Specific interaction between placental protein 5 and heparin.

Levels of placental protein 5 (PP5) in paired serum and plasma samples obtained during the second half of pregnancy were measured by radioimmunoassay. The apparent level of PP5 was significantly higher in serum than in heparin-derived plasma. The addition of heparin to late pregnancy serum resulted in a dose-dependent decrease in the apparent PP5 concentrations, the maximal effect occurring at heparin concentrations greater than 4 U/ml. The same phenomenon was observed using purified PP5 and placental homogenate. The effect of heparin was readily reversed by the addition of protamine sulphate. The specificity of the interaction between heparin and PP5 was demonstrated by affinity chromatography with heparin as the ligand. The functional significance of these results are discussed in relation to observations on other placental proteins and proteins with affinity to heparin.