Biosynthesis and Main Biological Activities of Prostanoid Mediators from Arachidonic Acid.

A: Cyclooxygenases catalyze two reactions: (1) cyclooxygenase activity, the conversion of arachidonic acid to prostaglandin (PG) G2; and (2) peroxidase activity, the conversion of PGG2 to PGH2. B: The cell-specific synthases that are involved in the conversion of PGH2 to the five principal prostaglandins (PGI2, PGD2, PGE2, PGF2α, TxA2), collectively named prostanoids. Prostacyclin (PGI2) protects the stomach mucosa from damage and prevents aggregation of blood platelets. PGD2 is a mediator in brain and mast cells. PGE2 is mostly involved in pain, fever, and inflammation. PGF2α is involved in labor and parturition. Thromboxane A2 (TxA2) is mainly found in blood platelets and promotes clotting of blood by inducing platelet aggregation.