Structure Function and Nutritional Requirements

Omega-6 (n-6) fatty acids are a class of polyunsaturated fatty acids (PUFA). They have two or more cis double bonds, with the position of the first double bond six carbon atoms from the methyl end of the molecule. The general formula of n-6 fatty acids is CH3 (CH2)4(CH=CHCH2)x(CH2)yCOOH [where x = 2-5]. Linoleic acid (cis-9, cis-12-octadecadienoic acid, 18:2n-6, LA) and a-linolenic acid (cis-9, cis-12, cis-15-octadecatrienoic acid, 18:3n-3, ALA) are the precursor fatty acids of the n-6 and omega-3 (n-3) fatty acids, respectively. These two fatty acids cannot be made by mammals and are therefore termed essential fatty acids (EFA). In addition, mammals are unable to interconvert LA and ALA, or any of the n-6 and n-3 fatty acids, because mammalian tissues do not contain the necessary desaturase enzyme. Plant tissues and plant oils tend to be rich sources of LA. ALA is also present in plant sources such as green vegetables, flaxseed, canola, and some nuts. Once consumed in the diet, LA can be converted via chain elongation and desaturation to 7-linolenic acid (GLA, 18:3n-6), dihomo-7-lino-lenic acid (DGLA, 20:3n-6), and arachidonic acid (AA, 20:4n-6) (Figure 1). The same enzymes involved in elongation and desaturation of the n-6 fatty acids are common to the n-3 series of fatty acids (Figure 1). Thus, ALA can be converted to eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). EPA and DHA are found in relatively high proportions in marine oils.

The n-6 and n-3 fatty acids are metabolically and functionally distinct and often have important opposing physiological functions. Indeed, the balance of EFA is important for good health and normal development. Historically, human beings evolved on a diet in which the ratio of n-6 to n-3 fatty acids was about 1:1. In contrast, Western diets have a ratio of

Linoleic Acid (18:2 w 6)

Linoleic Acid (18:2 w 6)

A 6Desaturase a-Linolenic Acid (18:3 w 3)

Octadecatetraenoic Acid (18:4 w 3)

a-Linolenic Acid (18:3 w 3)

Octadecatetraenoic Acid (18:4 w 3)

+ 2C Elongation

COOH

COOH

Dihomo-7-Linolenic Acid (20:3 w 6)

Dihomo-7-Linolenic Acid (20:3 w 6)

COOH

COOH

Eicosatetraenoic Acid (20:4 w 3)

Eicosatetraenoic Acid (20:4 w 3)

A 5Desaturase

COOH

Arachidonic Acid (20:4 w 6)

Cytochrome P-450

Arachidonic Acid (20:4 w 6)

COOH

COOH

Eicosapentaenoic Acid (20:5 w 3)

Lipoxygenase

HETEs 3-Series PGs HETEs Docosahexaenoic Acid

Eicosapentaenoic Acid (20:5 w 3)

Cyclooxygenase 2-Series PGs

Cytochrome P-450

EETs DiHETEs HETEs

Lipoxygenase Cyclooxygenase

Elongation + Desaturation

4-Series LTs

Figure 1 Essential fatty acid metabolism.

Lipoxygenase

HETEs 3-Series PGs HETEs Docosahexaenoic Acid

approximately 15:1. Evidence for this change in diet through history comes from studies on the evolutionary aspects of diet, modern-day hunter-gatherers, and traditional diets. Modern agriculture has led to a substantial increase in n-6 fatty acids at the expense of n-3 fatty acids, which has resulted in excessive consumption of n-6 fatty acids by humans.

The n-6 EFAs have two main functions. First, they act as structural components of membranes forming the basis of the phospholipid component of the lipid bilayer of plasma membranes in every cell in the body, thus providing a membrane impermeable to most water-soluble molecules. The length and degree of saturation of the fatty acids determine how the phospholipid molecules pack together and consequently affect membrane fluidity, signal trans-duction, and the expression of cellular receptors. The second role of n-6 fatty acids is as precursors to the eicosanoids (Figure 1). The eicosanoids are a family of 'hormone-like' compounds including prostaglandins (PGs), leukotrienes (LTs), and hydroxy- (HETEs), dihydroxy- (DiHETEs), and epoxy- (EETs) fatty acids. Eicosanoids, however, are distinct from most hormones in that they act locally, near their sites of synthesis, and they are catabolized extremely rapidly. Thus, they are considered to be locally acting hormones. The eicosanoids modulate renal and pulmonary function, vascular tone, and inflammatory responses. The enzymes involved in AA metabolism include the cyclooxygenases and lipoxygenases, which yield the 2-series PGs and 4-series LTs, respectively. Lipoxygenase also utilizes AA for the formation of the HETEs. A third pathway for the utilization of AA involves the cytochrome P-450 enzymes found in the liver, kidney, lung, intestines, heart, small blood vessels, and white blood cells. AA metabolized via cytochrome P-450 yields EETs, DiHETEs, as well as HETEs. The cytochrome P-450 metabolites play an important role as paracrine factors and second messengers in the regulation of pulmonary, cardiac, renal, and vascular function and modulate inflammatory and growth responses.

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