Radioimmunochemical measurement of the transferrin receptor in human trophoblast and reticulocyte membranes with a specific anti-receptor antibody.

A radioimmunoassay was developed to directly assay the presence of transferrin receptors in human tissues. Antisera developed in a goat against purified human placental transferrin binding protein was purified by fractional sodium sulfate precipitation and adsorption against Sepharose-bound transferrin to remove trace anti-transferrin activity. The antisera immunoprecipitates a Mr 94,000 peptide on 125I-iodinated syncytial trophoblast membranes from placentae. This polypeptide has been identified previously as the transferrin binding protein of the placenta [Wada, H. G., Hass, P. E. & Sussman, H. H. (1979) J. Biol. Chem. 254, 12629-12635]. A standard curve using purified 125I-iodinated placental transferrin receptor and various amounts of the purified noniodinated receptor is sensitive from 5 to 900 ng. A reticulocyte-enriched membrane ghost preparation (5% reticulocyte) gives a value of 9.5 micrograms of receptor per mg of protein. Normal erythrocyte membrane ghosts show binding (0.57 micrograms of receptor per mg of protein) proportional to the amount of reticulocytes normally present in blood (0.5-1.0%). In other tissues in which the transferrin receptor binding has been reported, purified syncytial trophoblastic membranes are found to have 34.5 micrograms of receptor per mg of protein, and BeWo cells, a choriocarcinoma cell line, are found to have 15.7 micrograms of receptor per mg of protein. In contrast, normal breast tissue, which has no demonstrated transferrin binding, contains only 0.18 micrograms of receptor per mg of protein by this method.