Prostaglandins formation from essential fatty acids

Both leukotrienes and prostaglandins can originate from arachidonic acid. Lipooxygenase initiates the formation of leukotrienes from arachidonic acid. Eicosapentaenoic acid is a precursor of series 3 prostaglandins, that is, prostaglandins with three double bonds in the side chains. Eicosapentaenoic acid is generated by linolenic acid, an essential fatty acid. 

Palmitic acid may be converted to stearic acid (C1K 0) by elongation of the carbon chain. Desaturation of stearic acid produces oleic acid (C18 1 A9). Linoleic acid (Ci8 2A9,12), however, cannot be synthesized in mammalian tissues. Therefore, it is an essential fatty acid for animals and must be obtained from the diet it has two important metabolic roles. One is to maintain the fluid state of membrane lipids, lipoproteins, and storage lipids. The other role is as a precursor of arachidonic acid, which has a specialized role in the formation of prostaglandins (Sec. 13.9). 

Finally a few remarks should be made about essential fatty acid (EFA) deficiency (section 5.2.2). Linoleic acid is the precursor of the eicosanoids, leukotrienes, prostaglandins, thromboxanes and related compounds (section 3.4). Until recently it was thought that the classic skin symptoms resulting from a deficiency were due to a lack of eicosanoids. However, the specific role of n-6 acids (such as linoleate) as opposed to n-3 acids (such as a-linolenate) in membrane fluidity and their function cannot explain their essentiality. For example, aspirin ingestion which effectively prevents prostanoid formation does not result in the appearance of EFA-deficiency symptoms. It is also generally accepted that at least some cell lines in culture do not have an EFA requirement - thus suggesting that EFAs are not essential for the formation and function of cellular membranes in general. 

Certain polyunsaturated fatty acids are essential in the human diet (see Box 21-B). One of these, arachidonic acid (which may be formed from dietary linoleic acid), serves as a precursor for the formation of the hormones known as prostaglandins and a series of related prostanoids. Lipids of animal origin also... 

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