Arachidonic Acid (AA) (⑤，8,11,14 eicosatetraenoic acid) COOH CH3 .20 carbon,unsaturated fatty acid .Gives rise to PGs,thromboxanes and leukotrienes .Phospholipase A2(PLA2)and phospholipase C(PLC)generate AA from phospholipids and diacylglycerols .Fatty acid cyclooxygenases(COX-1 and COX-2)initiate the biosynthesis of the PGs and TXs(prostanoids) Various lipoxygenases give rise to leukotrienes and lipoxins (inhib.of lipoxygenase are potential anti-asthmatic drugs)