Studies of the interaction between human transferrin and specific receptors on the trophoblast membrane

The concept of specific receptors for maternal transferrin on the human syncytiotrophoblast membrane has been generally accepted for many years, but definitive evidence of their existence has been established only recently. In this study, experiments were performed to characterize transferrin receptors further, both on intact membranes and after solubilization. Intact, isolated membrane fragments were examined for transferrin binding, both qualitatively by immunofluorescence and quantitatively by radiobinding. The amounts of ligand bound varied inversely with the quantities of residual maternal transferrin remaining at the time of testing. Scatchard plots revealed that the calculated affinity (K alpha) and the number of receptors increased substantially when transferrin was removed by initial washing with chaotropic agents. After dissociation from the membrane by detergents, occupied receptors largely retained bound transferrin, and transferrin binding by unoccupied receptors was also preserved. The stability of such ligand:receptor complexes was considerably enhanced at pH 5.0, and was apparently unaffected by prior treatment with chaotropic agents. Specific immunoprecipitation of solubilized radioiodinated membrane with rabbit antiserum to human transferrin, followed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate under reducing conditions, indicated a relative molecular mass for the unoccupied receptor of 90,000. These results confirm the existence of high-affinity receptors for transferrin on the trophoblast, and also delineate certain potential pitfalls in studies of their interactions with ligand.