An investigation of placental transferrin processing: influence of N-ethylmaleimide

The uptake and processing of 125I-59Fe labelled diferric transferrin was studied with the single bolus technique using the isolated guinea-pig placenta. Following preperfusion of the placenta with the SH-alkylating agent N-ethylmaleimide preferential iron uptake was inhibited and a total blockade of transplacental iron transfer was obtained. It can be shown by means of subcellular fractionation studies that neither transferrin binding nor endocytosis of the transferrin receptor complex were affected by N-ethylmaleimide. Additional experiments performed with microvillous membrane vesicles isolated from control placentas or from placentas preperfused with N-ethylmaleimide demonstrated that N-ethylmaleimide does not affect the affinity of the transferrin receptor for its ligand. The number of receptors per mg membrane protein remains unchanged as well. Ka = 1.4 x 10(8) M-1, n = 3.6 x 10(12). The results show that the blockade is located at the level of endosomal iron release. Since it is known that N-ethylmaleimide inhibits the endosomal proton pump, our results strongly suggest that the endocytotic pathway is a necessary route in transferrin mediated transplacental iron transfer.