| Popis: |
Evidence is presented demonstrating the existence of a receptor specific for transferrin on the brush border membrane of the human syncytial trophoblast. Equilibrium-binding studies were conducted using a placental vesicle preparation to demonstrate the presence of a high affinity, saturable binding site for ‘251-ferrotransferrin on placental membranes. The K, for this binding site was determined by Scatchard analysis to be 3.6 + 1.5 S.D. x lo7 M-‘. The specificity of the high affinity transferrin binding site was examined by competition studies showing that purified human IgG, human albumin, and ovalbumin did not compete for the binding sites. The protein nature of the binding site was suggested by its sensitivity to trypsinization of the membrane vesicles. Solubilization of the membranes with Triton X-100 did not impair the binding of “‘I-transferrin or the K, ‘of the soluble receptor. The transferrin binding of the complex was stable at pH 5.0 and reversible at pH 7.4. Using stabilization at pH 5.0, the solubilized membrane l transferrin complex was immunoprecipitated with anti-transferrin antibodies in the presence of saturating concentrations of transfer-r-in using Staphylococcus aweus as an immunoadsorbant. Polyacrylamide gel electrophoresis of this complex in the presence of sodium dodecyl sulfate demonstrated a single polypeptide of M, = 90,000 + 5,000 and p1 of 6.6 associated with transferrin in the immunoprecipitates of the membrane extracts. These values for the putative receptor were the same as for sialoglycoprotein 15b previously identified in a study characterizing the surface sialoglycoproteins of the term human placenta (Wada, H. G., Gornicki, S. Z., and Sussman, H. H. (1977) J. Supramol. Struct. 6,473-484). The demonstration of the transferrin receptor in the human placenta is of potential importance for studying the molecular mechanism involved in transplacental iron transport during fetal development. |
