Plants essential fatty acids from

Commodity soybean oil is composed of 61% polyunsaturated fatty acids, 25% monounsaturated fatty acid and 15% saturated fatty acids. The essential fatty acids linoleic (18 2, n-6) and a-linolenic (18 3, n-3) acids account for 89 and 11 % of the total essential fatty acids from this source. The n-6 acid content in soybean oil is slightly lower than that in com and sunflower oils, but it is more than double that in canola oil. Soybean and canola are the only two common plant oils that have a considerable amount of the n-3 linolenic acid. 

Some fatty acids are not synthesized by mammals and yet are necessary for normal growth and life. These essential fatty aeids include llnoleic and y-linolenic acids. These must be obtained by mammals in their diet (specifically from plant sources). Arachidonic acid, which is not found in plants, can only be synthesized by mammals from linoleic acid. At least one function of the essential fatty acids is to serve as a precursor for the synthesis of eicosanoids, such as... 

Drugs of particularly complex structure are often prepared commercially by partial synthesis from some abundant, structurally related, natural product obtained from plants. The majority i)f steroid drugs are in fact prepared in just this way. Prostaglandins are unique in that no prostanoid compounds have yet been found in plants, a perhaps surprising finding in view of the wide distribution of essential fatty acids in plant materials. 

Because they are necessary precursors for the synthesis of other products, linoleate and linolenate are essential fatty acids for mammals they must be obtained from dietary plant material. Once ingested, linoleate may be converted to certain other polyunsaturated acids, particularly y-linolenate, eicosatrienoate, and arachidonate (eicosatetraenoate), all of which can... 

Linoleic acid and linolenic acid are essential fatty acids that cannot be made by animals and must be obtained by dietary intake from plant sources. When metabolized in animals, these two acids each give rise to a family of Cig, C20, and C22 n-6 and n-3 polyunsaturated fatty acids thus ... 

Edible fats and oils (lipids) are derived from plant, animal, and marine sources. Fats and oils differ in that fats are solids at normal room temperature whereas oils are liquids under similar conditions. Lipids are recognized as essential nutrients in both human and animal diets. They provide the most concentrated source of energy of any foods. The caloric value of lipids (9 kcal/g) exceeds twice that of proteins and carbohydrates (4 kcal/g). Lipids not only contribute to flavor, color, odor, and texture of foods, but also confer a feeling of satiety after eating. Lipids also act as carriers of fat-soluble vitamins, supply essential fatty acids, and increase the palatability of foods. Dietary fats are often categorized as visible or invisible ... 

The answer is d. (Murray, pp 258-297. Scriver, pp 2705-2716. Sack, pp 121-138. Wilson, pp 362-367.) Infants placed on chronic low-fat formula diets olten develop skin problems, impaired lipid transport, and eventually poor growth. This can be overcome by including linoleic acid to make up 1 to 2% ol the total caloric requirement. Essential fatty acids are required because humans have only A", A , A , and A fatty acid desaturase. Only plants have desaturase greater than A . Consequently, certain fatty acids such as arachidonic acid cannot be made from scratch (de novo) in humans and other mammals. However, linoleic acid, which plants make, can be converted to arachidonic. acid. Arachidonate and eicosapentaenoate are 20-carbon prostanoic acids that are the starting point of the synthesis of prostaglandins, thromboxanes, and leukotrienes. 

Although most hpids required for cell structure, fuel storage, or hormone synthesis can be synthesized from carbohydrates or proteins, we need a minimal level of certain dietary hpids for optimal health. These hpids, known as essential fatty acids, are required in our diet because we cannot synthesize fatty acids with these particular arrangements of double bonds. The essential fatty acids a-linoleic and a-linolenic acid are supphed by dietary plant oils, and eicosapentaenoic acid (ERA) and docosa-hexaenoic acid (DHA) are supplied in fish oils. They are the precursors of the eicosanoids (a set of hormone-like molecules that are secreted by cells in small quantities and have numerous important effects on neighboring cells). The eicosanoids include the prostaglandins, thromboxanes, leukotrienes, and other related compounds. 

The most abundant and therefore the most common precursor of the eicosanoids is arachidonic acid (eicosatetraenoic acid, w6,20 4,A5,8,ll,14), a polyunsaturated fatty acid with 20 carbons and 4 double bonds (see Fig. 35.1). It is esterified to phospholipids located in the lipid bilayer that constitutes the plasma membrane of the cell. Because arachidonic acid cannot be synthesized de novo in the body, the diet must contain arachidonic acid or other fatty acids from which arachidonic acid can be produced. The major dietary precursor for arachidonic acid synthesis is the essential fatty acid linoleate, which is present in plant oils (see Chapter 33). 

Seaweed has a very little lipid content, ranging from 1% to 5% of dry matter (Khotimchenko, 2005). Neutral lipids and glycolipids are the major lipid classes in all seaweeds, and the proportion of essential fatty acids in seaweed is higher than that in land plants. Seaweed s)mthesizes higher amoimts of polyunsaturated fatty acids (PUFAs) especially imder the cool climates, and the total lipid content is elevated dining the hot seasons (Narayan et ah, 2006). However, the content and the composition of fat can be greatly varied depending on the t) e of seaweed. 

The human body can synthesize all except two of the fatty acids it needs. These two, linoleic acid and linolenic acid, are polyunsaturated fatty acids that contain 18 carbon atoms (Table 8.1). Because they are not synthesized within the body and must be obtained from the diet, they are called essential fatty acids. Both are widely distributed in plant and fish oils. In the body, both acids are used to produce hormonelike substances that regulate a wide range of functions and characteristics, including blood pressure, blood clotting, blood lipid levels, the immune response, and the inflammation response to injury and infection. 

Fatty acid synthase (FAS) constitutes a multisubunit complex in the plastid where it catalyzes ordered synthesis of fatty acids, initiated from acetyl CoA and malonyl CoA [146, 147]. Stepwise FAS activity generates the products 16 0-acyl carrier protein (AGP) and 18 0-ACP. Most of the 18 0-ACP is desaturated by a soluble stearoyl-ACP desaturase, yielding 18 1 D9-ACP [148]. Acyl-ACP thioesterases release AGP from 16 0-AGP and 18 0-AGP the deacylated fatty acids exit the plastid and are then esterified with coenzyme A (GoA) to form respective acyl-GoAs [148]. These acyl moieties are then esterified to phosphatidylcholine (PQ and then undergo desaturation by D12- and D15-desaturases to yield the essential fatty acids, linoleic acid, and a-linolenic acid [148-150], All higher plants have the enzymes for synthesizing the G18 PUFAs linoleic acid and a-linolenic acid. The primary genes involved in PUFA biosynthesis have been reviewed [151],... 

Fig. 56.3 Generalized pathway for the biosynthesis of long-chain PUFA. The conventional A -desaturase/elongase pathway for the synthesis of arachidonic acid and EPA from the essential fatty acids linoleic acid and a-linolenic acid in plants (Source [154])...

In attempting to answer this question, let us first of all consider the extent of variation in the intake of essential fatty acids likely to be encountered in different countries of the world. Data compiled by the Food and Agriculture Organization of the United Nations (1971) indicate that the fat component of human diets may contribute anywhere from just over 10% to more than 40% of the total calories. The various fats and oils that go to make up this dietary fat differ widely in their content of linoleic acid, the major dietary essential fatty acid. The diunsaturated (18 2) fatty acid content of different dietary fats and oils is shown in Table 2. In some cases, this is not all linoleic acid (e.g. butter contains only about 0.8% linoleic acid), but in most cases, the diunsaturated fatty acid can be equated with linoleic acid. Animal fats generally contain less linoleic acid than plant fats, but there is considerable overlap, as seen in Table 2. 

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