Placental anticoagulant protein-I: measurement in extracellular fluids and cells of the hemostatic system.

Placental anticoagulant protein-I (PAP-I), a member of the lipocortin protein family, is a potent in vitro anticoagulant whose in vivo function is unknown. Very low levels of PAP-I were present in plasma of normal volunteers (0 to 5 ng/ml) and in randomly chosen plasma specimens from hospitalized patients (0 to 28 ng/ml). Review of selected hospital records did not reveal any single clinical entity that correlated with plasma levels. PAP-I was also found in amniotic fluid (12 to 107 ng/ml) and in conditioned medium of cultured endothelial cells (49 +/- 20 ng/ml). Gel filtration experiments showed that PAP-I was intact and uncomplexed in plasma and amniotic fluid. The protein was fairly abundant intracellularly: 4080 +/- 2560 ng/mg total protein in cultured umbilical vein endothelial cells; 178 +/- 109 ng/mg in platelets; 564 +/- 384 ng/mg in leukocytes; and 8.4 +/- 4.3 ng/mg in erythrocytes. The levels of PAP-I increased in platelet-rich plasma after stimulation of platelets with arachidonic acid but not after stimulation with ADP, epinephrine, thrombin, ristocetin, or collagen. These data suggest that PAP-I probably does not function as a circulating natural anticoagulant in normal persons.