Molecular basis for the inhibition of prostanoid biosynthesis by nonsteroidal anti-inflammatory agents

Many nonsteroidal anti-inflammatory drugs exert their effects by inhibiting the synthesis of prostanoids. More specifically, these agents block the synthesis of prostaglandin endoperoxide G2 from arachidonic acid by competing with arachidonate for binding to the cyclooxygenase active site of prostaglandin endoperoxide synthase. Studies of the molecular biology of prostaglandin endoperoxide synthase indicate that there is a single gene for the enzyme. Thus, tissue-specific effects of nonsteroidal anti-inflammatory drugs probably result from differences in drug distribution and/or metabolism and not from the existence of tissue-specific prostaglandin endoperoxide synthase isozymes. Aspirin causes inactivation of prostaglandin endoperoxide synthase by first binding to the cyclooxygenase active site and then acetylating the protein at Ser530. Although the cyclooxygenase activity is inactivated, the hydroperoxidase activity of prostaglandin endoperoxide synthase is unaltered by Aspirin or other nonsteroidal anti-inflammatory drugs. Replacement of Ser530 of the native enzyme with an alanine residue by site-directed mutagenesis yields a prostaglandin endoperoxide synthase with unaltered catalytic and substrate binding activities. Thus, the hydroxyl group of Ser530 is not essential for enzyme activity. Instead, it appears likely that acetylation of prostaglandin endoperoxide synthase by Aspirin simply places a bulky acetyl group at or near the cyclooxygenase active site, thereby interfering with arachidonic acid binding.