Somatomedin-C/insulin-like growth factor I-binding proteins in human amniotic fluid and in fetal and postnatal blood: evidence of immunological homology.

By using the chemical cross-linking agent dis-succinimidyl suberate and [125I]somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) to affinity label Sm-binding proteins, we identified a 30,000- to 40,000-dalton (30-40K) [125I] Sm-C-binding protein complex in midterm amniotic fluid and cord blood. An antibody raised against a Sm-binding protein purified from midterm amniotic fluid recognized this labeled complex not only in amniotic fluid but also in fetal serum and term cord and several postnatal human plasmas, indicating that the 30-40K Sm-binding proteins in each are similar or identical. The binding proteins in amniotic fluid appear to possess binding sites specific for Sm-C, because under the conditions employed, unlabeled Sm-C was at least 10-fold more potent than IGF-II or multiplication-stimulating activity in competing with [125I]Sm-C for binding. Although [125I]GF-II could be cross-linked to similarly sized proteins, unlabeled Sm-C also competed for this binding better than either unlabeled IGF-II or MSA (at least 5-fold greater potency). These findings suggest that this amniotic fluid Sm-binding protein is primarily a carrier of Sm-C, but does not exclude the possibility that a binding site or a distinct binding protein exists which is specific for IGF-II but not amenable to cross-linking by the procedure used. Because unlabeled Sm-C was less potent in inhibiting the binding of [125I]Sm-C to amniotic fluid than to cord plasma proteins, the amniotic fluid binding protein is either more abundant or less avidly binds [125I]Sm-C than the cord plasma binding protein.