based on the name of particular molecules which are linked to the phosphoric acid molecule. Examples of some phosphoglycerides are depicted in Fig. 7.
Fig. 7 - Schematic illustration of some phosphoglycerides
Phosphoglycerides are the commonest of all phospholipids and are found in both animals and plants. The phosphatidylcholines are the lecithins found in soybean oil and are the common emulsifying agents used in the industrial preparation of many foods rich in fat or oil. In other phosphoglycerides, choline is replaced by serine, ethanolamine or inositol. In general the fatty acid R2 in position is unsaturated and most of the time the fatty acid in the position is saturated.
In these phospholipids, glycerol is replaced by sphingosine which contains a chain of 18 carbons on which are found one trans double bond, two hydroxyl groups and one amino group (-NH2). The phosphoric acid is esterified with the hydroxyl group of choline. The structure of sphingolipids is shown in Fig. 8.
Fig. 8 - Schematic illustration of a sphingomyelin molecule
Sphingomyelins are important constituents of the myelin sheath which surrounds nerve axons. The fatty acids R which are part of these molecules are long-chain fatty acids - 18 to 26 carbon atoms. Both the chain length and degree of saturation of the fatty acid R may alter the physical and physiological properties of these substances
Sphingomyelins are important components of the myelin sheath which surrounds the nerve axons. The difference with respect to sphingomyelins lies in the nature of fatty acids R which have a chain length varying from C16 to C24 carbon atoms. Lignoceric acids (C24:0) and nervonic acid (C24:2) are the common ones (4). The brain is rich in sphingolipids. The nature of the fats consumed may influence the ratio of the different fatty acids incorporated in the different sphingolipids of the body. The metabolic consequences of this influence are not well known at present.
The cerebrosides contain a sphingosine molecule joined with a fatty acid on one end and a sugar-like molecule, or more correctly, a galactose molecule joined at the other end (Fig. 9). The fatty acid is always a long chain, usually stearic acid (C18:0). The cerebrosides are important constituents of the myelin sheath.
The gangliosides contain molecules of sphingosine joined with a complex carbohydrate on one end and a fatty acid on the other. The grey matter of the brain is rich in these lipids. The structure of gangliosides is shown in Fig. 10.
Fig. 9 - Schematic illustration of a cerebroside
Cerebrosides are constituents of the myelin sheath of the nerve axons. Stearic acid (C18:0) is the predominant fatty acid R
Fig. 10 - Schematic illustration of a ganglioside
Gangliosides are lipid compounds containing fatty acids, complex carbohydrates and sphingosine. The gray matter of the brain is rich in gangliosides. The nature of the constituent fatty acid R can be influenced by the type of fat consumed in the diet, but the physiological consequences of such a change are not well understood
What is Cholesterol?
Cholesterol is a yellowish-white fatty substance and is a component of all body cells of animals including humans. The group of lipids that are soluble in organic solvents and are converted to water-soluble substances upon hydrolysis with alkali are called saponifiable lipids. However, the extraction of animal tissues with organic solvents may yield an appreciable quantity of lipid material that is resistant to saponification. Such unsaponifiable lipids may include one or more of a variety of substances belonging to a group of crystalline alcohols known as sterols (Greek 'Stereos', solid). In the tissues of vertebrates, the principal sterol is the C27 alcohol cholesterol; its fundamental carbon skeleton is the cyclopentanoperhydrophenanthrene ring. It will be noted that cholesterol has a double bond in the 5,6 position and a hydroxyl group at position 3 (21). The illustrations of cholesterol molecule are given in Fig. 11 and Fig. 12.
Fig. 11 - Schematic illustration of the cholesterol molecule
The cholesterol molecule contains mainly carbon and hydrogen. The carbon atoms are linked to one another to form four rings designated as rings A, B, C and D. There is a hydroxyl group on carbon 3 (Ring A) which reacts with fatty acids to form cholesterol esters. Both cholesterol and cholesterol esters are practically insoluble in water and body fluids. Cholesterol and its fatty acid esters are essential for the proper functioning of body cells
Fig. 12 - The planar nature of the cholesterol molecule
As we understand it, chemical evolution preceded the biological evolution of living organisms as proposed by Darwin. Cholesterol and structurally related sterols of fungi and plants are, as far as we know, not universal in living organisms. We can, therefore, state with certainty that the sterol structure is not essential for life processes as a whole. The appearance of oxygen in the biosphere was essential for the biosynthetic pathway of sterols to develop. Vertebrates without exception synthesise cholesterol, and cholesterol synthesis in these animals is universally present. Perhaps it is not without surprise then that, the more advanced the organism, the more diverse is the role of the sterol molecule.
Fig. 13 - Some derivatives of cholesterol
Cholesterol is the precursor of many substances essential to life, such as bile acids, hormones and Vitamin D3. All body cells must be constantly supplied with cholesterol. This is ensured by a supply from the diet or by its biosynthesis in the liver
Cholesterol is an essential precursor of bile acids, corcosteroide and sex hormones and Vitamin D-derived hormones in all vertebrates (Fig. 13) (5).
Cholesterol in the Human Body
The average adult male weighing about 70kg, contains just about 0.2% or 145 g of cholesterol, out of which about 5.5% or 8 g is present in blood plasma. An average man in western countries would consume close to 0.45 g of cholesterol every day in addition to assorted amounts of related sterol. The average daily synthesis of cholesterol, working at only a fraction of full capacity, might be about 1.0 g, giving a total of 1.45 g. The average daily metabolic requirement should be no more than 350 mg, even less if the recycling efficiency is higher. Man's plight is actually even worse than this manifold excess suggests. Every cell of man's body not only contains cholesterol and has ready access to a large extracellular supply, but also, in addition, every cell can manufacture cholesterol (with the exception of red blood cells). In other words, every body cell must have 26 enzymes responsible for converting acetyl CoA to cholesterol. Such extravagance, on such a grand scale, is virtually unknown elsewhere in the mammalian world. As a result towards the later part of life, and sometimes very much earlier the body may contain more cholesterol than it should. The final irony of all this cholesterol imbalance is that it may lead to undesirable health consequences and the ultimate death of the average human being.
The information above gives a brief description of the lipids which are important in food and nutrition. On the Lipoproteins link I shall describe the transportation and metabolism of the lipids in the body in the form of lipoproteins. With many thanks for your interest.
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