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Metabolism of Leukotrienes: The Linear Biosynthetic Pathway David Goad Department of Infectious Diseases and Physiology, Oklahoma State University Introduction Leukotrienes (LT) are short-lived, highly biologically active eicosanoid products of the linear pathway of arachidonic acid (AA) metabolism, originating with the 5-lipoxygenase (5-LO)-catalyzed catalyzed oxidation of arachidonate. LT biosynthesis, metabolism, and degradation have been extensively reviewed (1-5). LT are synthesized in a variety of cell types including neutrophils, eosinophils, monocytes, mast cells, and lymphocytes and in a number of tissues, most notably the lung, as well as brain, heart, and spleen. Two distinct types of enzymatically-produced LT have been identified: Cysteinyl (or peptidyl) leukotrienes LTC4, LTD4, and LTE4 (originally identified as the slow reacting substances of anaphylaxis), and the dihydroxy leukotriene LTB 4 (4). The pathophysiological effects of LT have been reviewed (1) and the biological effects of both classes of LT are summarized in table 1. In addition, recent data suggests LTD4 and LTE4 may have substantial effects upon the histamine H1 receptor, with the potential to exacerbate the physiological effects of histamine (6). Biosynthesis of LT is not inhibited by non-steroidal antiinflammatory drugs (NSAIDS) which decrease production of the bioactive products of cyclic arachidonate metabolism, the prostaglandins, prostacyclins, and thromboxanes. However, the physiological relationship between cyclic and linear AA metabolites is demonstrated by recent evidence suggesting inhibition of LT production may alleviate gastric ulceration associated with the administration of NSAIDS (7). When one considers the bioactivity of LT with respect to pathophysiological processes it is not surprising that the biosynthesis of LT, and the search for inhibitors of LT synthesis or receptor function remains an area of highly active research (8). This review will outline the linear biosynthetic pathway of bioactive LT, with particular emphasis on recent experimental observations. Table 1. Biological effects of LT. Cysteinyl LT Vasoconstriction Increase of vascular permeability in postcapillary venules Bronchoconstriction Stimulation of mucus secretion Intestinal contraction (ileum) Plasma extravasation Decrease of blood pressure Reduction of myocardial contractility and coronary blood flow Decrease of renal blood flow LTB4 Aggregation; chemokinesis Chemotaxis; release of lysosomal enzymes; stimulation of superoxide anion production Adhesion and transendothelial migration of neutrophils Increase of vascular permeability Modulation of lymphocyte function Affector of the production and action of cytokines Release of intracellular calcium Increase of cAMP and cGMP synthesis (Adapted from (1).) Mobilization of Arachidonic Acid and the 5-Lipoxygenase Reaction LT production originates with a phospholipase A2 (PLA2) catalyzed cleavage of AA from membrane phospholipids (figure 1)(8). PLA2 may exist as either a cytosolic 85 kDa enzyme, or in an extracellular or cellassociated 14 kDa form (9). Activity of neutrophilic 85 kDa PLA2 has been shown to be stimulated by a downstream product of the linear pathway, LTB4. A positive feedback model was proposed wherein increased AA mobilization in concert with LTB4 receptor stimulation and increased Ca2+ levels helped drive activation of PLA2 via enzyme phosphorylation (10). However, in monocytes which display both the 85 and 14 kDa forms of PLA2, inhibition or depletion of the 85 kDa form did not significantly decrease the production of LT, suggesting the 14 kDa form may be more important in LT formation (9). Figure 1. 5-Lipoxygenase pathway to leukotriene generation. The key enzymatic and biochemical steps in the synthesis of products of the 5-lipoxygenase pathway are shown. LTA4 is the common intermediate in both branches of this pathway. The heavy arrows indicate unidirectional steps by which LTB4 and LTC4 are exported from cells. FLAP denotes the 5-lipoxygenase activating protein. (From Lewis et al. (2).) Following AA mobilization the next step in LT biosynthesis, indeed the determining step, involves the membrane-associated 5-LO oxidation of AA to 5S-hydroperoxy-6,8-trans-11,14-cis-eicosatetraenoic acid (5HPETE) followed by 5-LO catalyzed dehydration to the epoxide form LTA4. The activity and function of 5-LO has been comprehensively reviewed (11). 5-LO pools may be cytosolic or nuclear depending upon cell type and are translocated to the nuclear envelope where the reaction occurs (12), with 5-LO in complex with a required 5lipoxygenase activating protein (FLAP)(11,13). The 5-LO reactions are the determining factors committing AA to linear metabolism to LT which, in combination with the requirement of FLAP for 5-LO activity, have made both of these proteins the subject of intensive research to generate inhibitors of LT biosynthesis (8). Transient LTA4 is rapidly converted to LTB4 by LTA4 hydrolase, or to peptidyl LTC4 by LTC4 synthase (figure 1). In addition to intracellular metabolism, LTA4 may also be secreted allowing transcellular metabolism by homologous or heterologous cell types, irrespective of native 5-LO activity (6,8,14,15). Mobilization of AA, 5-LO production of LTA4, and intracellular and transcellular metabolism is shown in a simplified schematic representation (figure 2). Production of LTB4, Isoleukotrienes, and Bioactive LTB4 Metabolites Stereospecific hydration of LTA4 by LTA4 hydrolase yields the dihydroxy-AA metabolite LTB4. The active site of LTA4 hydrolase and LTA4 hydrolase-targeted inhibitors have been recently reviewed (16). LTA4 hydrolase has been shown to be activated by dephosphorylation in endothelial cells, although specific LTA4 hydrolase kinases or phosphatases have not been identified (17). Polymorphonuclear leukocytes (PMN) are the major LTB4-producing cells in blood (18), and adenosine has recently been shown to down-regulate LTB4 production in PMN cultures. This effect was minimized by red blood cell-mediated removal of adenosine from these cultures (15). These results taken together with transcellular LT metabolism and the bioactivity of LTs hint at the complexity of the LT cell signalling system. The occurrence of biologically active structural isomers of LTB 4, termed B4-isoleukotrienes, resulting from nonenzymatic peroxidation of LTA4 has been reported (19). These iso-LTs may have similar yet less potent activity compared to LTB4. However, precise measurements of bioactivity and metabolism of these isomers remains to be determined. C. Figure 2. A model for cellular leukotriene synthesis and transcellular metabolism. A. In a model insect cell system a FLAP/5-LO membrane-bound complex metabolizes exogenously-supplied AA to LTA4 and HPETE. B. In a leukocyte model the addition of membrane-bound phospholipase allows mobilization of AA from membrane phospholipids. AA presentation via FLAP to 5-LO allows cellular production of LTB4 which can be secreted from cell. C. In transcellular metabolism LTA4 is transferred from cells expressing 5-LO to cells containing LTA4 hydrolase or LTC4 synthase activity. Recipient cells need not express 5-LO for LT production to occur. Nonenzymatic breakdown of LTA4 also occurs. (A, B from Abramovitz et al. (13), C from Lewis et al. (2).) Production of Peptidyl Leukotrienes LTA4 can also follow another enzymatic pathway to create the major bioactive peptidyl leukotrienes LTC 4, LTD4, and LTE4. The reaction sequence begins with the conjugation of reduced glutathione to form LTC 4 (figure 1), catalyzed by LTC4 synthase. The glutathione S-transferase (GST) function of LTC4 synthase is unusual in its high specificity for LTA4 and glutathione as reaction substrates (4), as well as high amino acid sequence homology of LTC4 synthase with FLAP (20). Further similarities to FLAP include LTC4 synthase sensitivity to FLAP inhibitor MK-886, and both are integral membrane proteins (20). However, because no catalytic function has been assigned to FLAP it is assumed that the primary FLAP function is to direct substrate in 5-LO reactions (21). The FLAP-like moiety of LTC4 synthase is likely responsible for binding LTA4 during the glutathione transferase reaction (20). In addition to the originally characterized LTC4 synthase, another human microsomal glutathione S-transferase (MGSTII) sharing similar properties has been identified and partially characterized (22). Experiments utilizing proteinspecific polyclonal antibodies have indicated substantial expression in liver tissue and endothelial cells. Furthermore, MGST-II expression in endothelial cells may be responsible for substantial production of LTC 4 in this cell type (23). Production of LTD4 involves the -glutamyl transpeptidase catalyzed removal of glutamate from LTC4. In addition to -glutamyl transpeptidase, a similar LTC4 cleavage activity has been reported in humans (24). Recent studies using recombination-derived totally -glutamyl transpeptidase-deficient mice indicate that alternative pathways for LTC4-specific glutamyl cleavage exist for this species as well (25). The biosynthesis of the final LT of bioactive importance involves the cleavage of the glycine moiety from LTD4 to produce LTE4. This reaction is catalyzed by a variety of dipeptidases which are likely membrane-bound (2, 4, 5). An additional leukotriene, LTF4, has been demonstrated as an in vitro product of -glutamyl transpeptidase glutamyl addition to LTE4, although no in vivo evidence for this reaction exists. Further Metabolism of Leukotrienes Although LT are rapidly metabolized to inactivity, some metabolites retain a fraction of original bioactivity (typically more than 100-fold activity decrease compared to precursor molecules). LTB 4 is converted within neutrophils to 20-OH LTB4 via -oxidation by LTB4-20-hydroxylase, a cytochrome p450-like enzyme (2, 4, 5). Subsequent oxidation yields an inactive product. LTB4 may also be converted to less bioactive12-oxo-LTB4 by LTB4 12-dehydrogenase (26). In addition to the peptidyl LT biosynthetic conversions, LTE 4 may become Nacetylated. Peptidyl LT are also subject to peroxidative inactivation by conversion to diasteromeric sulfoxides, and LTC4 may be converted to 6-trans-LTB4 isomers which are degraded by -oxidation (2, 27) (figure 3). Figure 3. Metabolism of leukotrienes. Rapid, localized metabolism of LTB4 and LTC4 is shown in the right-hand panel. The progressive inactivation of LTB4 by the sequential oxidation of its terminal carbon is shown on the far right. The left-hand panel shows the peptidolytic cleavage of LTC4 and the potential acetylation of LTE4. (From Lewis et al. (2).) Future Directions Intensive research involving LT metabolism over the last 10 years has hastened the development of synthetic inhibitors of LT formation. The application of recombinant DNA technology to cloning and characterizing LT biosynthetic enzymes has been of tremendous importance, and it has logically followed that these enzymes are ideal targets for specific inhibitors to allow switching off a metabolic pathway. The further application of these techniques to allow mutagenesis studies of enzyme active sites or potential allosteric or inducer sites is likely to be an area of highly active research for years to come. References (1)* Mayatepek, E., and Hoffman, G. F. (1995) Pediatric Res. 37, 1-9 (2)* Lewis, R. A., Austen, K. F., and Soberman, R. J. (1990) New England J. Med. 323, 645-655 (3)* Samuelsson, B., Dahlen, S-E., Lindgren, J., Rouzer, C. A., and Serhan, C. N. (1987) Science 237, 1171-1176 (4)* Needleman, P., Turk, J., Jakschik, B. A., Morrison, A. R., and Lefkowth, J. B. (1986) Ann. Rev. Biochem. 55, 69-102 (5)* Hammerstrom, S. (1983) Ann. Rev. Biochem. 52, 355-377 (6) Bloemers, S. M., Verheule, S., Peppelenbosch, M. P., Smit, M. J., Tertoolen, L. G. J., and de Laat, S. (1998). J. Biol. Chem. 237, 2249-2255 (7) Kirchner, T., Aparicio, B., Argentieri, D. C., Lau, C. Y., and Ritchie, D. M. (1997) Prostaglandins Leukot. Essent. Fatty Acids 56, 417-423 (8)* Brooks, C. D. W., and Summers, J. B. (1996) J. Med. Chem. 39, 2629-2654 (9) Marshall, L. A., Bolognese, B., Winkler, J. D., and Roshak, A. (1997) J. Biol. Chem. 272, 759-765 (10) Wijkander, J., O’Flaherty, J. T., Nixon, A. B., and Wykle, R. L. (1995) J. Biol. Chem. 270, 26543-26549 (11)* Ford-Hutchinson, A. W., Gresser, M., and Young, R. N. (1994) Ann. Rev. Biochem. 63, 383-417 (12) Brock, T. G., McNish, R. W., and Peters-Golden, M. (1995) J. Biol. Chem. 270, 21652-21658 (13) Abramovitz, M., Wong, E., Cox, M. E., Richardson, C. D., Li, C., and Vickers, P. J. (1993) Eur. J. Bioch. 215, 105-111 (14) Sala, A., Bolla, M., Zarini, S., Muller-Peddinghaus, R., Folco, G. (1996) J. Biol. Chem. 271, 17944-17948 (15)* Krump, E., Picard, S., Mancini, J., and Borgeat, P. (1997) J. Exp. Med. 186, 1401-1406 (16)* Wetterholm, A., Mueller, M. J., Blomster, M., Samuelsson, B., and Haeggstrom, J. Z. (1997) Adv. Exp. Med. Biol. 407, 1-7 (17) Rybina, I. V., Liu, H., Gor, Y., and Feinmark, S. J. (1997) J. Biol. Chem. 272, 31865-31871 (18) Fradin, A., Zirolli, J. A., Maclouf, J., Vausbinder, L., Henson, P. M., and Murphy, R. C. (1989) J. Immunol. 143, 3680-3685 (19) Harrison, K. A., and Murphy, R. C. (1995) J. Biol. Chem. 270, 17273-17278 (20) Penrose, J. F., Spector, J., Baldasaro, M., Xu, K., Boyce, J., Arm, J. P., Austen, K. F., and Lam, B. K. (1996) J. Biol. Chem. 271, 11356-11361 (21) Goetzl, E. J., An, S., and Smith, W. H. (1995) FASEB J. 9, 1051-1058 (22) Jakobsson, P-J., Mancini, J. A., and Ford-Hutchinson, A. W. (1996) J. Biol. Chem. 271, 22203-22210 (23) Scoggan, K. A., Jakobsson, P-J., and Ford-Hutchinson, A. W. (1997) J. Biol. Chem. 272, 10182-10187 (24) Heisterkamp, N., Rajpert-De Meyts, E., Uribe, L., Forman, H. J., and Groffen, J (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 6303-6307 (25) Carter, B. Z., Wiseman, A. L., Orkiszewski, R., Ballard, K. D., Ou, C-N., and Lieberman, M. W. (1997) J. Biol. Chem. 272, 12305-12310 (26) Yokomizo, T., Ogawa, Y., Uozumi, N., Kume, K., Izumi, T., and Shimizu, T. (1996) J. Biol. Chem. 271, 28442850 (27) Wheelan, P., and Murphy, R. C. (1995) J. Biol. Chem. 270, 19845-19852 *review article