fig5

Figure 5. Overview of eicosanoid biosynthesis and receptor signaling. Arachidonic acid (AA) is liberated from the cellular membranes by cytoplasmic phospholipase A2 (cPLA2). Free AA can be metabolized by two main enzymes: cyclooxygenases (COXs) to form various prostanoids and 5-lipoxygenase (5-LOX) to form leukotrienes. In the COX pathway, the intermediate PGH2 is sequentially metabolized into prostaglandins and thromboxanes by specific prostaglandins and thromboxane synthases. In the 5-LOX pathway, AA is converted into the unstable leukotriene A4 (LTA4), which is subsequently converted to leukotriene B4 by LTA4 hydrolase (LTA4H) or to cysteinyl leukotriene LTC4 by LTC4 synthase (LTC4S). Each of the prostaglandins and leukotrienes exerts its biological effects through its cognate G protein-coupled receptor.