Arachidonic acid from phospholipids

Manoalide (164), a marine natural product which inhibits the release of arachidonic acid from phospholipids by phospholipase A2 [397,398], showed topical anti-inflammatory activity in mouse ear models [399]. Activity in ISN and cRBL (< 1 M) have also been reported [400]. A series of analogues consisting of the furanone ring of manoalide bearing simple unsaturated 16-20 carbon chains showed similar activity in rabbit neutrophils and isolated guinea-pig neutrophil 5-LO [401] interestingly, however, topical anti-inflammatory activity was seen in phorbol ester ear oedema but not in AAE [399]. The importance of 5-LO inhibition to the anti-inflammatory activity of manoalide is unknown effects on phospholipase C and calcium channels have also been shown [402, 403]. 

The acid-soluble SH-groups in platelets are mainly those of glutathione (GSH). GSH is a cofactor for enzymes such as peroxidase. If feverfew is able to interfere with this cofactor, enzyme function may be impaired. One pathway that may be affected in this way is the metabolism of arachidonic acid (Figure 6.1). In the presence of feverfew extract an increase was found in lipoxygenase product formation and impaired conversion of HPETE to HETE, for which GSH is a cofactor [52]. Inhibition of the liberation of [ " C]arachidonic acid from phospholipids was also found [53], which implies impairment of phospholipase A2 activity and for which SH-groups are thought to be important. 

Another important aspect of the inflammatory cascade is arachidonic acid metabolism, leading to the synthesis of the proinflammatory prostaglandins and leukotrienes. Through the formation of Upocortin, an inhibitor of phospholipase A2, glucocorticoids depress the release of arachidonic acid from phospholipids and hence the production of arachidonic acid metabolites. 

Phospholipases are crucial enzymes in the arachidonic acid pathway and appear to be primarily responsible for the esterolytic action that releases arachidonic acid from phospholipids (209). Once released, this acid is converted into several mediators of inflammation. The PLA2 s are small (MW 14,000) stable proteins that require Ca2 + ions for the specific hydrolysis of the 2-acyl group of 3-sn-phospholipids. 

The involvement of arachidonic acid metabolites in endotoxic and septic shock is further supported by studies with essential fatty acid-deficient (EFAD) rats Rats raised on a diet deficient in arachidonic acid become depleted of this substrate necessary for eicosanoid formation. Studies have shown that EFAD rats are significantly more resistant to lethal endotoxic shock than normal rats and do not exhibit plasma elevations in iTxB2 in response to endotoxin This impaired iTxB2 synthesis may be attributed to a depletion of arachidonic acid from phospholipid pools or an increase in (co —9)-eicosatrienoic acid. The latter increases in EFA deficiency and has been reported to inhibit fatty acid cyclo-oxygenase. This possibility is, however, unlikely. Ethyl arachidonic acid supplementation restored the ability of EFAD rats to synthesize iTxB2 in response to endotoxin ". These deficient rats... 

Fig. 5. Mechanism of action of corticosteroids and a non-steroidal anti-inflammatory drug (indomethacin) on cellular metabolism of arachidonic acid. Treatment with corticosteroids induces synthesis of lipomod-ulin or macrocortin, which inhibits the release of arachidonic acid from phospholipids. Indomethacin inhibits the enzyme fatty acid cyclo-oxygenase and thus the formation of all prostaglandins and thromboxanes.

Chemoattractants induce release of arachidonic acid from phospholipids in PMNL [244] and the arachidonate metabolism is stimulated in a manner similar to that of ionophore A23187 [235,245], Inhibitors of arachidonate metabolism inhibit leukocyte migration [246-248], However, with inhibition mainly of the cyclooxygenase pathway, an increase of PMNL migration is observed in vitro [ 150,247,249] and in vivo [6,250,251], Formation of LTB and 5-HETE is significantly increased in synovial fluid in rheumatoid arthritis [40], These data support the concept that an endogenous lipoxygenase product is involved in the PMNL chemotactic response. 

Fig. 3. Release of arachidonic acid from phospholipids a) by phospholipase A2, b) by combined actions of phospholipase c and diglyceride lipase.

With respect to vasodilation, niacin-elicited vasodilation requires the activation of GPR109A in skin Langerhans cells [34,35], which then triggers the release of arachidonic acid from membrane phospholipids and its subsequent metabolism to PGD2. The production of PGD2 then activates DPI receptors in dermal blood vessels to cause vasodilation [36]. 

Enzyme that releases arachidonic acid from membrane phospholipid... 

Arachidonic acid is not present in significant amounts in tissues as the free acid but is stored as a fatty acid at the sn-2 position of phospholipids. Prostaglandin biosynthesis is initiated by the interaction of a stimulus with the cell surface. Depending on the cell type, the stimulus can take the form of a hormone, such as angiotensin II or antidiuretic hormone, or a protease such as thrombin (involved in blood clotting), or both hormone and protease. These agents bind to a specific receptor that activates a phospholipase A2 that specifically releases the arachidonic acid from a phospholipid such as phosphatidylcholine. The release of arachidonic acid by phospholipase A2 is believed to be the rate-limiting step for the biosynthesis of eicosanoids. 

The levels of cardiovascular factors could also be influenced by the ability of vitamin E to affect activation of arachidonic acid from membrane phospholipids by phospholipase A2. Vitamin E, either given in the diet or by incubation with platelets themselves, was found to inhibit phospholipase A2 in a dose-dependent manner. a-Tocopheryl acetate had little or no effect on the activity of this enzyme, but tocol, without methyl groups in the chroman ring, was more potent than either (+)- or ( )-a-tocopherol, suggesting that the methyl groups were not important for the inhibition but the hydroxy group in the ring was critical for activity [131]. 

The aldehydes are generated at temperatures as low as 60 C. O-f particular note is the oxidation o-f phospholipids and polyunsaturated -fatty acids. It is the oxidative products o-f arachidonic acid -from the phospholipid -fraction in chicken, yielding cis—4-decenal, trans-2-cis—5—undecadienal, and trans-2-cis—4— trans-7—tridecatrienal that are responsible -for the species identity o-f chicken. At temperatures o-f 200 to 300 C the thermal oxidative reactions allow the -formation o-f ketones and lactones as well as aldehydes. 

Antiinflammatory steroids inhibit the release of arachidonic acid from membrane phospholipids by inhibiting a phospholipase A2. If arachidonate is not released, it is not converted to either leukotrienes or prostaglandins. Aspirin, a synthetic antiinflammatory agent, inhibits cyclooxygenase but not lipooxygenase. 

Phospholipase A2, not C, is involved in the release of arachidonic acid from membrane phospholipids. 

A major platelet response during activation is liberation of arachidonic acid fix>m jdio holipids and its subsequent oxygenation to TxA2. Patients have been described with impaired liberation of arachidonic acid from membrane phospholipids during platelet stimulation (32,54,46). In the patients described by Rao et al (32) platelet TxA2... 

PG synthesis involves four steps (Figme 2). The first two steps are common to all cells involved in prostaglandin synthesis while the final two steps are cell-specific (14-16). Release of the substrate, arachidonic acid, from membrane phospholipid stores by phospholipase is the initial event in prostaglandin synthesis, and this is followed by formation of the common PG intermediate, PGHj catalyzed by PGH synthase. At this point, rearrangement of PGH to form either stabk (PGD / Ej/ F, ) or unstable (platelet thromboxane - TxA, endothelial prostacyclin - PGy products takes place. The final step, also cell-specific, involves breakdown of the active compounds to irractive metabolites. 

Feverfew s mechanism of action in the prevention of migraine headaches is not known. It is speculated that feverfew affects platelet activity or inhibits vascular smooth-muscle contraction, perhaps by inhibiting prostaglandin synthesis (4). Results of in vitro studies suggest that rather than acting as a cyclooxygenase inhibitor, feverfew inhibits phospholipase A2, thus inhibiting release of arachidonic acid from the cell membrane phospholipid bilayer (11,12). 

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