Arachidonate oxygenase activity

A useful insight into the subtle differences between the catalytic sites of the two enzymes was provided by the observations that while the cyclooxygenase activities of both were inhibited by aspirin, aspirin-acetylated Cox-2 has an increased 15-oxygenase activity, which can be inhibited by some Cox inhibitors (Mancini et al., 1994 Lecomte et al., 1994). Aspirin is known to acetylate Ser-530 of Cox-1 and Ser-516 of Cox-2. Apparently, in the acetylated Cox-2 catalytic site there is still room for arachidonate to enter in an extended conformation, which permits catalysis of 15-hydroperoxyeicosatetraenoic acid formation but not of PGG2 formation. This led to the conclusion that the catalytic site of Cox-2 must be slightly larger than that of Cox-1, a conclusion later... 

Figure 3. The mechanism ofa irin s effect on platelets The 599 amino acid polypeptide chain of PGH synthase (center wavy line NHj-terminal methionine-1, COOH-terminal leucine-599) exerts cyclo-oxygenase activity as shown above (oxygenation/cycli-zation of arachidonate to PGGj) and interacts with aspirin as shown below. The serine residue located at position 529 of the polypeptide chain of cyclo-oxygenase is acetylated through transfer of aspirin s acetyl groiq> as indicated in bold face. Covalently-modified, acetylated PGH synthase carries a single acetyl group in its active site and lacks all cyclo-oxygenase activity.

COX-1 [Cyclo-oxygenase-1] An enzyme that catalyzes the synthesis of prostaglandins from the polyunsaturated fatty acid arachidonic acid. COX-1 is responsible for baseline levels of prostaglandins. Cyclo-oxygenase activity is inhibited by aspirin-like drugs, accounting for their antiinflammatory effects. 

There is little information on prostaglandin formation by human cardiac tissue, most of which, based on measurements of thromboxane B2, prostaglandin E2 and 6-keto-prostaglandin in aortic and coronary sinus blood, is likely to be highly artifactual. In a recent experiment, Nowak and coworkers infused radiolabelled arachidonic acid into the coronary arteries of male volunteers 3 days following a single dose of aspirin, 600 mg, by which time cyclo-oxygenase activity in endothelial cells, but not in platelets, would... 

Lipoxygenases also metabolize their hydroperoxy products to epoxides of LTA4 type by an apparent dehydration reaction. In addition to the arachidonate 5-oxygenase activity, 5-lipoxygenase possesses LTA synthase activity, converting 5-HpETE into LTA [22-25], Such a dual activity of 5-lipoxygenase was confirmed with recombinant enzymes expressed in osteosarcoma cells [26], Escherichia coli [27], and Sf9 insect cells [28,29]. As shown in Table 1, the formation of LTA is... 

Steps in prostaglandin (PG) biosynthesis PG synthesis involves four steps 1) Release of the substrate, arachidonic acid (20 4), firom membrane phospholipids by a phospholipase. 2) Formation of the common intermediate, PGHj, from arachi-donate by PGH synthase throu sequential cyclo-oxygenase and peroxidase activities, both ofwWch are present within the same enzyme. 3) Fonnation of cell-specific PG products, either "stable" (Dj, E, Fj,) or "imstable" (thromboxane, prostacyclin), by distinct enzymes formd in different cells and tissues. 4) Breakdown of active compoimds to inactive metabolites, using TxAj -> TxB, and PGI -> 6keto PGF as examples. 

Prostaglandin H synthase is the first enzyme involved in the arachidonic acid cascade . The prostaglandins can cause both relaxation and tension in smooth muscle and so have vital physiological roles. Prostaglandin H synthase acts as both a peroxidase and an oxygenase its so-called cyclooxygenase activity involves addition of two equivalents of oxygen to a free radical derived from arachidonic acid to form an endoperoxide... 

The eicosanoids are locally active hormones (autocoids) that are derived from precursor polyunsaturated fatty acids. The rate-limiting step in the synthesis of eicosanoids is the phospholipase-regulated release of arachidonic add from membranes. Arachi-donic acid metabolism may follow one of three possible pathways. In the first, the cydooxygenase- peroxidase pathway leads to the formation of the prostenoids - prostaglandins and thromboxanes. In the second, the lipoxygenase pathway yields the leuko-trienes and lipoxins. A third pathway, the cytochrome P-450 mono oxygenase pathway is also involved in the metabolism of arachidonic add. 

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