A-linolenic acids

Lipids. Representative fatty acid compositions of the unprocessed triglyceride oils found in the four oilseeds are given in Table 4 (see Fats and FATTY oils). Cottonseed, peanut, and sundower oils are classified as oleic—linoleic acid oils because of the high (>50%) content of these fatty acids. Although the oleic and linoleic acid content of soybean oils is high, it is distinguished from the others by a content of 4—10% of linolenic acid, and hence is called a linolenic acid oil. 

Write properly balanced chemical equations for the oxidation to COg and water of (a) myristic acid, (b) stearic acid, (c) a-linolenic acid, and (d) arachidonic acid. 

Linoleic and a-linolenic acids are the only fatty acids known to be essential for the complete nutrition of many species of animals, including humans, and are known as the nutritionally essential fatty acids. In most mammals, arachidonic acid can be formed from linoleic acid (Figure 23-4). Double bonds can be intro-... 

Figure 23-1. Structure of some unsaturated fatty acids. Although the carbon atoms in the molecules are conventionally numbered—ie, numbered from the carboxyl terminal—the co numbers (eg, co7 in palmitoleic acid) are calculated from the reverse end (the methyl terminal) of the molecules. The information in parentheses shows, for instance, that a-linolenic acid contains double bonds starting at the third carbon from the methyl terminal, has 18 carbons and 3 double bonds, and has these double bonds at the 9th, 12th, and 15th carbons from the carboxyl terminal. (Asterisks Classified as "essential fatty acids.")...

The essential fatty acids in humans are linoleic acid (C-18 2 N-6) and a-linolenic acid (C18 3 N-3). Arachidonic acid (C20 4 N-6) is also essential but can be synthesized from linoleic acid. Administration of 2% to 4% of total daily calories as linoleic acid should be adequate to prevent essential fatty acid deficiency in adults (e.g., infusion of 500 mL of 20% intravenous lipid emulsion once weekly).7 Biochemical evidence of essential fatty acid deficiency can develop in about 2 to 4 weeks in adult patients receiving lipid-free PN, and clinical manifestations generally appear after an additional... 

Dietary polyunsaturated fatty acids (PUFAs), especially the n-3 series that are found in marine fish oils, modulate a variety of normal and disease processes, and consequently affect human health. PUFAs are classified based on the position of double bonds in their lipid structure and include the n-3 and n-6 series. Dietary n-3 PUFAs include a-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) whereas the most common n-6 PUFAs are linoleic acid, y-linolenic acid, and arachidonic acid (AA). AA is the primary precursor of eicosanoids, which includes the prostaglandins, leukotrienes, and thromboxanes. Collectively, these AA-derived mediators can exert profound effects on immune and inflammatory processes. Mammals can neither synthesize n-3 and n-6 PUFAs nor convert one variety to the other as they do not possess the appropriate enzymes. PUFAs are required for membrane formation and function... 

Figure 11.2 Structures of commonly occurring unsaturated fatty acids (i) oleic acid, C18 1 (ii) linoleic acid, C18 2 (iii) a-linolenic acid, C18 3.

Figure 11.12 Outline of the pathway by which a-linolenic acid is converted to eicosapentaenoic (ERA) and docosahexaenoic acid (DHA).

Figure 11.13 The pathways in Figures 11.11 and 11.12 are presented side by side for comparison. The pathways are known as the omega-6 and the omega-3 pathways. Both linoleic and a-linolenic acids are, in general, present in sufficient amounts in the diet of humans to provide adeguate amounts of arachidonic and eicosapentaenoic acids, but the enzymes in the two pathways must be sufficiently active for conversions to occur (see below).

The sources of these fatty acids in the cells are those that are present at position 2 of the membrane phospholipids. The proportion of these two in the phospholipid depends to a large extent on the type of fatty acids in the triacylg-lycerol in the diet, that is, the amount of the omega-6 (lin-oleic acid) and that of the omega-3 (a-linolenic acid). 

The volatiles produced by the LOX pathway and autoxidation are typically volatile aldehydes and alcohols responsible for fresh and green sensorial notes. In the LOX pathway these volatile compounds are produced in response to stress, during ripening or after damage of the plant tissue. The pathway is illustrated in Scheme 7.2. Precursors of the LOX (EC 1.13.11.12) catalysed reactions are Cis-polyunsaturated fatty acids with a (Z,Z)-l,4-pentadiene moiety such as linoleic and a-linolenic acids that are typically oxidised into 9-, 10- or 13-hydro-peroxides depending on the specificity of the LOX catalyst. These compounds are then cleaved by hydroperoxide lyase (HPL) into mainly C, C9 and Cio aldehydes, which can then be reduced into the corresponding alcohols by alcohol dehydrogenase (ADH EC 1.1.1.1) (Scheme 7.2) [21, 22]. The production of volatile compounds by the LOX pathway depends, however, on the plants as they have different sets of enzymes, pH in the cells, fatty acid composition of cell walls, etc. 

Dietary intake is of great importance. Linoleic acid (C18 2o)6) and a-linolenic acid (C18 3o)3) are the parent essential fatty acids for humans. Both fatty acids derive from vegetable oils. Higher fatty acids are then produced by chain elongation and desaturation. In addition, some of the prime essential fatty acids, AA (C20 4o)6), EPA (C20 5w3) and DHA (C22 6w3), can be obtained directly from the diet. Meat and eggs are rich in AA, whereas fish is a rich source of EPA and DHA [14]. 

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