The ionized form of aspirin, acetylsalicylate (ASA-) reacts spontaneously with one subunit of PTGS1 dimer to acetylate serine residue 516. The modified enzyme is no longer capable of catalyzing the conversion of arachidonic acid to PGH2. The identity of the acetylated residue is inferred from data for the humann PTGS2 enzyme (Lecomte et al. 1994) and the ovine PGHS1 enzyme (Loll et al. 1995).