Type‐2 plasminogen‐activator inhibitor is a substrate for trophoblast transglutaminase and Factor XIII a

Plasminogen‐activator inhibitor type‐2 (PAI‐2), a serine‐proteinase inhibitor, suppresses fibrinolysis by blocking both urokinase and tissue‐type plasminogen activators. The 43‐kDa PAI‐2 molecule is an abundant cytosolic protein in certain cell types, but can upon appropriate stimulation be secreted as an approximately 60–70‐kDa glycoprotein. However, in trophoblast membranes PAI‐2 activity is associated with large covalent complexes (Jensen, P. H., Nykjær, P., Andreasen P. A., Lund, L., Åstedt, B. Lecander, I. & Gliemann, J. (1989) Biochim. Biophys. Acta 986 , 135–140). This study shows that PAI‐2 can act as a substrate for both tissue transglutaminase and activated plasma factor XIII. In the presence of Ca 2+ , either of these will catalyze the incorporation of primary amines, such as putrescine, into PAI‐2. Moreover, in reactions with tissue transglutaminase, PAI‐2 homopolymers and, in conjunction with other biological substrates, heteropolymers were observed. As judged by the test of incorporating 125 I‐urokinase into SDS‐resistant 125 I‐urokinase/PAI‐2 complexes, polymerized PAI‐2 retained its inhibitory activity. Furthermore, syncytiotrophoblast microvillous membranes and trophoblast detergent extracts incorporated 125 I‐PAI‐2 into large structures in a reaction inhibited by putrescine and a synthetic inhibitor of transglutaminase. Trophoblast transglutaminase was identified as a tissue transglutaminase by non‐denaturing gel electrophoresis and dansylcadaverine activity staining, fibronectin binding and Western blotting with a specific antibody. The transglutaminase‐catalyzed and Ca 2+ ‐dependent anchoring of PAI‐2 to extracellular membrane structures might have the purpose of focally regulating fibrinolysis.