Phospholipase A2 in human placental serum

Intervillous blood was collected from term placentae at delivery, and sera were tested for phospholipase A2 under various experimental conditions. Enzyme activity was found to develop upon extended storage in the cold or at 37 degrees C. The enzyme is reversibly inhibited by dithiothreitol, requires Ca++ ions for activity, and tolerates various detergents. The apparent molecular weight is 42 kDa. In all these parameters the serum enzyme behaves similar to the 42 kDa phospholipase A2 which we recently purified to homogeneity from thoroughly washed placental tissue. Serum phospholipase A2 appears to be generated by proteolytic processing from a slightly larger inactive precursor which was detected immunochemically. Most likely this protein originates from fetal cells and may be released by membrane damage. We conclude that both placental serum and tissue harbour a novel type of phospholipase A2 which is distinct from cytosolic and secretory phospholipases A2. Preference for arachidonate containing substrate suggests a role in eicosanoid production within gestational tissues.