Pregnancy zone protein-tissue-type plasminogen activator complexes bind tolow-density lipoprotein receptor-related protein (LRP)

Tissue-type plasminogen activator (t-PA), is a serine proteinase that catal yzes the initial and rate-limiting step in the fibrinolytic cascade. Its pl asma activity is determined by the rate of release into the bloodstream,, t he rate of inhibition by plasminogen-activator inhibitor type 1 (PAI-1) and the rate of hepatic clearance. Two receptor systems contribute to the clea rance of t-PA: the mannose receptor and the low-density lipoprotein recepto r-related protein (LRP) that removes free t-PA as well as t-PA-PAI-1 comple xes from the blood. During pregnancy a significant rise in the plasma level s of pregnancy zone protein (PZP) is observed, while alpha (2)-macroglobuli n (alpha (2)-M) remains constant. Interestingly, the fibrinolytic activity is decreased during this period. In this context, we have recently demonstr ated the in vitro formation of PZP-t-PA complexes. Here, we purified LRP fr om human placenta by affinity chromatography and then analyzed the binding specificity and affinity of PZP-proteinase complexes to the receptor by enz yme immunoassay (EIA), Our results clearly established that the binding of PZP-t-PA complexes to LRP was specific, saturable, and with K-d = 337 +/- 3 1 nM, Moreover, by using the same EIA, we further observed that this bindin g was inhibited by receptor-associated protein. These data suggest that PZP , by binding to t-PA and promoting its clearance via LRP, might contribute in vivo to the downregulation of the fibrinolytic activity during pregnancy , (C) 2001 Academic Press.