Secretory component in human amniotic fluid and gestational tissues: a potential endogenous phospholipase A(2) inhibitor

OBJECTIVES: Prostaglandins (PGs) are essential mediators of labor during hu man pregnancy. Phospholipase A(2) (PLA(2)) provides the essential substrate for PG synthesis through the liberation of arachidonic acid from membrane phospholipid stores. Nonlaboring amniotic fluid (NL-AF) contains secretory component (SC)-like protein(s) that suppress in vitro PLA(2) activity. This study characterizes the biologic activity, identity, and tissue distributi on 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 PLA(2) ass ay. Identity of the PLA(2) inhibitor in NL-AF was confirmed by Western blot and antibody neutralization studies. Secretory component-immunoreactive pr oteins were purified by immunoaffinity chromatography and visualized by sod ium dodecyl sulfate-gel electrophoresis. Tissue distribution of SC in gesta tional tissues was determined by immunohistochemistry. RESULTS: The 100% pellet and supernatant fractions of NL-AF suppressed PLA( 2) 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 ra nge in the 100% pellet fraction of NL-AF. Two SC-reactive proteins were det ected 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. Immunohistochemi cal studies revealed SC in placental trophoblast, amniotic membranes, and d ecidual epithelium. CONCLUSIONS: These results demonstrate that proteins homologous to SC are p resent in human gestational tissues and possess anti-PLA(2) activity. These proteins may contribute to the maintenance of pregnancy by suppressing loc al PG production. (J Soc Gynecol Investig 1999;6:311-7) Copyright (C) 1999 by the Society for Gynecologic Investigation.