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The Chemistry of Lipids

WartaIPTEK.com - Lipids are a group of heterogeneous compounds which are classified together because they share the property of insolubility in water and solubility in organic solvents such as ethyl ether, petroleum ether and chloroform. Also, most often somewhere in the molecular structure of lipids there is a fatty acid or fatty acid derivative. Lipids include the following:
  • Triglycerides
  • Phospholipids
  • Sterols and sterol esters
  •  Glycolipids (cerebrosides)
  • Lipoproteins
  • Fat soluble vitamins are also included in this group
Although lipids are present in all cells and tissues, the they are not sufficiently polar to circulate in an aqueous medium such as plasma and are therefore dependent for dispersion and mobility on combination with a carrier protein to form lipoproteins.

Fatty Acids
The fatty acids which usually are found in natural triglycerides in foods and in human lipids have an even number of carbon atoms arranged in a straight chain with a carboxyl group at one end and may be saturated or unsaturated.

The most common fatty acids in mammalian tissues and most vegetables are the longer-chain fatty acids, primarily 16 to 18 carbon atoms in length. These fatty acids may be saturated, monounsaturated or polyunsaturated. Saturated fatty acids have all the four bonds surrounding each carbon atom in the hydrocarbon part of the fatty acid either attached to hydrogen atoms or shared with other carbon atoms. The more common fatty acids are shown in figures 1, 2, and 3.

Stearic acid is found in considerable amounts in triglycerides of adipose tissued and is particularly abundant in cerebrosides. In food lipids, it is prevalent in beef tallow. Monounsaturated oleic acid is probably the predominant fatty acid in nature. Usually mofe than half of the adipose tissue triglyce fatty acid is oleic acid. Olive oil has approximately 85% oleic acid. Linoleic acid is also present in adipose tissue triglycerides and in some species of animals is probably the most abundant fatty acid combined with cholesterol to form cholesterol esters. Most seed and nut oils contain a large amount of linoleic acid, which is the essential FA the body is unable to synthesize. Other important fatty acids include palmitic acid, palmitolic, butyric, lauric, myristic and another essential fatty acid, arachidonic.

Iodine value of FA
Fatty acids in fats will combine with iodine and other halogens in proportion to the number of double bonds they contain. The "iodine number" or "iodine value" of a fat is constant which gives information concerning the degree of unsaturation of fats (Figure 4). Thus coconut oil, which is composed largely of saturated fatty acids has an iodine value of 8 to 11 and polyunsaturated corn oil one of 115 to 129.
Figure 4. Determination of the iodine number of fats (iodine number = number of grams of iodine absorbed by 100 grams of fat)

Triglycerides, also called neutral fats, are fatty acid esters of glycerol. If R is used to designate the hydrocarbon part of the fatty acid residue, a typical triglyceride, which contains of one molecule of glycerol and three molecules of fatty acid has the formula as seen in Figure 5. The fatty acids in the triglyceride molecule may be different. Indentification of the triglyceride may be made by locating the fatty acid on the glyceride molecule.

Figure 5. Triglyceride

Chemical properties
The physical state of the triglycerides depends on the chain length and degree of unsaturation of their fatty acid components, the more unsaturated the fatty acids and the shorter the chain length, the lower the melting point of the fat. Fats which are liquid at room temperature are called oils.

The double bonds of unsaturated fatty acids contained in triglycerides of oils are very susceptible to oxidation, with the resulting formatiea of oxides and peroxides. These oxidized products eventually give rise to shorter-chain fatty acids and aldehydes.

Lipid peroxides are very powerful oxidizing agents that may destroy fat-soluble vitamins which are usually present in fats. However, the spontaneous autooxidation of unsaturated fats, referred to as rancidity, is inhibited by the presence of natural antioxidants in the fats. The most important of these are the tocopherols, which are widely distributed in vegetable oils.

Triglycerides may be hydrolyzed to form glycerol and fatty acids by heating with alkali. Glycerol is released, and the fatty acids are transformed to water-soluble soaps. This saponification of fats may be used to provide information about the average molecular weight of the fatty acids present in the fat.

Saponification can also be effected at body temperature under physiological conditions by Upases (lipolytic enzymes) which are secreted into the small intestine. Here, the hydrolysis of triglycerides takes place in a stepwise fashion with the sequential release of fatty acids. In the lumen of the small intestine where digestion of triglyceride takes place, extensive hydrolysis takes place, forming diglycerides, monoglycerides, free fatty acids and glycerol.

Since triglycerides are fat soluble (water insoluble) and enzymes are water soluble, in order for enzyme-substrate interaction takes place, neutral fat must be emulsified. Phospholipids contain glycerol, two fatty acids, at least one of which is KBsaturated, phosphate and a nitrogeneous base, which may be ethanolamine, serine or most frequently choline. The structure for the phospholipid, lecithin, which is the principal phospholipid in eggs and in the body is seen in figure 6.

Figure 6. The structure for the phospholipid, lecithin

Cholesterol and its products
The main site of cholesterol manufacture and metabolism is the liver. The blie acids and their salts which are also important emulsifying agents for-fat, are major metabolic products of cholesterol, as seen in figure 7. Cholesterol is also synthesized in the animal body from small molecules resulting from the metabolism of other nutrients. It is absorbed from the intestine in its free form, combines with fatty acids in the liver and circulates in the blood, as two thirds cholesterol ester and one third unes-terified cholesterol.

Figure 7. Cholesterol

Are the largest and lightest lipoproteins. They consist of 80 to 95% exogenous triglyceride, 2 to 7% cholesterol, 3 to 6% phospholipid and 1 to 2% protein.

Chylomicrons are formed in the intestine following fat ingestion. They serve primarily to transport exogenous triglycerides to tissue sites for storage and utilization.

After triglycerides are hydrolyzed by lipases in the lumen of the intestine, they are resynthesized in the mucosa and appear in the lymph as chylomicron containing largely long-chained FA triglyceride cores with outer coatings composed of protein, cholesterol and its esters and phospholipid (lecithin). Most of the fatty acids in chylomicron triglycerides are derived from the diet.

Fate of fatty acids
All triglycerides fatty acids may undergo a variety of energy-producting processes, the two main important being fatty acid oxidation and fatty acid synthesis. Fatty acids become available for oxidation in the liver from 3 sources:
1. synthesis in liver from excess dietary carbohydrate
2. hydrolysis of chylomicron triglycerides
3. mobilization of fatty acids from adipose tissue.

Fatty acid oxidation is an energy-producing process which produces in a stepw^i fashion through successive beta-oxidations, The resulting 2-carbon fragments (acetyl CoA's) which are formed are further oxidized through the Krebs cycle to C02, H20 and energy. Both saturated and unsaturated fatty acids may undergo fatty acid oxidation. The rates of oxidation for both of these types of fatty acids are the same.

Linoleic acid
Linoleic acid (C18:2) is an essential fatty acid required for growth and maintenance. The essential dienoic acid, linoleic, cannot be formed by the human body, and so depend on dietary sources for an adequate supply of this fatty acid.

Fatty acid functions
Essential fatty acids and their derivatives are components of cell membranes (in phospholipids), are used to esterify cholesterol in plasma, are functionally important in gonadal tiifaies and are major components of human adrenal cholesterol esters among others.

Essential fatty acids are in some way required for the metabolism of cholesterol and other lipids, since in animal experiments essential fatty acid deficiency results in abnormal accumulations of cholesterol and triglycerides in the liver.

Recently it has been established that essential fatty acids are precursors of the hormone like prostaglandins, physiologically active substances which play important roles in many metabolic processes. [Source: Kartari DS, Puslit Penyakit Tidak Menular, Badan Litbang Dep.Kes, Jakarta]
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