Prostaglandins and Related Compounds 1Dr. Nikhat Siddiqi

Prostaglandins, and the related compounds thromboxanes and leukotrienes, are collectively known as eicosanoids to reflect their origin from polyunsaturated fatty acids with 20 carbons. They are extremely potent compounds that elicit a wide range of responses, both physiologic and pathologic. Although they have been compared to hormones in terms of their actions, eicosanoids differ from the true hormones in that they are produced in very small amounts in almost all tissues rather than in specialized glands. Their biologic actions are mediated by plasma membrane G protein–coupled receptors, which are different in different organ systems. 2Dr. Nikhat Siddiqi

Examples of prostaglandin structures Prostaglandins are named as follows: PG plus a third letter (for example, A, D, E, F), which designates the type and arrangement of functional groups in the molecule. PGI 2 is known as prostacyclin. The subscript number indicates the number of double bonds in the molecule. Thromboxanes are designated by TX and leukotrienes by LT. 3Dr. Nikhat Siddiqi

Synthesis of prostaglandins and thromboxanes The dietary precursor of the prostaglandins is the essential fatty acid, linoleic acid. It is elongated and desaturated to arachidonic acid, the immediate precursor of the predominant class of prostaglandins (those with two double bonds) in humans. 4Dr. Nikhat Siddiqi

Synthesis of PGH 2 The first step in the synthesis of prostaglandins is the oxidative cyclization of free arachidonic acid to yield PGH 2 by prostaglandin endoperoxide synthase (PGH synthase). This enzyme is an endoplasmic reticulum membrane-bound protein that has two catalytic activities: fatty acid cyclooxygenase (COX), which requires two molecules of O 2, and peroxidase, which is dependent on reduced glutathione. 5Dr. Nikhat Siddiqi

PGH 2 is converted to a variety of prostaglandins and thromboxanes, as shown in, by cell- specific synthases. 6Dr. Nikhat Siddiqi

Isozymes of PGH synthase Two isozymes, usually denoted as COX-1 and COX-2, of the synthase are known. COX-1 is made constitutively in most tissues, and is required for maintenance of healthy gastric tissue, renal homeostasis, and platelet aggregation. COX-2 is inducible in a limited number of tissues in response to products of activated immune and inflammatory cells. 7Dr. Nikhat Siddiqi

Inhibition of prostaglandin synthesis The synthesis of prostaglandins can be inhibited by a number of unrelated compounds. For example, cortisol (a steroidal anti- inflammatory agent) inhibits phospholipase A 2 activity and, therefore, the precursor of the prostaglandins, arachidonic acid, is not available. 8Dr. Nikhat Siddiqi

Aspirin, indomethacin, and phenylbutazone (all nonsteroidal anti- inflammatory agents [NSAIDS]) inhibit both COX-1 and COX-2 and, therefore, prevent the synthesis of the parent prostaglandin, PGH 2. 9Dr. Nikhat Siddiqi

Synthesis of leukotrienes Arachidonic acid is converted to a variety of linear hydroperoxy acids by a separate pathway involving a family of lipoxygenases. For example, neutrophils contain 5-lipoxygenase, which converts arachidonic acid to 5-hydroxy- 6,8,11,14 eicosatetraenoic acid (5-HPETE). 5-HPETE is converted to a series of leukotrienes, the nature of the final products varying according to the tissue. Leukotrienes are mediators of allergic response and inflammation. 10Dr. Nikhat Siddiqi

Role of prostaglandins in platelet homeostasis In addition to their roles in mediating inflammation, fever, and allergic response, and ensuring gastric integrity and renal function, eicosanoids are involved in a diverse group of physiologic functions, including ovarian and uterine function, bone metabolism, nerve and brain function, smooth muscle regulation, and platelet homeostasis. Thromboxane A 2 (TXA 2 ) is produced by activated platelets. It promotes adherence and aggregation of circulating platelets, and contraction of vascular smooth muscle, thus promoting formation of blood clots (thrombi). Prostacyclin (PGI 2 ), produced by vascular endothelial cells, inhibits platelet aggregation and stimulates vasodilation, and so impedes thrombogenesis. The opposing effects of TXA 2 and PGI 2 limit thrombi formation to sites of vascular injury. Aspirin has an antithrombogenic effect. It inhibits TX A 2 synthesis from arachidonic acid in platelets by irreversible acetylation and inhibition of COX-1. 11Dr. Nikhat Siddiqi

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