Purification and characterization of a uterine phospholipase inhibitor that loses activity after labor onset in women.

Gravidin, a protein that inhibits release of arachidonic acid from human decidual cells, was purified from amniotic fluid. The protein has a molecular weight of 58 to 60 kilodaltons, an isoelectric point of 8.4, and physical characteristics that are indistinguishable from those of inhibitor II previously described. Activity was determined in a dispersed decidual cell system that released arachidonic acid in response to either histamine or calcium ionophore and in a cell-free assay of phospholipase A2. Protein purified from incubates of chorion obtained after the onset of labor was significantly less active than that from chorion obtained before the onset of labor.