Contribution of type II phospholipase A2 to in vitro phospholipase A2 enzymatic activity in human term placenta

Although phospholipase A2 (PLA2) enzymatic activities have been implicated in the regulation of phospholipid metabolism and eicosanoid formation in human gestational tissues, the role and contribution made by individual PLA2 isozymes has not been established. The aim of this study, therefore, was to determine the contribution made by Type II PLA2 to PLA2 enzymatic activity present in human term placenta. The experimental paradigm used to establish the contribution made by Type II PLA2 to total in vitro PLA2 enzymatic activity present in placental extracts was to remove Type II PLA2 by immunoaffinity extraction and then to quantify residual PLA2 enzymatic activity. Before immunoaffinity extraction, Type II PLA2 immunoactivity and total PLA2 enzymatic activity present in placental extracts averaged 28.0 +/- 10.0 ng/mg protein and 1040 +/- 367 pmol/h per mg protein (n = 3) respectively. After solid-phase immunoaffinity batch extraction of placental extracts, immunoreactive Type II PLA2 was not detectable by ELISA, and PLA2 enzymatic activity was decreased by 82 +/- 1% (P < 0.001). Residual (i.e. non-Type II) PLA2 enzymatic activity was further characterised by Western blot analysis and enzyme activity assay. The data obtained are consistent with a contribution by both cytosolic PLA2 and other secretory PLA2 isozymes (i.e. non-Type II) to residual PLA2 enzymatic activity. The results obtained in this study support the conclusion that Type II PLA2 is quantitatively the primary PLA2 isozyme that contributes to in vitro PLA2 enzymatic activity present in extracts of human term placenta, accounting for at least 80% of total activity. These data further support the involvement of this extracellularly active isozyme in the regulation of placental phospholipid metabolism and eicosanoid formation during late gestation.