NUMBER AND AFFINITY OF TRANSFERRIN-RECEPTORS AT THE PLACENTAL MICROVILLOUS PLASMA-MEMBRANE OF THE GUINEA-PIG - INFLUENCE OF GESTATIONAL-AGEAND DEGREE OF TRANSFERRIN GLYCAN CHAIN COMPLEXITY

Transferrin receptors (TfR's) on the syncytiotrophoblast mediate trans ferrin (Tf) dependent Fe uptake and transfer to the fetus. We studied TfR number and density at the microvillous membrane isolated from guin ea 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 cha racteristics 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 chang e 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 Fe transfer during preg nancy is directly related to number and density of the TfR's at the sy ncytial border, and that adaptive adjustment of K-a values does not pl ay a role in the maturation of the transfer process. The pregnancy dep endent shift to iso-transferrins with a higher degree of glycosylation offers no explanation for the increase of Fe transfer during pregnanc y. Desialylation of the microvillous membranes did not effect the bind ing parameters of Tf(slow) and Tf(fast), unless desialylation surpasse d the 50% level. Then K-a values decreased and TER number increased (P <0.05). Desialylation of Tf(slow) and Tf(fast) had no effect on K-a no r on the number of TfR. The maternal fetal interface therefore lacks a n asialo-glycoprotein receptor.