Arachidonate/arachidonic acid

Eicosanoids, so named because they are all derived from 20-carbon fatty acids, are ubiquitous breakdown products of phospholipids. In response to appropriate stimuli, cells activate the breakdown of selected phospholipids (Figure 25.27). Phospholipase Ag (Chapter 8) selectively cleaves fatty acids from the C-2 position of phospholipids. Often these are unsaturated fatty acids, among which is arachidonic acid. Arachidonic acid may also be released from phospholipids by the combined actions of phospholipase C (which yields diacyl-glycerols) and diacylglycerol lipase (which releases fatty acids). 

Cyclooxygenases. Figure 1 Pathways for the formation of prostanoids from arachidonic acid. Arachidonic acid is converted by cyclooxygenase to endoperoxides, which are acted upon by various synthesases to form the prostanoids. Prostacyclin and thromboxane are relatively unstable and break down rapidly to form the inactive metabolites 6-oxo-PGF1a and thromboxane B2, respectively. 

Prostaglandins are a subgroup of a larger family of compounds known collectively as eicosanoids, which are synthesized from arachidonic acid (arachidonate) this is a 20-carbon omega-6 unsaturated fatty acid (C20 4). The source of the arachidonic acid for PG synthesis is the cell membrane. Most membrane phospholipids have an unsaturated fatty acid as arachidonate at carbon 2 on the glycerol backbone to help maintain membrane fluidity. The arachidonic acid released from the membrane by the... 

T FIGURE 10-18 Arachidonic acid and some eicosanoid derivatives. (a) In response to hormonal signals, phospholipase A2 cleaves arachidonic acid-containing membrane phospholipids to release arachidonic acid (arachidonate at pH 7), the precursor to various eicosanoids. (b) These compounds include prostaglandins such as PCE, in which C-8 and C-12 of arachidonate are joined to form the characteristic five-membered ring. In thromboxane A2/ the C-8 and... 

Correct answer = E. Prostaglandins are synthesized from arachidonic acid. Arachidonic acid is synthesized from linoleic acid, an essential fatty acid obtained by humans from dietary lipids. The teenager would be able to synthesize all other compounds, but presumably in somewhat depressed amounts. 

Fig. 4.4 Hypothetical model showing the modulation of glutamate transporter by arachidonic acid. Interactions of glutamate with its receptor result in depolarization and Ca2+ entry into the cell. Ca2+-mediated stimulation of PLA2 results in breakdown of neural membrane phospholipids and the release of arachidonic acid. Arachidonic acid not only modulates proton conductance associated with neuronal excitability, but also provides eicosanoids, which may control the glutamate transporter (modified from Fairman and Amara, 1999)...

The problem states that PGI i2 has one more double bond than PGE and that it is biosynthesized from arachidonic acid. Arachidonic acid (text Table 26.1) has a double bond at C-5, and thus PGI i2 has the structure shown. 

Prostaglandins, thromboxanes and leucotrienes are eicosanoids deriving from oxidation of arachidonic acid. Arachidonic acid (deriving from PLA2 action on phospholipids) is cyclized by constitutive cyclooxygenase (COX-1) or inducible cyclooxygenase (COX-2) to yield... 

Phospholipids — arachidonic acid - Arachidonic acid metabolites appear not to be stored within cells. Their biosynthesis depends upon the appearance of substrate at or near the microsomal synthetase complex(es). According to current thinking, arachidonic acid is stored in the phospholipid fraction of the cell from which the free fatty acid is liberated by the action of a phospholipase... 

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. 

The eicosanoids are an important group of endogenous fatty acid derivatives that are produced from arachidonic acid. Arachidonic acid is derived from cell membrane lipids. 

Ans. Fatty acids containing more than one unsaturated bond past carbon 9 of a saturated chain counting from the carboxyl end cannot be synthesized by humans. Prostaglandins, a family of lipid-soluble organic acids which are regulators of hormones, are synthesized by mammals from arachidonic acid. Arachidonic acid is a polyunsaturated fatty acid which mammals can synthesize using the dietary polyunsaturated fatty acids of plant origin as precursors. 

It is clear that oxylipin formation occurs in these organisms, and that the production is sensitive to life stage. It is difficult to interpret the significance of these oxylipins to the reproductive biology of Dipodascopsis because virtually all work has been done with exogenous arachidonic acid. Arachidonic acid comprised only 0.1% of the endogenous fatty acids in Dipodascopsis while linoleic and linolenic acids represented 25.6 and 3.3% respectively [27]. It would be interesting to see if 3-HPODE and aspirin-sensitive linoleic-acid-based oxylipins are produced in Dipodascopsis. 

The fragrance of jasmine is due to several compounds including jasmone, methyl jasmonate, benzyl acetate, and indole. Jasmone and its relatives derive from an important lipid called arachidonic acid. Arachidonic acid is the starting compound from which a number of physiologically important compounds including prostaglandins are obtained. 

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