Specific interaction between placental protein 5 and thrombin.

The affinity of placental protein 5 (PP5) for thrombin was examined by the application of placental homogenate, late pregnancy serum, plasma and amniotic fluid to thrombin-Sepharose column chromatography. A specific interaction between PP5 in these fluids and thrombin was seen, revealing heterogeneity of PP5 with respect to thrombin affinity. Placental homogenate contained non-binding (species I), interacting (species II) and strongly binding (species III) subpopulations of PP5. By contrast, serum contained predominantly the strongly binding form (species III) and a minor proportion of the non-binding form (species I). The interacting form (species II) was the major form in plasma and amniotic fluid. These observations imply that the interacting form (species II) is the dominant form produced by the placenta and secreted into the circulation, but is converted to species III, i.e. high affinity for thrombin during coagulation. These results suggest that PP5 may be involved locally in the regulation of the coagulation system during pregnancy.