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Secretory Component in Human Amniotic Fluid and Gestational Tissues: A Potential Endogenous Phospholipasa A2 Inhibitor

Objective: Prostaglandins (PGs) are essential mediators of labor during human pregnancy. Phospholipase A2 (PLA2) provides the essential substrate for PG synthesis through the libertion of arachidonic acid from membrane phospholipid stores. Nonlaboring amniotic fluid (NL-AF) contains secretory component (SC)-like protein(s) that suppress in vitro PLA2 activity. This study characterizes the biologic activity, identity, and tissue distribution of these protein(s) in NL-AF and gestational tissues. Methods: Third-trimester NL-AF was collected by amniocentesis, fractionated by ammonium sulfate precipitation, and submitted to an in vitro PLA2 assay. Identity of the PlA2 inhibitor in NL-AF was confirmed by Western blot and antibody neutralization studies. Secretory component-immunoreactive proteins were purified by immunoaffinity chromatography and visualized by sodium dodecyl sulfate-gel electrophoresis. Tissue distribution of SC in gestational tissues was determined by immunohistochemistry. Results: The 100% pellet and supernatant fractions of NL-AF suppressed PLA2 activity, and this activity was neutralized by a polyclonal antibody to SC. Western blot studies revealed an SC-reactive protein in the 70-80-kD range in the 100% pellet fraction of NL-AF. Two SC-reactive proteins were detected in the 60-80-kD range in the eluate from the SC immunoaffinity column, along with minor proteins of 30 and greater than 100 kD. Immunohistochemical studies revealed SC in placental trophoblast, amniotic membranes, and decidual epithelium. Conclusions: These results demonstrate that proteins homologous to SC are present in human gestational tissues and possess anti-PLA2 activity. These proteins may contribute to the maintenance of pregnancy by suppressing local PG production.