Uptake of vitamin A and retinol-binding protein by human placenta in vitro

The mechanisms involved in the transplacental passage of vitamin A bound to serum retinol-binding protein (RBP) were investigated. Samples of full-term human placenta were incubated for up to 3 h with 125I-RBP or [3H]retinol-RBP. The tissue 125I activity was monitored in washed, whole samples, while [3H]retinol was extracted from homogenized samples and separated by high-pressure liquid chromatography before counting. An accumulation of [3H]retinol and, to a much lesser degree, of 125I-RBP was noted. The placental uptake of both compounds was dependent on temperature and could be inhibited by adding unlabelled retinol-RBP. About 40 per cent of the tritium activity consisted of retinyl esters formed after the uptake of [3H]retinol. When washed placental specimens were postincubated for a few hours in a tracer-free medium, about 25 per cent of the 125I activity and 80 per cent of the tritium activity were released into the medium, provided that it was supplemented with fetal serum. The results illustrate two possible mechanisms by which vitamin A can pass the placental membranes: a direct transfer of retinol-RBP involving cellular uptake and release of the protein-ligand complex; cellular uptake of retinol by a cell-surface receptor specific for RBP, transient esterification of retinol, and release of the vitamin into fetal serum.