Metabolism and Detection of Arachidonic Acid

Arachidonic acid (AA) is an important essential fatty acid and the most abundant and widely distributed polyunsaturated fatty acid in human body, which is of great significance in maintaining the structure and function of cell membranes of the body. At present, arachidonic acid is widely used in health food, cosmetics and medicine.

Arachidonic acid is a 20-carbon unsaturated fatty acid, the majority of which is bound in cell membrane phospholipids, and the concentration of free AA inside and outside the cell is very low. When the cell membrane is stimulated by certain stimuli (such as inflammatory stimuli), membrane phospholipids are catalyzed by phospholipase A2 and phospholipase C system to hydrolyze and release AA, which is oxidized into different metabolites by two pathways: cyclooxygenase (COX) and lipoxygenase (LOX).

Schematic representation of the arachidonic acid metabolic pathway (Palumbo et al, 2017).

Fatty acid cyclooxygenase pathway

Cyclooxygenase (COX) is present in the endoplasmic reticulum of various mammalian cells and is highly active. AA is catalyzed by COX and converted to PGG2, which is then degraded by prostaglandin peroxidase to PGH2, while releasing oxygen free radicals. PGG2 and PGH2 are unstable and are metabolized into various prostaglandins and thromboxane in two major systems in different cells, respectively. In macrophages, neutrophils and lymphocytes, PGH2 is converted to PGD2 by the action of 11-ketoisomerase or to PGE2 by the action of 9-ketoisomerase, which is converted to PGF2α by the action of 9-keto reductase. PGH2 is also converted to PGI2 (prostacyclin) by the action of prostaglandin synthase. PGI2 is rapidly and spontaneously hydrolyzed to 6- PGH2 is converted to thromboxane A2 (TXA2) in platelets by the action of thromboxane synthase.

Role of metabolites of the cyclooxygenase pathway:

Prostaglandins (PG) are synthesized by a variety of mammalian cells, except for erythrocytes. PG is generally not stored intracellularly, but is only synthesized and released in response to some stimulus.

PGE2 and PGI2 have stronger vasodilating effects, reduce vascular tone, increase vascular permeability, enhance bradykinin and histamine-induced edema, stimulate leukocyte chemotaxis, and inhibit platelet aggregation.

During the metabolism of AA, the generation of PG is accompanied by the production of various oxygen radicals, including superoxide ions, hydroxyl radicals, epoxide radicals and hydrogen peroxide, all of which can cause tissue damage.

In addition, different PGs have opposite effects among themselves and with TXA2 and other inflammatory mediators, such as PGF2α increases vascular tone and decreases vascular permeability, PGI2 inhibits leukocyte chemotaxis, and TXA2 increases vascular tone and platelet aggregation capacity.

PGE1 and PG2 do not cause pain but sensitize pain perception.

Lipoxygenase metabolic pathway

AA is first formed into unstable hydroperoxyeicosatetraenes (HPETE) by lipoxygenase action. 5-Lipoxygenase is the main metabolic enzyme for the production of leukotrienes (LTs). Unlike cyclooxygenases, 5-lipoxygenases are found only in neutrophils, basophils, eosinophils, monocytes and some mast cells. The 5-HPETE formed by the action of 5-lipoxygenase is first converted to the unstable LTA4, and then further converted to LTB4 or LTC4 sulfated polypeptide, which can be progressively metabolized by a series of transpeptidases to LTD4, LTE4, and LTF4 sulfated polypeptide. LTB4 is mainly produced by neutrophils. Monocytes and macrophages produce LTB4 and LTC4, while basophils, eosinophils and some mast cells mainly produce leukotriene sulfated polypeptides. In platelets, AA is acted upon by 12-lipoxygenase to form 12-HPETE, which is further broken down to 12-hydroxyeicosatetraenoic acid (12-HETE). In addition, 15-lipoxygenase converts AA to 15-HPETE, which is further converted to 15-HETE and trihydroxyeicosatetraenoic acid.

Metabolic pathway of arachidonic acid via 5-lipoxygenase (Knab et al., 2016).

Many peroxisomal fatty acids produced in the lipoxygenase metabolic pathway have a strong activating effect on cyclooxygenase, and thus the lipoxygenase metabolic pathway indirectly affects the metabolism of the cyclooxygenase pathway.

Role of lipoxygenase pathway metabolites:

Leukotrienes enhance vascular permeability and cause edema at the site of inflammation.

LTB4, C4, D4, and E4 all cause increased capillary and subsequent microvascular exudation.

LTB4 has strong chemotactic effects on neutrophils, monocytes and eosinophils, causing leukocytes, especially neutrophils, to accumulate at the site of inflammation, promoting leukocyte adhesion to capillaries and microvascular endothelium, and accelerating leukocyte leakage across the postcapillary venous wall.

Analysis of arachidonic acid metabolites

Arachidonic acid metabolites are biomarkers for a variety of diseases, and accurate qualitative and quantitative analysis is of great value in studying the development mechanism, diagnosis and prediction of related diseases. Currently, four major classes of AA metabolites have been identified: prostaglandins, thromboxanes, leukotrienes, and hydroxy fatty acids.

Since the majority of AA metabolites are present at extremely low levels in biological samples, they cannot be accurately characterized and quantified by conventional detection techniques. With the development of technology, liquid chromatography-mass spectrometry (LC-MS) analytical techniques have shown great advantages in the detection and analysis of endogenous small molecules, as demonstrated by the powerful qualitative and quantitative capabilities of high-resolution time-of-flight mass spectrometry (Q-TofMS) and triple quadrupole linear ion trap mass spectrometry (Q-TrapMS), respectively.

Creative Proteomics is equipped with an advanced mass spectrometry platform and has extensive experience in metabolite analysis to provide you with arachidonic acid metabolite analysis services.

References

1. Palumbo, S. (2017). Pathogenesis and progression of multiple sclerosis: the role of arachidonic acid-mediated neuroinflammation. Exon Publications, 111-123.
2. Knab, L. M., Grippo, P. J., & Bentrem, D. J. (2014). Involvement of eicosanoids in the pathogenesis of pancreatic cancer: the roles of cyclooxygenase-2 and 5-lipoxygenase. World journal of gastroenterology: WJG, 20(31), 10729.