Arachidonic acid leukotriene biosynthesis from

Figure 1.9. Overview of the biosynthesis of ecosanoids. The 20 carbon fatty acid arachidonic acid is released from cell membrane phospholipids by the actions of phospholipase A2. Free arachidonic acid forms the precursor of prostaglandins and thromboxanes via the multi-enz5une cyclooxygenase pathway, while leukotrienes are formed via the lipoxygenase pathway...

Detailed accounts of the biosynthesis of the prostanoids have been pubUshed (14—17). Under normal circumstances arachidonic acid (AA) is the most abundant C-20 fatty acid m vivo (18—21) which accounts for the predominance of the prostanoids containing two double bonds eg, PGE2 (see Fig. 1). Prostanoids of the one and three series are biosynthesized from dihomo-S-linolenic and eicosapentaenoic acids, respectively. Concentrations ia human tissue of the one-series precursor, dihomo-S-linolenic acid, are about one-fourth those of AA (22) and the presence of PGE has been noted ia a variety of tissues (23). The biosynthesis of the two-series prostaglandins from AA is shown ia Eigure 1. These reactions make up a portion of what is known as the arachidonic acid cascade. Other Hpid products of the cascade iaclude the leukotrienes, lipoxins, and the hydroxyeicosatetraenoic acids (HETEs). Collectively, these substances are termed eicosanoids. 

Figure 8.9 Prostaglandins and leukotrienes are potent eicosanoid lipid mediators, derived from phospholipase-released arachidonic acids, that are involved in numerous homeostatic biological functions and inflammation. They are generated by cyclooxygenase isozymes and 5-lipoxygenase, respectively, and their biosynthesis and pharmacological actions are inhibited by clinically relevant nonsteroidal anti-inflammatory drugs.

Figure 3 Simple schema of the biosynthesis of prostaglandins (PG), thromboxanes (TX), and leukotrienes from arachidonic acid consequent to cell injury (modified after Bonica, 1990).

Overview of the biosynthesis and function of some important prostaglandins, leukotrienes, and a thromboxane from arachidonic acid. 

Arachidonic acid is of particular importance, being the initial compound for eicosanoid biosynthesis. Prostaglandins, prostacyclins and thromboxanes are synthesized from the eicosanoids via the cyclo-oxygenase pathway. The important mediators of inflammation — such as leukotrienes and leukotetraenes — are synthesized via the lipoxygenase pathway, (s. fig. 3.10)... 

Assay methods using combined gas chromatography-mass spectrometry with selected ion monitoring and addition of stable isotopes as carriers and internal standards have been published for the different HETE isomers [208,236,385]. However, so far no GC-MS-assay for LTB4 and related dihydroxy acids or the cysteinyl-containing leukotrienes have been described. Deuterated standards of HETEs were prepared from octadeuterated arachidonic acid either by biosynthesis in mammalian tissues [236,385] or by chemical methods [208]. Preparation of internal standards by chemical methods resulted in deuterated standards with minimal contamination of unlabeled material. This resulted in assays with a detection limit of about 1 pmol for the different HETEs. A GC-MS assay without addition of deuterated internal standards has also been described for analysis of the ratio between different HETEs in the same sample [210]. 

In addition to fatty acid cyclooxygenase (11,15-bis-lipoxygenase) there are several other lipoxygenases in mammalian tissues (Fig. 12). Leukocytes and mast cells contain a 5-lipoxygenase which leads to the biosynthesis of leukotrienes from arachidonic acid [176,177]. 12-Lipoxygenase, rich in platelets, initiates a bio.synthetic... 

Fig. 3.8 Biosynthesis of cysteinyl leukotrienes from arachidonic acid showing the pathways to the formation of LTAi, LTB4, LTC4, LTD4, and LTE4...

Leukotriene biosynthesis depends upon the availability of arachidonic acid (8) as the free carboxylic acid as the 5-LOX substrate, which typically requires the action of cytosolic phospholipase to release arachidonic acid (8) from membrane phospholipids [27]. The name leukotriene was conceived to capture two unique attributes of these molecules. The first relates to those white blood cells derived from the bone marrow that have the capacity to synthesize this class of eicosanoids, for example, the polymorph nuclear leukocyte. The last part of the name refers to the unique chemical structure, a conjugated triene retained within these eicosanoids [29,30]. The first step for the leukotriene biosynthesis is the insertion of molecular oxygen at position 5 of arachidonic add (8) to produce 5-HPETE (17) that can be converted to leukotriene (18) by the second catalytic activity... 

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