Binding of human isotransferrin variants to microvillous and basal membrane vesicles from human term placenta.

Transferrin (Tf)-dependent iron transfer from mother to fetus is mediated by Tf receptors (TfRs) which are present on both microvillous and basal membranes of human placental syncytiotrophoblast. We used microvillous and basal membrane vesicles, both isolated from the same human term placenta, to investigate the binding of [125I]-labelled diferric bi-bi antennary tetra-sialo Tf (bb Tf), bi-tri-antennary penta-sialo Tf (bt Tf) and tri-tri-antennary hexa-sialo Tf (tt Tf). To diminish the effect of endogenous Tf, membrane vesicles were washed before binding of [125I]-Tf. The number of TfRs on microvillous membranes was 6.1 +/- 2.4 (mean +/- s.d., n = 15) times higher than that on basal membranes, whereas the affinity of TfRs on basal membranes was 3.9 +/- 0.4 (mean +/- s.d., n = 15) times higher than that of TfRs on microvillous membranes, irrespective the isoTf used. The affinity constants of TfRs on both microvillous and basal membranes were higher for bb Tf than for bt Tf and higher for bt Tf than for tt Tf. However, these latter differences were rather small and probably not of physiological importance.