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Prostaglandins and Thromboxanes

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Prostaglandins and Thromboxanes
Page 431
Figure 10.61 ­Hexosaminidase reaction.
normal. The enzyme assays have been used to detect affected fetuses and carriers in utero, using cultured fibroblasts obtained by amniocentesis as a source of enzyme.
Except for Gaucher's disease, there is no therapy for the sphingolipidoses; the role of medicine at present is prevention through genetic counseling based on enzyme assays of the type discussed above. A discussion of the diagnosis and therapy of Gaucher's disease is presented in Clin. Corr. 10.4.
10.5— Prostaglandins and Thromboxanes
Prostaglandins and Thromboxanes Are Derivatives of Twenty­Carbon, Monocarboxylic Acids
In mammalian cells two major pathways of arachidonic acid metabolism produce important mediators of cellular and bodily functions: the cyclooxygenase and the lipoxygenase pathways. The substrate for both pathways is unesteri­
Figure 10.62 Structures of the major prostaglandins.
Page 432
Figure 10.63 Structure of prostanoic acid.
fied arachidonic acid. The cyclooxygenase pathway leads to a series of compounds including prostaglandins and thromboxanes. Prostaglandins were discovered through their effects on smooth muscle, specifically their ability to promote the contraction of intestinal and uterine muscle and the lowering of blood pressure. Although the complexity of their structures and the diversity of their sometimes conflicting functions often create a sense of frustration, the potent pharmacological effects of the prostaglandins have afforded them an important place in human biology and medicine. With the exception of the red blood cell, the prostaglandins are produced and released by nearly all mammalian cells and tissues; they are not confined to specialized cells. Unlike most hormones, prostaglandins are not stored in cells but instead are synthesized and released immediately.
Figure 10.64 Synthesis of E and F prostaglandins from fatty acid precursors.
Page 433
Figure 10.65 Cyclooxygenase reaction.
There are three major classes of primary prostaglandins, the A, E, and F series. The structures of the more common prostaglandins A, E, and F are shown in Figure 10.62 (p. 431). All are related to the hypothetical parent compound, prostanoic acid (Figure 10.63). Note that the prostaglandins contain a multiplicity of functional groups; for example, PGE2 contains a carboxyl group, a b ­hydroxyketone, a secondary alkylic alcohol, and two carbon–carbon double bonds. The three classes (A, E, and F) are distinguished on the basis of the functional groups about the cyclopentane ring (Figure 10.64): the E series contains a b ­hydroxyketone, the F series are 1,3­diols, and those in the A series are a b­unsaturated ketones. The subscript numerals 1, 2, and 3, refer to the number of double bonds in the side chains. The subscript a refers to the configuration of the C­9 OH group: an a ­hydroxyl group projects "down" from the plane of the ring.
The most important dietary precursor of the prostaglandins is linoleic acid (18:2), which is an essential fatty acid. In adults linoleic acid is ingested daily in amounts of about 10 g. Only a very minor part of this total intake is converted by carbon chain elongation and desaturation in liver to arachidonic acid (eicosatetraenoic acid) and to some extent also to dihomo­ g­linolenic acid. Since the total daily excretion of prostaglandins and their metabolites is only about 1 mg, it is clear that the formation of prostaglandins is a quantitatively unimportant pathway in the overall metabolism of fatty acids. At the same time, however, the metabolism of prostaglandins is completely dependent on a regular and constant supply of linoleic acid. When the diet is deficient in linoleic acid, there is decreased production of prostaglandins. The diet also provides arachidonic acid.
Synthesis of Prostaglandins Involves a Cyclooxygenase
The immediate precursors to the prostaglandins are C20 polyunsaturated fatty acids containing 3, 4, and 5 carbon–carbon double bonds. Since arachidonic acid and most of its metabolites contain 20 carbon atoms, they are referred to as eicosanoids. During their transformation into various prostaglandins they are cyclized and take up oxygen. Dihomo­ g­linolenic acid (20:3(8,11,14)) is the precursor to PGE1 and PGF1a; arachidonic acid (20:4(5,8,11,14)) is the precursor to PGE2 and PGF2a; and eicosapentaenoic acid (20:4(5,8,11,14,17)) is the precursor to PGE3 and PGF3a (see Figure 10.64).
Compounds of the 2­series derived from arachidonic acid are the principal prostaglandins in humans and are of the greatest significance biologically. The
Figure 10.66 Conversion of PGG2 to PGH2; PG hydroperoxidase (PGH synthase) reaction.
Page 434
Figure 10.67 Major routes of prostaglandin biosynthesis.
central enzyme system in prostaglandin biosynthesis is the prostaglandin synthase (PGS) complex, which catalyzes oxidative cyclization of polyunsaturated fatty acids. Arachidonic acid is derived from membrane phospholipids by the action of the hydrolase phospholipase A2. This cleavage step is the rate­limiting step in prostaglandin synthesis and some agents that stimulate prostaglandin production act by stimulating the activity of phospholipase A2. Cholesterol esters containing arachidonic acid may also serve as a source of arachidonic acid substrate.
The cyclooxygenase component of the prostaglandin synthase complex catalyzes the cyclization of C­8–C­12 of arachidonic acid to form the cyclic 9,11­
endoperoxide 15­hydroperoxide, PGG2. The reaction requires two molecules of oxygen (Figure 10.65; see p. 433). PGG2 is then converted to prostaglandin H2 (PGH2) by a reduced glutathione (GSH)­dependent peroxidase (PG hydroperoxidase) (Figure 10.66; see p. 433). Details of the additional steps leading to individual prostaglandins remain to be elucidated. Reactions that cyclize polyunsaturated fatty acids are found in the membranes of the endoplasmic reticulum. Major pathways of prostaglandin biosynthesis are summarized in Figure 10.67. Formation of primary prostaglandins of the D, E, and F series and of thromboxanes or prostacyclin (PGI2) is mediated by different specific enzymes, whose presence varies depending on the cell type and tissue. This results in a degree of tissue specificity as to the type and quantity of prostaglandin produced. In kidney and spleen PGE2 and PGF2a are the major prostaglandins formed. In contrast, blood vessels produce mostly PGI2 and PGF2a. In the heart PGE2, PGF2a, and PGI2 are formed in about equal amounts. Thromboxane A2 (TXA2) is the main prostaglandin endoperoxide formed in platelets.
There are two forms of cyclooxygenase (COX) or prostaglandin synthase (PGS). COX­1, or PGS­1, is a constitutive enzyme found in gastric mucosa,
Page 435
platelets, vascular endothelium, and kidney. COX­2, or PGS­2, is inducible and is generated in response to inflammation. It is expressed mainly in activated macrophages and monocytes when they are stimulated by platelet­activating factor (PAF), interleukin­1, or bacterial lipopolysaccharide (LPS), and in smooth muscle cells, epithelial and endothelial cells, and neurons. PGS­2 induction is inhibited by glucocorticoids. The two forms of PGS catalyze both oxygenation of arachidonic acid to PGG2 and the reduction of PGG2 to PGH2, which is the peroxidase reaction.
Prostaglandins have a very short half­life. Soon after release they are rapidly taken up by cells and inactivated either by oxidation of the 15­hydroxy group or by b ­
oxidation from the C1­COOH end of the fatty acid chain. The lungs appear to play an important role in inactivating prostaglandins.
Figure 10.68 Synthesis of TXB2 from PGH2.
Thromboxanes are highly active metabolites of the PGG2­ and PGG2­type prostaglandin endoperoxides that have the cyclopentane ring replaced by a six­membered oxygen­containing (oxane) ring. The term thromboxane is derived from the fact that these compounds have a thrombus­forming potential. Thromboxane A synthase, present in the endoplasmic reticulum, is abundant in lung and platelets and catalyzes conversion of endoperoxide PGH2 to TXA2. The half­life of TXA2 is very short in water (t1/2 ~ 1 min) as the compound is transformed rapidly into inactive thromboxane B2 (TXB2) by the reaction shown in Figure 10.68.
Prostaglandin Production Is Inhibited by Steroidal and Nonsteroidal Anti­inflammatory Agents
Two types of drugs affect prostaglandin metabolism and are therapeutically useful. The nonsteroidal, anti­inflammatory drugs (NSAIDs), such as aspirin (acetylsalicylic acid), indomethacin, and phenylbutazone, block prostaglandin production by inhibiting cyclooxygenase. In the case of aspirin, irreversible inhibition occurs by acetylation of the enzyme. Other NSAIDs inhibit cyclooxygenase but do so by binding noncovalently to the enzyme instead of acetylating it; they are called ''non­aspirin NSAIDs." Certain NSAIDs inhibit COX­1 more than COX­2 and vice versa. These drugs are not without their undesirable side effects; aplastic anemia can result from phenylbutazone therapy. Steroidal anti­inflammatory drugs like hydrocortisone, prednisone, and betamethasone block prostaglandin release by inhibiting phospholipase A2 activity so as to interfere with mobilization of arachidonic acid (see Figure 10.69). The rate­limiting step in the synthesis of prostaglandins is release of arachidonic acid from membrane phospholipid stores in response to phospholipase A2 activation.
Factors that govern the biosynthesis of prostaglandins are poorly understood, but, in general, prostaglandin release seems to be triggered following hormonal or neural excitation or after muscular activity. For example, histamine stimulates an increase in the prostaglandin concentration in gastric perfusates. Also, prostaglandins are released during labor and after cellular injury (e.g., platelets exposed to thrombin, lungs irritated by dust).
Prostaglandins Exhibit Many Physiological Effects
Prostaglandins are natural mediators of inflammation. Inflammatory reactions most often involve the joints (rheumatoid arthritis), skin (psoriasis), and eyes, and inflammation of these sites is frequently treated with corticosteroids that inhibit prostaglandin synthesis. Administration of PGE2 or PGE1 induce the signs of inflammation that include redness and heat (due to arteriolar vasodilation) and swelling and edema resulting from increased capillary permeability. PGE2 generated in immune tissues (e.g., macrophages, mast cells, B cells) evokes chemokinesis of T cells. PGE2 in amounts that alone do not cause pain, prior to administration of the autocoids, histamine and bradykinin, enhance both the intensity and duration of pain caused by these two agents. It is thought that
Figure 10.69 Site of action of inhibitors of prostaglandin synthesis.
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