Uptake and metabolism of transferrin and albumin by rat yolk sac placenta.

The uptake of radiolabeled albumin and transferrin by the rat yolk sac and their subsequent transport to the embryo were studied. Transferrin uptake increases with incubation time whether the results are expressed in terms of the total amount accumulated or per milligram embryo or yolk sac protein, whereas albumin levels increase only in absolute terms. The fate of transferrin and albumin was examined by partitioning 125I into low- and high-molecular-weight fractions. Nearly all the embryonic radioiodine originally derived from transferrin is in the low-molecular-weight fraction, compared with only 60% albumin. These results have been extended by examining the uptake and hydrolysis of transferrin and albumin by the isolated yolk sac. Transferrin is taken up more rapidly than albumin. The release of hydrolyzed transferrin to the incubation medium occurred 40 min after the initiation of incubation, compared with 20 min for albumin. Transferrin uptake by the yolk sac at different transferrin concentrations showed an initial rapid phase followed by a slower linear phase, whereas albumin uptake increased linearly with concentration. There was no competition between the two proteins for uptake. Transferrin was released from the yolk sacs at approximately twice the rate of albumin. Results demonstrate at least two uptake mechanisms in the rat visceral yolk sac, one for transferrin, which probably involves receptor-mediated endocytosis, and one for albumin, by which transferrin can also be transported, which probably involves pinocytotic mechanisms.