Cyclooxygenase peroxidase

We conclude that so far eicosapentaenoic acid has proved to be a very poor substrate for the prostaglandin endoperoxide synthetase (cyclooxygenase + peroxidase) in every species or organ tested, irrespective of whether this enzyme is connected with prostaglandin-, thromboxane- or prostacyclin-biosynthesis. 

The enzyme system responsible for the biosynthesis of PGs is widely distributed in mammalian tissues and has been extensively studied (2). It is referred to as prostaglandin H synthase (PGHS) and exhibits both cyclooxygenase and peroxidase activity. In addition to the classical PGs two other prostanoid products, thromboxane [57576-52-0] (TxA ) (3) and prostacyclin [35121 -78-9] (PGI2) (4) are also derived from the action of the enzyme system on arachidonic acid (Fig. 1). 

In order for the cyclooxygenase to function, a source of hydroperoxide (R—O—O—H) appears to be required. The hydroperoxide oxidizes a heme prosthetic group at the peroxidase active site of PGH synthase. This in turn leads to the oxidation of a tyrosine residue producing a tyrosine radical which is apparendy involved in the abstraction of the 13-pro-(5)-hydrogen of AA (25). The cyclooxygenase is inactivated during catalysis by the nonproductive breakdown of an active enzyme intermediate. This suicide inactivation occurs, on average, every 1400 catalytic turnovers. 

All prostaglandins are cyclopentanoic acids derived from arachidonic acid. The biosynthesis of prostaglandins is initiated by an enzyme associated with the endoplasmic reticulum, called prostaglandin endoperoxide synthase, also known as cyclooxygenase. The enzyme catalyzes simultaneous oxidation and cyclization of arachidonic acid. The enzyme is viewed as having two distinct activities, cyclooxygenase and peroxidase, as shown in Figure 25.28. 

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. 

O Donnell et al. [70] found that LOX and not cyclooxygenase, cytochrome P-450, NO synthase, NADPH oxidase, xanthine oxidase, ribonucleotide reductase, or mitochondrial respiratory chain is responsible for TNF-a-mediated apoptosis of murine fibrosarcoma cells. 15-LOX activity was found to increase sharply in heart, lung, and vascular tissues of rabbits by hypercholesterolemia [71], Schnurr et al. [72] demonstrated that there is an inverse regulation of 12/15-LOXs and phospholipid hydroperoxide glutathione peroxidases in cells, which balanced the intracellular concentration of oxidized lipids. 

COX = cyclooxygenase LOX = lipoxygenase POD = peroxidase PGI2 = prostacyclin, PGX TxB2 = thromboxane B2... 

Similar to peroxidases (Chapter 22) and LOXs (see above), cyclooxygenases are capable of catalyzing the oxidation of substrates during the reduction of PGG2 to PGH2. Potter and Hinson [96] proposed that prostaglandin H synthase catalyzed the oxidation of acetaminophen by both one-electron and two-electron mechanisms. Formosa et al. [91] showed that... 

FIGURE 21-15 The "cyclic" pathway from arachidonate to prostaglandins and thromboxanes, (a) After arachidonate is released from phospholipids by the action of phospholipase A2/ the cyclooxygenase and peroxidase activities of COX (also called prostaglandin H2 synthase) catalyze the production of PGH2/ the precursor of other... 

FIGURE 1 Structures of COX-1 and COX-2, (a) COX-1 with an NSAID inhibitor (flurbiprofen, orange) bound (PDB ID 3PGH). The enzyme consists of two identical monomers (gray and blue) each with three domains a membrane anchor consisting of four amphipathic helices a second domain that somewhat resembles a domain of the epidermal growth factor and the catalytic domain, which contains the cyclooxygenase and peroxidase activities, as well as the hydrophobic channel in which the substrate (arachidonate) binds. The heme that is part of the peroxidase active sites is shown in red ... 

Synthesis of prostaglandins and thromboxanes begins with the oxidative cyclization of free arachidonic acid to yield PGH2 by prostaglandin endoperoxide synthase—a microsomal protein that has two catalytic activities fatty acid cyclooxygenase (COX) and peroxidase. There are two isozymes of the synthase COX-1 and COX-2. Leukotrienes are produced by the 5-lipoxygenase pathway. 

The anti-inflammatory effect of aspirin appears to be due to the acetylation of a serine residue at the active site of the cyclooxygenase, which irreversibly inactivates the enzyme (fig. 19.18). Aspirin has no effect on the peroxidase activity of the enzyme. 

Tylenol inhibits the peroxidase activity and aspirin inhibits the cyclooxygenase activity. 22.46 because dead heart muscles spill their enzyme contents into the serum 22.48 They are both basic amino acids. 

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