Two Immunoreactive Binding Proteins for Insulin-Like Growth Factors in Human Amniotic Fluid: Relationship to Fetal Maturity*

A binding protein for insulin-like growth factors (IGFs) has been purified from human amniotic fluid by IGF-I affinity chromatography and high performance reverse phase chromatography. This protein, with an apparent molecular mass of 28K nonreduced and 34K reduced, had an identical amino-terminus to a previously purified binding protein from amniotic fluid and to placental protein 12. The purified preparation (BP-28) bound both IGFs with high affinity [Ka, 6.55 +/- 2.24 (+/- SD) L/nmol for IGF-I and 3.23 +/- 1.05 L/nmol for IGF-II], with approximately 0.5 mol binding sites/mol BP-28 for either ligand. A 53K IGF-binding protein purified from human plasma (BP-53) did not cross-react in a RIA for BP-28, and BP-28 had less than 0.1% molar cross-reactivity in a RIA for BP-53. Human amniotic fluid reacted strongly in both assays. Fractionation of amniotic fluid samples by reverse phase chromatography showed that BP-28 and BP-53 immunoreactivities were present on separate proteins. In 40 third trimester amniotic fluid samples selected to cover a wide range of lecithin to sphingomyelin ratios, the mean concentrations of BP-28 and BP-53 were 37.6 +/- 17.6 (+/- SD) and 4.6 +/- 1.6 mg/L, respectively. Significant negative correlations were found between the levels of both BP-28 and BP-53 and the lecithin to sphingomyelin ratio, suggesting an association between the levels of both proteins and the degree of fetal maturity. A significant positive association was also found between the levels of BP-28 and BP-53. We conclude that the 28K IGF-binding protein from amniotic fluid, like the previously purified 53K binding protein, has high affinity for both IGF-I and IGF-II, that it coexists in amniotic fluid with BP-53 or a related protein, and that the levels of both proteins decline with increasing fetal maturity.