Acetylation of prostaglandin synthase by aspirin

When microsomes of sheep or bovine seminal vesicles are incubated with [acetyl-3H]aspirin (acetyl salicylic acid), 200 Ci/mol, we observe acetylation of a single protein, as measured by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The protein has a molecular weight of 85,000 and corresponds to a similar acetylated protein found in the particulate fraction of aspirin-treated human platelets. The aspirin-mediated acetylation reaction proceeds with the same time course and at the same concentration as does the inhibition of prostaglandin synthase (cyclo-oxygenase) (EC 1.14.99.1; 8,11,14-eicosatrienoate, hydrogen-donor:oxygen oxidoreductase) by the drug. At 100 muM aspirin, 50% inhibition of prostaglandin synthase and 50% of maximal acetylation are observed after 15 min at 37 degrees. Furthermore, the substrate for cyclo-oxygenase, arachidonic acid, inhibits protein acetylation by aspirin at concentrations (50% inhibition at 10-30 muM) which correlate with the Michaelis constant of arachidonic acid as a substrate for cyclooxygenase. Arachidonic acid analogues and indomethacin inhibit the acetylation reaction in proportion to their effectiveness as cyclo-oxygenase inhibitors. The results suggest that aspirin acts as an active-site acetylating agent for the enzyme cyclo-oxygenase. This action of aspirin may account for its anti-inflammatory and anti-platelet action.