Uptake of retinol and retinoic acid from serum retinol-binding protein by retinal pigment epithelial cells.

Isolated pigment epithelial cells from bovine eyes were shown to be capable of taking up retinol and retinoic acid. The uptake was dependent on the ligands being presented to the cells as complexes with serum retinol-binding protein. Neither free retinol nor retinol or retinoic acid presented as serum albumin complexes were taken up by the pigment epithelial cells. The transport of retinal and retinoic acid was a relatively rapid, temperature-dependent process. The uptake of retinol and retinoic acid was accompanied by dissociation of the ligand retinal-binding protein complex, with the ligand being taken up by the cells while the cariier protein remained outside the cell. Retinol which was taken up by the pigment epithelial cells was found in the cytosol as retinal. About a third of the transported retinol was found to be bound to a high molecular weight protein (>1.5 x 10”) while the rest was found as free retinol. These results were similar to those previously obtained by the direct addition of retinol to pigment epithelial cytosol (Heller, J. (197Q.I. Biol. Chem. 251, 2952). The retinoic acid which was taken up by the pigment epithelial cells was found as retinal-like material, partly bound to a high molecular weight cytosol protein and partly free. This retinol-like material was not retinol since it was not incorporated into rhodopsin in ciao. Fresh leucocytes and red cells from either human or bovine blood did not show an3 uptake of either retinol or retinoic acid when presented to the cells as their respective serum retinol-binding protein complexes.