A NOVEL PHOSPHOLIPASE A(2) FROM HUMAN PLACENTA

A major soluble phospholipase A(2) of human term placenta was characterized and purified about 15 000-fold to homogeneity. The apparent molecular mass as determined in SDS/polyacrylamide gels is 42 kDa. The enzyme is inhibited by dithiothreitol indicating the presence of disulphide bridges which are essential for activity. Studies with known phospholipase A(2) inhibitors revealed no immediate relationship to either secretory or cytosolic phospholipases A(2). The placental enzyme prefers liposomes of phosphatidylcholine and has a distinct preference for arachidonic acid in the sn-2 position. It tolerates various detergents. Roughly 10 mu M Ca2+ is required for activity, but it cannot be replaced by Mg2+ or Mn2+; Zn2+, Cu2+ and Fe3+ are inhibitory. In immunoblots, the placental enzyme was not detected by two separate antisera specific for type-II phospholipases A(2) but reacted very weakly with antisera directed against cytosolic phospholipase A(2). From these data we suggest that this enzyme is a novel form of phospholipase A(2) which may be involved in arachidonic acid mobilization both during the course of pregnancy and at parturition.