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 media ted by Tf receptors (TfRs) which are present on both microvillous and basal membranes of human placental syncytiotrophoblast. We used microv illous and basal membrane vesicles, both isolated from the same human term placenta, to investigate the binding of [I-125]-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 th e effect of endogenous Tf, membrane vesicles were washed before bindin g of [I-125]-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, w hereas 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, i rrespective the isoTTf used. The affinity constants of TfRs on both mi crovillous and basal membranes were higher for bb Tf than for bt Tf an d higher for bt Tf than for tt Tf. However, these latter differences w ere rather small and probably not of physiological importance. (C) 199 7 W. B. Saunders Company Ltd.