Molecular heterogeneity of placental protein 5 (PP5) in late pregnancy serum and plasma: evidence for a heparin-PP5 polymer.

The apparent levels of placental protein 5 (PP5) measured by radioimmunoassay are substantially greater in serum than plasma. Using molecular sieve chromatography we have shown that PP5 exists in a number of different forms, and that in the presence of heparin or endogenous heparin-like substances PP5 forms complexes and polymers. These findings add weight to the concept that PP5 may be involved with the coagulation system and, in particular, with coagulation mechanisms in the intervillous space of the human placenta.