Animal tissues have a metabolic system called the arachidonic acid cascade. Arachidonic acid, which is stored in the form of phospholipid, is released by the action of an esterase, phospholipase.
Arachidonic acid is converted into 5-hydroxyperoxy-6, 8, 11, 14-eicosatetraenoic acid (5-HPETE) by the addition of molecular oxygen by the action of 5-lipoxygenase. Part of the peroxy acid 5-HPETE is converted to form 5-hydroxy-6, 8, 11, 14-eicosatetraenoic acid (5-HETE). Further, 5-HPETE is converted into leukotriene A.sub.4 (LTA.sub.4) and leukotriene B.sub.4 (LTB.sub.4). Furthermore, LTA.sub.4 is convened into leukotriene C.sub.4 (LTC.sub.4), leukotriene D.sub.4 (LTD.sub.4), and leukotriene E.sub.4 (LTE.sub.4).
On the other hand, arachidonic acid is convened into prostaglandin G.sub.2 (PGG.sub.2) by cyclooxygenase. PGG.sub.2 is converted into prostaglandin H.sub.2 (PGH.sub.2) by PG hydroperoxytase. PGH.sub.2 is convened into thromboxane A.sub.2 (TXA.sub.2) by thromboxane (TX) synthetase. It is known that TXA.sub.2 thus obtained is very unstable, and therefore, immediately convened into thromboxane B.sub.2 (TXB.sub.2). ##STR4##
LTB.sub.4 has migratory and adhesive activities on leukocyte, and degranulation activity on leukocyte. LTC.sub.4, LTD.sub.4 and LTE.sub.4 have contractile activity on pulmonary and bronchial muscles, and artery.
It is known that TXA.sub.2 has platelet aggregation activity, and contractile activity on vascular smooth muscle and on bronchial smooth muscle.