Arachidonic acid sites

Animal cells can modify arachidonic acid and other polyunsaturated fatty acids, in processes often involving cyclization and oxygenation, to produce so-called local hormones that (1) exert their effects at very low concentrations and (2) usually act near their sites of synthesis. These substances include the prostaglandins (PG) (Figure 25.27) as well as thromboxanes (Tx), leukotrienes, and other hydroxyeicosanoic acids. Thromboxanes, discovered in blood platelets (thrombocytes), are cyclic ethers (TxBg is actually a hemiacetal see Figure 25.27) with a hydroxyl group at C-15. 

In this bromoaspirin-inactivated structure, Ser , which lies along the wall of the tunnel, is bromoacetylated, and a molecule of salicylate is also bound in the tunnel. Deep in the tunnel, at the far end, lies Tyr, a catalytically important residue. Heme-dependent peroxidase activity is implicated in the formation of a proposed Tyr radical, which is required for cyclooxygenase activity. Aspirin and other NSAIDs block the synthesis of prostaglandins by filling and blocking the tunnel, preventing the migration of arachidonic acid to Tyr in the active site at the back of the tunnel. 

Dehydroarachidonic acid analogs in which one Z-olefinic unit is replaced by a triple bond are irreversible inhibitors of the lipoxygenasses which normally deliver dioxygen to the corresponding site of arachidonic acid. The inactivation appears to be a consequence of dioxygenation at the acetylinic unit to from a vinyl hydroperoxide which undergoes rapid 0-0 homolysis. Synthetic routes to these interesting enzyme inhibitors are outlined below. 

The occupation of all available coordination sites by phytate suppresses other iron-mediated processes, such as lipid peroxidation ( 0). Figure 6 demonstrates that 0.24 mM phytate prevents the peroxidation of arachidonic acid driven by ascorbic acid and iron, whereas substantial amounts of malondialdehyde arise in the presence of free iron or of an iron-ADP chelate. 

Metabotropic receptors, in contrast, create their effects by activating an intracellular G protein. The metabotropic receptors are monomers with seven transmembrane domains. The activated G protein, in turn, may activate an ion channel from an intracellular site. Alternately, G proteins work by activation or inhibition of enzymes that produce intracellular messengers. For example, activation of adenylate cyclase increases production of cyclic adenosine monophosphate (cAMP). Other effector mechanisms include activation of phospholipases, diacylglycerol, creation of inositol phosphates, and production of arachidonic acid products. Ultimately, these cascades can result in protein phosphorylation. 

NSAIDs are drugs related to acetyls alley lie acid which inhibit cyclooxygenase (COX), the enzyme in the synthesis of PGs and thromboxanes from arachidonic acid. There are two isoforms of cyclooxygenase, COX-1 and COX-2 [121].The former is constitutively expressed in blood vessels, stomach and kidney, while COX-2 is normally not present at these sites. It can,... 

Glucocorticosteroids are the most potent antiinflammatory agents available. They stabilize lysosomal membranes and reduce the concentration of proteolytic enzymes at the site of inflammation. They promote the synthesis of proteins called lipocortins which inhibit phospholipase-A2 and thus inhibit production of arachidonic acid, leukotrienes and prostaglandins. Furthermore, the expression of COX-II and through that the inflammatory effects of the licosanoids is inhibited. Glucocorticosteroids reduce the release of histamine from basophils, decrease capillary permeability and cause vasoconstriction. Glucocorticosteroids stimulate the loss of calcium with the urine and inhibit the resorption of calcium from the gut. 

Metabolites of arachidonic acid, including prostaglandins (PG), thromboxanes, and leukotrienes, are considered strong candidates as mediators of the inflammatory process. Steroids may exert a primary effect at the inflammatory site by inducing the synthesis of a group of proteins called lipocortins. These proteins suppress the activation of phospholipase A2, thereby decreasing the release of arachidonic acid and the production of proinflammatory eicosanoids (Fig. 60.6). 

Arachidonic acid is the biosynthetic precursor of metabolites (such as prostaglandins) that play a basic role in cell signal processes. Fluorine atoms have been introduced on the strategic oxidation sites of arachidonic acid in order to study the... 

Triethanolamine has been clinically tested with other model irritant compoimds for potency to stimulate signal release of proinflammatory mediators in hrnnan skin in order to find biomarkers of irritancy. Neat or aqueous triethanolamine was applied to the lower arm of 12 male volimteers after 24 h, suction blister fluid specimens were taken from the site of treated skin. Triethanolamine caused no significant increase in arachidonic acid and prostaglandin concentrations in suction blister fluid samples, in... 

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