Salicylic acid inhibition of the irreversible effect of acetylsalicyclic aicd on prostaglandin synthetase may be due to competition for the enzyme cationic binding site.

Salicylic acid (SA), a weak inhibitor of the prostaglandin endoperoxide synthetase or fatty acid cyclooxygenase enzyme, is known to prevent irreversible enzyme inhibition by acetylsalicylic acid (ASA). The interaction of arachidonic acid with ferrous sulfate was used as a model to study the reaction of the fatty acid with the postulated enzymic cationic binding site on Fe2+-heme. SA was as potent as ASA in inhibiting the cooxygenation of arachidonic acid and ferrous sulfate. The results suggests that SA could complete effectively for the enzyme cationic site with ASA. Thus SA may block ASA acetylation of the cyclooxygenase by preventing ASA from binding to this site.