Prostaglandin-endoperoxide synthase (PTGS), also known as cyclooxygenase (COX), is the key enzyme in prostaglandin biosynthesis. It converts free arachidonic acid, released from membrane phospholipids at the sn-2 ester binding site by the enzymatic activity of phospholipase A2, to prostaglandin (PG) H2. The reaction involves both cyclooxygenase (dioxygenase) and hydroperoxidase (peroxidase) activity. The cyclooxygenase activity incorporates two oxygen molecules into arachidonic acid or alternate polyunsaturated fatty acid substrates, such as linoleic acid and eicosapentaenoic acid. Metabolism of arachidonic acid forms a labile intermediate peroxide, PGG2, which is reduced to the corresponding alcohol, PGH2, by the enzyme's hydroperoxidase activity. COX-1 promotes the production of the natural mucus lining that protects the inner stomach and contributes to reduced acid secretion and reduced pepsin content. COX-1 is normally present in a variety of areas of the body, including not only the stomach but any site of inflammation.