Metabolism of Unstable Eicosanoids: Role of Prostaglandin E Synthases

 

The pathways responsible for the generation of eicosanoids in animals and jasmonates in plants involve unstable intermediates and specific enzymes acting on these intermediates. Examples are thromboxane synthase, leukotriene A₄ hydrolase and allene oxide cyclase, which catalyze the biosynthesis of thromboxane A₂, leukotriene B₄, and 12-oxophytodienoic acid, respectively. Prostaglandin E₂ (PGE₂) is formed from the unstable cyclic peroxide prostaglandin H₂ (PGH₂) by action of prostaglandin E synthases, a group of related glutathione-dependent isomerases (EC 5.3.99.3). These enzymes are shortly treated in the following.

PGH₂ is chemically unstable (t_1/2, 5 min at 37^oC) and is spontaneously converted into a mixture of the isomeric keto-hydroxy cyclopentane prostaglandins PGE₂ and PGD₂ upon standing in aqueous medium (1). In many tissues, a specific rate-acceleration of the reaction PGH₂ -> PGE₂ due to enzyme activity is observed. Thus, the sheep vesicular gland, a rich source of cyclooxygenase (COX), also contains a microsomal prostaglandin E synthase (PGES), which catalyzes isomerization of the 9a,11a-epidioxycyclopentane structure of PGH₂ into the 9-keto-11a-hydroxycyclopentane structure typical of PGE₂. A bovine PGES was solubilized and partially purified almost 25 years ago and characterized as a glutathione-requiring enzyme (2). Depending on which of the two cyclooxygenase products, prostaglandin G₂ or H₂, is isomerized, two pathways leading to the formation of PGE₂ are possible (3):

 

During the last three years, molecular biological techniques have allowed significant progress in our understanding of PGESs and their relationship to cyclooxygenases. The need of drugs which selectively inhibit PGES to be used as an alternative to aspirin and other NSAIDs makes studies of PGES an important task for the pharmaceutical industry.

Microsomal prostaglandin E synthase (mPGES) is a 16-kDa protein which belong to the so-called MAPEG (membrane associated proteins involved in eicosanoid and glutathione metabolism) superfamily of enzymes (4,5). mPGES and COX-2 are inducible by the proinflammatory cytokine IL-1b, and a functional relationship between these two enzymes has been proposed (4,6). Recent studies have shown that the induction occurs at a pre-translational level and can be blocked by dexamethasone (7). Interestingly, it was also reported that SC58125, a selective COX-2 inhibitor, attenuated the induction of mPGES, suggesting a dependence of this enzyme on COX-2 activity.

Cytosolic prostaglandin E synthase (cPGES) is a constitutively expressed glutathione-dependent enzyme (8). Expression of this enzyme is not affected by IL-1b or TNF-a, but a modest induction in rat brain by lipopolysaccharide has been observed. It has been suggested that cPGES is identical to p23, a putative chaperone which stabilizes steroid receptor complexes. In analogy with the proposed association between mPGES and COX-2 mentioned above, a functional association of cPGES with COX-1, the constitutive prostaglandin H synthase, has been suggested (8).

Several mechanisms for the glutathione-dependent isomerization of PGH₂ by PGES are conceivable and all involve the nucleophilic thiolate anion of glutathione as the attacking species. Lands et al. in an early study (9) proposed that C-9 of the endoperoxide was attacked by thiolate resulting in the formation of an unstable thiohemiketal intermediate. Another possibility would consist of removal of the proton attached to C-9 by glutathione thiolate acting as a base followed by isomerization into the 9-keto-11a-hydroxy structure by concerted bond shifts. An attractive mechanism for PGES involves attack by the thiolate on the peroxide oxygen attached to C-9. The adduct thus formed will decompose into PGE₂ and glutathione thiolate following enzyme-assisted removal of the C-9 proton:

 

It is interesting to note that this mechanism may have relevance also for glutathione-dependent PGF synthases, which catalyze the reduction of the 9a,11a-epidioxy structure of PGH₂ into a 9a,11a-diol. This stems from the fact that the glutathionyl-intermediate would be expected to partition between isomerization (formation of PGE₂) and reduction (formation of PGF_2a) depending on whether or not it is attacked by a second molecule of glutathione at the GS-O bond.

References

1. Hamberg, M., Svensson, J., Wakabayashi, T. and Samuelsson, B. (1974) Isolation and structure of two prostaglandin endoperoxides that cause platelet aggregation. Proc. Natl. Acad. Sci. USA 71, 345-349.

2. Ogino, N., Miyamoto, T., Yamamoto, S. and Hayaishi, O. (1977) Prostaglandin endoperoxide E isomerase from bovine vesicular gland microsomes, a glutathione-requiring enzyme. J. Biol. Chem. 252, 890-895.

3. Samuelsson, B. and Hamberg, M. (1974) Role of endoperoxides in the biosynthesis and action of prostaglandins. Proceedings of an International Symposium on Prostaglandin Synthetase Inhibitors, ed. H.J. Robinson and J.R. Vane, Raven Press, New York. pp. 107-119.

4. Jakobsson, P.-J., Thorén, S., Morgenstern, R. and Samuelsson, B. (1999) Identification of human prostaglandin E synthase: A microsomal, glutathione- dependent, inducible enzyme, constituting a potential novel drug target. Proc. Natl. Acad. Sci. USA 96, 7220-7225.

5. Jakobsson, P.-J., Morgenstern, R., Mancini, J., Ford-Hutchinson, A. and Persson, B. (1999) Common structural features of MAPEG - a widespread superfamily of membrane associated proteins with highly divergent functions in eicosanoid and glutathione metabolism. Prot. Sci. 8, 689-692.

6. Murakami, M., Naraba, H., Tanioka, T., Semmyo, N., Nakatani, Y., Kojima, F., Ikeda, T., Fueki, M., Ueno, A., Oh-ishi, S. and Kudo, I. (2000) Regulation of prostaglandin E₂ biosynthesis by inducible membrane-associated prostaglandin E₂ synthase that acts in concert with cyclooxygenase-2. J. Biol. Chem. 275, 32783-32792.

7. Han, R., Tsui, S. and Smith, T.J. (2002) Up-regulation of PGE₂ synthesis by IL-1b in human orbital fibroblasts involves coordinate inductions of prostaglandin endoperoxide H synthase-2 and glutathione-dependent PGE₂ synthase expression. J. Biol. Chem. 277, 16355-16364.

8. Tanioka, T., Nakatani, Y., Semmyo, N., Murakami, M. and Kudo, I. (2000) Molecular identification of cytosolic prostaglandin E₂ synthase that is functionally coupled with cyclooxygenase-1 in immediate prostaglandin E₂ biosynthesis. J. Biol. Chem. 275, 32775-32782.

9. Lands, W., Lee, R., and Smith, W. (1971) Factors regulating the biosynthesis of various prostaglandins. Ann. N.Y. Acad. Sci. 180, 107-122.