An elucidation of the arachidonic acid cascade. Discovery of prostaglandins, thromboxane and leukotrienes.

Abstract

Arachidonic acid is normally stored in membrane-bound phospholipids and released by the action of phospholipases. Enzymatic conversion of released arachidonic acid into biologically active derivatives proceeds through one of several routes. Cyclo-oxygenase converts arachidonic acid to unstable cyclic endoperoxides from which prostaglandins, prostacyclin and thromboxanes are derived. Formation of the leukotrienes from arachidonic acid is initiated by the action of 5-lipoxygenase producing leukotriene A4. Hydrolysis of leukotriene A4, or the incorporation of glutathione results in the formation of leukotriene B4 and C4, respectively. In addition, 12- and 15-lipoxygenase can catalyse arachidonic acid conversion and lipoxins A and B are amongst the possible products. Many of these metabolites of arachidonic acid feature prominently in the development of inflammation. Prostaglandin E2 and prostacyclin are potent vasodilators, while leukotriene D4 causes cellular adhesion, chemotaxis of neutrophils and degranulation. Leukotrienes C4, D4 and E4 contribute to inflammation by increasing vascular permeability. Leukotrienes are also believed to play an important pathophysiological role in allergic broncho-constriction of asthma. Through pharmacological intervention in the arachidonic acid cascade various anti-inflammatory agents have been developed. These include aspirin-like drugs, which inhibit cyclo-oxygenase. Corticosteroids appear to indirectly inhibit phospholipases thus preventing release of arachidonic acid. Future progress in this field is likely to produce drugs which antagonise arachidonic acid derivatives or inhibit the enzymes involved in their synthesis with greater specificity.