Transferrin binding by microvillar vesicles isolated from rat placenta.

Microvillar vesicles have been isolated from rat placentae. The vesicles have been characterized with respect to their microscopic appearance, their capacity to accumulate amino acids, and their enzyme activities. Alkaline phosphatase activity in the vesicle fraction was threefold greater than in the homogenate, and cytochrome c reductase activity was decreased fourfold. The vesicles were used to characterize transferrin binding to the trophoblast membrane in vitro. Transferrin binding was specific (K alpha = 8 X 10(7) M-1), and could not be replaced by apotransferrin. Apotransferrin binding showed lower affinity than the diferric transferrin, and did not appear to saturate over the range 0 to 500 micrograms/ml.