Number and affinity of transferrin-receptors at the placental microvillous plasma membrane of the guinea pig: influence of gestational age and degree of transferrin glycan chain complexity.

Transferrin receptors (TfR's) on the syncytiotrophoblast mediate transferrin (Tf) dependent Fe uptake and transfer to the fetus. We studied TfR number and density at the microvillous membrane isolated from guinea pig placentas at day 40, 50 and 64 (near term), together with the K(a) values for the main serum isotransferrins being biantennary Tf(slow) and triantennary Tf(fast). The effect of desialylation of either the microvillous membranes or of Tf(slow) and Tf(fast) on the binding characteristics was also studied. The number of TfR's per mg placenta- or membrane protein increased significantly from day 40 to term (P < 0.01 resp. P < 0.025). The K(a) values for Tf(slow) and Tf(fast) did not change during pregnancy. K(a)Tf(slow) = 0.3 nM-1, K(a)Tf(fast) = 0.19 nM-1 (P < 0.01). It is suggested that the increase in F(e) transfer during pregnancy is directly related to number and density of the TfR's at the syncytial border, and that adaptive adjustment of K(a) values does not play a role in the maturation of the transfer process. The pregnancy dependent shift to iso-transferrins with a higher degree of glycosylation offers no explanation for the increase of F(e) transfer during pregnancy. Desialylation of the microvillous membranes did not effect the binding parameters of Tf(slow) and Tf(fast), unless desialylation surpassed the 50% level. Then Ka values decreased and TfR number increased (P < 0.05). Desialylation of Tf(slow) and Tf(fast) had no effect on K(a) nor on the number of TfR. The maternal fetal interface therefore lacks an asialo-glycoprotein receptor.