In vitro uptake of vitamin A from the retinol-binding plasma protein to mucosal epithelial cells from the monkey's small intestine.

The in vitro uptake of retinol from its plasma carrier protein, the retinol-binding protein (RBP), to the cells of the monkey's small intestine has been studied. [3H]Retinol was readily delivered from the RBP to the cells without a concomitant cellular uptake of the RBP. The [3H]retinol accumulation in the cells was dependent on the temperature and was virtually abolished at 0 degrees. Several metabolic inhibitors could not impede the uptake process. The cellular [3H]retinol accumulation was linear for about 45 min and exhibited characteristic saturation kinetics. The uptake of [3H]retinol by the cells could be inhibited by RBP containing unlabeled retinol, vitamin A-depleted RBP, and Fab' fragments against RBP. In contrast, free, unlabeled retinol and the metabolite form of RBP, lacking retinol and affinity for prealbumin, were inactive. It is therefore suggested that there is a receptor for vitamin A on the cell surface which recognizes the protein part of the protein-ligand complex.