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

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Abstract
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 94,000 MW peptide on 125I-iodinated syncytial trophoblast membranes from placentae. This polypeptide was identified previously as the transferrin binding protein of the placenta. A standard curve using purified 125I-iodinated placental transferrin receptor and various amounts of the purified noniodinated receptor is sensitive from 5-900 ng. A reticulocyte-enriched membrane ghost preparation (5% reticulocyte) gives a value of 9.5 .mu.g of receptor/mg of protein. Normal erythrocyte membrane ghosts show binding (0.57 .mu.g of receptor/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 .mu.g of receptor/mg of protein, and BeWo cells, a choriocarcinoma cell line, are found to have 15.7 .mu.g of receptor/mg of protein. Normal breast tissue, which has no demonstrated transferrin binding, contains only 0.18 .mu.g of receptor/mg of protein by this method.