Biosynthesis of prostanoids

Phospholipases A (PLA ) are a diverse class of enzymes catalysing the hydrolysis of the sn-2 ester bond of phospholipids. This growing superfamily of lipolytic enzymes has at least 19 mammalian enzyme members identified to date (Balsinde et ah, 2002). The PLA enzymes have recently been systematically classified on the basis of their nucleotide and amino acid sequences (Dennis, 1997 Six and Dennis, 2000). 

Although PLAj activity is the first step for prostanoid biosynthesis and can limit their availability, it is not the rate-limiting step in prostanoid formation. However, direct inhibition of PLA could potentially block the production of all eicosanoids, making it a desirable target for pharmacological intervention (Yedgar et al., 2000). 

The two COX-catalysed reactions take place at two different locations of the enzyme the cyclooxygenase reaction occurs in a hydrophobic channel in the core of the enzyme, while the peroxidase reaction takes place at a heme-containing active site located near the surface of the protein. However, the two activities are functionally coupled the overall reaction is initiated by the oxidation of the heme group of the peroxidase reaction by low levels of a 

An unusual kinetic feature of COX is the autoinactivation (suicide inactivation) of the enzyme both the cyclooxygenase and peroxidase activities are inactivated during catalysis as the result of non-productive breakdown of active enzyme intermediates. The chemical changes in the protein that accompany this process are unknown, and although the biological relevance of this inactivation is unclear, it may constitute a cmde regulatory mechanism of cellular prostaglandin biosynthesis (Smith et al, 2000). 

The stmcture of the cyclooxygenase active site is very similar in both COX-1 and COX-2. However, there are two important structural differences between the two isoenzymes. Firstly, the active site of COX-2 is larger and more accommodating than that of COX-1, a feature that has been exploited in developing COX-2-specific nonsteroidal anti-inflammatory drugs (NSAID). Secondly, COX-1 exhibits negative allosterism at low concentrations of arachidonic acid. This feature may result in greater prostanoid synthesis by COX-2 under conditions of low arachidonic acid concentration (Smith et al, 2000). 

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Fig. 1. Biosynthesis of prostanoids, where structures (5)—(8) are PGG2, PGH2, PGD2, and PGF2Q, respectively.

Finally, there has been speculation and recent experimental support for the involvement of a Nazarov-type cyclization in the biosynthesis of c/ s-jasmonic acid ° and marine-derived prostanoids. Radiolabel tracer studies have demonstrated Ae intermediacy of 8-HPETE (104) in the biosynthesis of prostanoid intermediate preclavulone A (lO ). This remarkable conversion was proposed to proceed by formation of allene oxide (105) followed by isomerization to (107) via the 2-oxidocyclopentadienyl cation (106 Scheme 41). To demonstrate the chemical feasibility of this proposal, Corey reported the transformation of epoxysilane (108) to, inter alia, the cyclopentenone (111 Scheme 42). The reaction is presumed to involve formation of the allene oxide (109) followed by isomerization to the 2-oxi-dopentadienylic cation (110). Conrotatory closure of (110) is expected to produce the cis isomer of (111) as observed. 

The biosynthesis of prostanoids involves three important enzymatic reactions. In the case of PGE2, formation begins with the release of AA from phospholipids by phospholipase A2 (PLA2), followed by the synthesis of PGH2 by COX, and conversion of PGH2 to specific prostanoids (e.g., PGE2) by terminal PG synthases (66). 

Figure 6.2. The biosynthesis of prostanoids from arachidonic acid. Free arachidonic acid is converted to the unstable intermediates PGG, and PGH, by cyclooxygenase (COX) enzymes. PGH, is then converted to the five primary prostanoids by specific synthases.

Although devil s claw has been studied for anti-inflammatory activity, notably in patients with arthritis, studies have indicated that devil s claw does not affect the biosynthesis of prostanoids, and thus is not expected to produce the adverse effects associated with nonsteroidal anti-inflammatory and glucocorticoid drugs (ESCOP 2003 Loew et al. 1996 Moussard et al. 1992 Whitehouse et al. 1983). 

Bruckner GG, Lokesh B, German B, Kinsella JE (1984) Biosynthesis of prostanoids, tissue fatty acid composition and thrombotic parameters in rats fed diets enriched with docosahexaenoic or eicosapentaenoic acid. Thromb Res 34 479-497... 

The essentialness of a-linolenic acid and of its longer derivatives EPA or DHA is supported by experiments in animals and observations in young patients. In infant rhesus monkeys after a diet deficient in co-3 fatty acids a visual loss could be observed [21]. In 1982 Holman et al. [15] described the case of a 6-year-old girl with a-linolenic acid deficiency. She experienced distal numbness and paresthesias, weakness, periodic inability to walk, and blurring of vision. The authors suggest that a-linolenic acid respectively co-3-polyenoic fatty acids are required for normal nerve function, at least in growing individuals. Meanwhile co-3 fatty acid deficiency could be observed in five adults, documented by biochemical changes of fatty acid composition, but not by clinical symptoms [3,22]. In a 90-year-old female with a-linolenic acid deficiency, the effects of ethyl a-linolenate on biosynthesis of prostanoids could be demonstrated [4]. 

Bjerve KS, Fischer S, Alme K (1987) Alpha-linolenic acid deficiency in man effect of ethyl linolenate on plasma and erythrocyte fatty acid composition and biosynthesis of prostanoids. Am J Clin Nutr 46 570-576... 

Figure 20.3 Biosynthesis of prostanoids by a 2-oxydopentadienyl cation pathway.

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