Purification of placental protein PP12 from human amniotic fluid and its comparison with PP12 from placenta by immunological, physicochemical and somatomedin-binding properties.

Human amniotic fluid was found to contain a protein which is immunochemically indistinguishable from placental protein PP12. This protein was purified by gel filtration, hydrophobic interaction high performance liquid chromatography and anion-exchange chromatography. The relative molecular mass as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis was 34,000, and the isoelectric point was 4.9. Tryptic peptides of amniotic fluid PP12 as determined by reversed phase high performance liquid chromatography were similar to those of placental PP12. Both had the N-terminal amino acid sequence Ala-Pro-Trp-Gln-, which is the same as previously reported for a somatomedin-binding protein. Both placental PP12 and amniotic fluid PP12 were found to bind somatomedin C (IGF-I) with high affinity, Ka = 1 X 10(9) l/mol). Amniotic fluid is an ideal source of this somatomedin-binding protein, and the purification method described allows rapid isolation of PP12 under mild conditions which are essential for studies on its biological function.