Arachidonic acid metabolism Pathway
Arachidonic acid is a polyunsaturated fatty acid with a chemical formula of C20H32O2, 20:4(ω-6). In this chemical formula, 20:4 refers to its 20 carbon atom chain with four Cis double bonds, while (ω-6) refers to the position of the first double bond from the last, omega carbon atom. The four double bonds of Arachidonic acid makes it susceptible to oxygenation, which leads to excessive number of metabolites of significant importance for the proper function of the immune system, promotion of allergies and inflammation, resolving of inflammation, mood, and appetite.
Arachidonic acid is often referred to as Omega-6 fatty acids. It is usually present in phospholipids of cellular membranes of the body and also abundantly present in the brain, muscles, and liver. Arachidonic acid is also involved in cellular signaling and regulation of enzymes involved in cell signaling. It is also a vasodilator and can be beneficial in the management of cardiovascular related diseases.
Arachidonic acid precursors
Arachidonic acid can be essential or nonessential fatty acid depending on the ability or inability of the animal to utilize it. It is derived from ϒ-Linolenic acid, which is also synthesized from linoleic acid. Linoleic acid is an essential fatty acid, which can be derived from eaten plant sources. The fatty acid is then transformed by the enzyme 6– desaturase.
Arachidonic Acid metabolism
There are three major pathways involved in Arachidonic acid metabolism, these three pathways are Cyclooxygenase (COX) enzyme pathway, Lipoxygenase enzyme pathway and Cytochrome P450 pathway. The cyclooxygenase catalyzes the transformation of arachidonic acid to prostaglandins and thromboxane, cytochrome P450 catalyzes the transformation to arachidonic acid to hydroeicosatetraenoic acids (HETEs),and Epoxyeicosatrienoic acids, while Lipoxygenase converts arachidonic acid to leukotrienes.
Arachidonic acid cyclooxygenase metabolism pathway
Before the catabolism of arachidonic acid to prostaglandins and thromboxanes can take place, arachidonic acid is first released from the phospholipids of the plasma membrane. This activity is catalyzed by the enzymes, phospholipases. Thereafter, arachidonic acid is being acted upon by the bifunctional enzyme, cyclooxygenase, also known as prostaglandin G/H synthase (PGHS) metabolizes the conversion of arachidonic acid to prostaglandin H2 (PH2) and prostaglandin G2 (PG2). This enzyme activity takes place in the endoplasmic reticulum (ER).
In the first step of the arachidonic acid metabolism through the cyclooxygenase pathway, the cyclooxygenase activity of COX introduces oxygen molecule to the arachidonic acid, thereby converting it to prostaglandin G2 (PGG2). This is further acted upon by the peroxidase activity of the COX enzyme, which then converts PGG2 to prostaglandin H2 (PGH2).
There are two major isoforms of COXs. COX-1 is constantly expressed in many mammalian cells, and tissues. It is responsible for the formation of Prostaglandins involved in general regulation of physiological activities. COX-2 on the other hand is present at low concentrations in inflammatory cells, and is strongly induced by inflammatory stimuli such as endotoxins, tumor promoters, cytokines and some lipids.
Biosynthesis of thromboxane (TX) through arachidonate pathway
Thromboxane is a vasoconstrictors. They are been produced by the thromboxane synthase present in the blood platelets (Thrombocytes), which catalyzes the conversion of prostaglandins H2 to thromboxane A2. From this thromboxane A2, other forms of thromboxanes are being formed. Thromboxane induces the early steps in the constriction of the blood vessels, and platelet aggregation.
Like prostaglandins, thromboxane contains a ring of five or six atoms. The pathway that leads from arachidonate to these two compounds is sometimes referred to as the “Cyclic” pathway, to distinguish it from the pathway that leads from arachidonic acid to leukotrienes, usually referred to as “Linear” pathway.
In the biosynthesis of thromboxane, arachidonate is first converted in a two step reaction to prostaglandin H2 (PGH2). PGH2 is then acted upon by thromboxane synthase to form thromboxane.
Cyclooxygenase 2 (COX-2) is responsible for inflammation in the body
Both isoforms of COX enzyme convert arachidonate to prostaglandins. However, they differ significantly in their distribution in the body, and their roles in health and disease. The COX-1 enzyme, which is the predominantly constitutive form of the enzyme, and expressed throughout the body is responsible for certain body homeostatic functions like maintenance of normal gastric mucosa, influencing renal blood flow, and aiding in blood clotting by stimulating platelet aggregation. COX-2, in contrast to COX-1, is the inducible form of the cyclooxygenase isoform. COX-2 is expressed in response to inflammatory and other physiologic stimuli, and growth factors. It is involved in the production of those prostaglandins that mediate pain and support the inflammatory process.
Aspirin and Ibuprofen are inhibitors of COX-2
Non-steroidal anti-inflammatory drugs (NSAIDs) such as Aspirin and Ibuprofen function as pain relievers by blocking the cyclooxygenase enzyme pathway. The blocking of the COX enzymes results in the suppression of certain inflammatory factors such as cytokines, endotoxins, Nf-kB, tumor necrosis factors and growth factors.
COX-2 has been identified as a major mediator of inflammatory responses, and also plays an important role in cancer progression. This is because the release of inflammatory molecules generates microenvironment which is highly favorable for development of tumor, cancer progression and metastasis. COX -2 has also been implicated in some virus-mediated tumorigenesis, thereby making COX-2 an important therapeutic target of some chemotherapy.
Aspirin causes ulcer by inhibiting COX-1
Although COX-1 is not involved in the mediation of inflammatory responses, most NSAIDs agents act by suppressing the activities of both COX-1 and COX-2. The inhibition of COX-1 by aspirin and ibuprofen lead to the suppression of certain physiological functions such as the production of gastric mucosa fluids, thereby resulting in ulceration of the mucosa wall due to increased acidity due to increased proton pump. This is because COX-1 functions as a housekeeping isoform of cyclooxygenase, and is constitutively expressed to serve functions such as control of renal blood flow, imparting protection to stomach against ulcers, production of prostaglandin E2 (PGE2) to maintain coherence and structure of gastric mucosal surface.
Thromboxane promotes blood clotting.
Thromboxane is involved in blood clotting and platelet aggregation, in addition to its vasoconstriction effect. To initiate blood clotting, thromboxane A2 (TXA2) would bind to its receptor. The binding of PXA2 to its receptor elicits the influx of Calcium ions, which directly increases the smooth muscle cells contractions. The vasoconstriction caused by TxA2 helps in the platelet aggregation, which leads to greater clot formation.
Biosynthesis of prostacyclin through arachidonic pathway
Prostacyclin is produced through the arachidonic acid metabolic pathway as one of the prostaglandins. Arachidonate is first converted to PGG2 by the cyclooxygenase activity of COX, and then to PGH2 by the endoperoxidation activity. Another enzyme, PGI synthase catalyzes the transformation of PGH2 to prostacyclin PGI2. PGI2 is an unstable 6,9α-epoxy compound and is nonenzymatically decomposed to 6-keto-PGF1α.
Prostacyclin (PG12) is a strong vasodilator, and the most potent inhibitor of platelet aggregation. It works in contrast to thromboxane, which is a vasoconstrictor. The inhibition of platelet aggregation by prostacyclin is through the stimulation of platelet adenyl cyclase leading to an increase in platelet cyclic AMP. The enzyme that synthesizes prostacyclin is predominantly concentrated in the endothelial layer of the vessel wall.
Prostacyclin can also function as a hormone; it acts on target cells mainly through its membrane G-protein coupled receptor, PTGIR. The binding of PGI2 to its cell surface G-coupled PTGIR leads to an increase of intracellular cAMP production and phosphatidylinositol response. This PGI2 signaling pathway is involved in vasodilation and inhibition of platelet aggregation, which is a crucial process for the maintenance of vascular homeostasis.
Relationship between prostacyclin and thromboxane
Both the thromboxane and prostacyclin are products of arachidonic acid metabolic pathway. Prostacyclin is synthesized in the endothelia layer of the vascular wall from prostaglandin H2 by the enzyme, PGI synthase. On the other hand, thromboxane is synthesized in the blood platelet by thromboxane synthase enzyme.
Both compounds share contrasting roles. Prostacyclin is a vasodilator, and a platelet aggregation inhibitor, while thromboxane is a vasoconstrictor and pro-platelet aggregator.
The balance between the activities of these compounds are important in the homeostatic interaction of the platelets and the vessel wall. A change in the balance between the two compounds may impact the development of thrombosis, atherosclerosis, and stroke. The inflammatory responses may also be implicated in the onset of diabetes mellitus.
Moncada S, Vane JR. Prostacyclin: its biosynthesis, actions and clinical potential. Philos Trans R Soc Lond B Biol Sci. 1981 Aug 18;294(1072):305-29.
Moncada S, Vane JR. Interrelationships between prostacyclin and thromboxane A2. Ciba Found Symp. 1980;78:165-83. doi: 10.1002/9780470720615.ch9. PMID: 6258879.
Simon LS. Role and regulation of cyclooxygenase-2 during inflammation. Am J Med. 1999 May 31;106(5B):37S-42S.
Williams, C. S., Mann, M., and DuBois, R. N. (1999). The role of cyclooxygenases in inflammation, cancer, and development. Oncogene 18, 7908-7916. doi: 10.1038/sj.onc.120328.
Leng, J., Han, C., Demetris, A. J., Michalopoulos, G. K., and Wu, T. (2003). Cyclooxygenase-2 promotes hepatocellular carcinoma cell growth through Akt activation: evidence for Akt inhibition in celecoxib-induced apoptosis. Hepatology 38 756-768.
Grann M, Comerma-Steffensen S, Arcanjo DD, Simonsen U. Mechanisms Involved in Thromboxane A2 -induced Vasoconstriction of Rat Intracavernous Small Penile Arteries. Basic Clin Pharmacol Toxicol. 2016 Oct;119 Suppl 3:86-95.