Structural characteristics of prostaglandin synthetase from sheep vesicular gland

Prostaglandin synthetase contains both oxygenase and peroxidase activity and catalyzes the first step of prostaglandin synthesis. Aspirin (acetylsalicylic acid) inhibits oxygenase activity by acetylating a serine residue of the enzyme. In the current study, we have investigated the subunit structure of this complex enzyme and the stoichiometry of aspirin-mediated acetylation of the enzyme. The enzyme was purified to near homogeneity in both active and aspirin-acetylated forms. The purified protein was analyzed for enzymatic activity, [3H]acetate content following treatment with [acetyl-3H]aspirin, NH2-terminal sequence, and amino acid composition. The results show first, that the enzyme can be purified to near homogeneity in an active form; second, that the enzyme consists of a single polypeptide chain (molecular weight 72,000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis) with a unique NH2-terminal sequence (Ala-Asp-Pro-Gly-Ala-Pro-Ala-Pro-Val-Asn-Pro-Met-Gly-); and third, that aspirin inhibits the enzyme by transfer of one acetate per enzyme monomer. Therefore, the two distinct enzymatic activities, oxygenation and peroxidation, are present in a single polypeptide chain. Experiments with a cross-linking agent indicate that in nonionic detergent the enzyme is a dimer of two identical subunits.