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The eicosanoids are an important group of endogenous fatty acid autacoids that are produced from arachidonic acid, a 20-carbon fatty acid lipid in cell membranes. Major families of eicosanoids include the straight-chain derivatives (leukotrienes) and cyclic derivatives (prostacyclin, prostaglandins, and thromboxane).

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AbortifacientA drug used to cause an abortion. Example: prostaglandin F
CyclooxygenaseEnzyme that converts arachidonic acid to PGG and PGH, the precursors of the prostaglandins, including PGE, PGF, prostacyclin, and thromboxane
DysmenorrheaPainful uterine cramping caused by prostaglandins released during menstruation
Great vessel transpositionCongenital anomaly in which the pulmonary artery exits from the left ventricle and the aorta from the right ventricle. Incompatible with life after birth unless a large patent ductus or ventricular septal defect is present
LipoxygenaseEnzyme that converts arachidonic acid to leukotriene precursors
NSAIDNonsteroidal anti-inflammatory drug, for example, aspirin, ibuprofen, celecoxib. NSAIDs are cyclooxygenase inhibitors
OxytocicA substance that causes uterine contraction
Patent ductus arteriosusAbnormal persistence after birth of the shunt between the pulmonary artery and the aorta; normal in the fetus
Phospholipase A2Enzyme in the cell membrane that generates arachidonic acid from membrane lipid constituents
Slow-reacting substance of anaphylaxis (SRS-A)Material originally identified by bioassay from tissues of animals in anaphylactic shock; now recognized as a mixture of leukotrienes, especially LTC4 and LTD4
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Classification

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The principal eicosanoid subgroups are the leukotrienes and a group of cyclic molecules, including prostaglandins, prostacyclin, and thromboxane. The leukotrienes retain the straight-chain configuration of arachidonic acid. Prostacyclin, thromboxane, and other members of the prostaglandin group are cyclized derivatives of arachidonic acid. There are several series for most of the principal subgroups, based on different substituents (indicated by letters A, B, etc) and different numbers of double bonds (indicated by a subscript number) in the molecule.

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Synthesis

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Active eicosanoids are synthesized in response to a wide variety of stimuli (eg, physical injury, immune reactions). These stimuli activate phospholipases in the cell membrane or cytoplasm, and arachidonic acid (a tetraenoic [4 double bonds] fatty acid) is released from membrane phospholipids (Figure 18–1). Arachidonic acid is then metabolized by several different enzymes. The 2 most important are lipoxygenase, which results in straight-chain leukotrienes, and cyclooxygenase (COX), which results in cyclization to prostacyclin, prostaglandins, or thromboxane. COX exists in at least 2 forms. COX-1 is found in many tissues; the prostaglandins produced by COX-1 appear to be important for a variety of normal physiologic processes (see later discussion). In contrast, COX-2 is found primarily in inflammatory cells; the products of its actions play a major role in tissue injury (eg, inflammation). In addition to these inflammatory functions, COX-2 is also responsible for synthesis of prostacyclin and of prostaglandins important in renal function. Thromboxane is preferentially synthesized in platelets, whereas prostacyclin is synthesized in the endothelial cells ...

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