Transferrin receptor affinity and iron transport in the human placenta

Transferrin receptor activity has been investigated on isolated human placental syncytiotrophoblast microvillous plasma membrane preparations following 3 M KCl washing of membranes to remove endogenous receptor-bound maternal transferrin. The calculated binding parameters for essentially apo- and diferric human transferrin, and also for rabbit transferrin, were closely comparable (ranges: Ka, 3.5 to 5.0 X 10(7) M-1; n, 2.6 to 4.0 X 10(14)/mg membrane protein). Iron transport in pregnancy is thus unlikely to involve a simple process of transferrin displacement following iron release at the placental trophoblast plasma membrane receptor site.