Affinity-Labeled Somatomedin-C Receptors and Binding Proteins from the Human Fetus

We investigated the ontogeny of the human placental membrane somatomedin-C (Sm-C)/insulin-like growth factor I (IGF-I) receptor by affinity labeling with the cross-linking agent disuccinimidyl suberate (DSS). Specific Sm-C receptors, identified from as early as 6 weeks gestation, demonstrated no apparent structural changes through the course of gestation. Second trimester human fetal brain membranes crosslinked to [125I]Sm-C exhibited an identical pattern of receptor binding. These findings in both placenta and fetal brain membrane are consistent with the proposed heterotetrameric structure of the Sm-C/IGF-I receptor. Using a similar DSS cross-linking technique, we identified Sm-C-binding proteins in second trimester amniotic fluid [apparent molecular mass (Mr), 38,000 and 35,000] and term cord plasma (Mr, 41,000, 38,000, and 35,000). Identically sized binding components (Mr, 35,000–45,000) were also found in membrane reparations of preterm placenta and brain after crosslinking. Evidence that these bindin...