Binding characteristics of insulin-like growth factor-binding proteins in maternal and cord sera

Insulin-like growth factor-binding proteins (IGFBPs) in sera from non-pregnant and pregnant women, and from the umbilical cord at the time of delivery, were separated by gel filtration chromatography. The binding characteristics of the 150 and 40 kDa binding proteins were examined by Scatchard analysis. In serum from pregnant women, the affinity of the 150 kDa binding proteins (IGFBP-3) decreased as pregnancy progressed, with an apparent increase in the maximum binding capacity. In contrast, the affinity of the 40 kDa binding proteins showed a slight increase. In umbilical cord serum, the binding affinity of both the 150 and 40 kDa binding proteins was similar, whereas the binding capacity was two-fold higher in the 40 kDa than in the 150 kDa group. Both maternal and umbilical cord serum (artery and vein) contained unoccupied IGFBPs. Following acid treatment, there was an increase in binding of 125I-IGF-I to umbilical cord serum but a decrease in that to maternal serum. These changes in the characteristics of the binding proteins in the pregnancy sera are probably due to partial degradation of the 150 kDa binding proteins by proteases. The present data confirm that IGFBP-3 is the main binding protein for IGFs during pregnancy, although its affinity is decreased. In fetal circulation, the 40 kDa binding proteins appear to be the major IGFBP with a high affinity and high capacity.