Specific binding of fluorescein labelled serum retinol-binding protein to its cell surface receptor in isolated,purified bovine pigment epithelial cells

Purified bovine pigment epithelial cells were labelled specifically with fluorescein tagged serum retinol-binding protein. This labelling was specific for serum retinol-binding protein and for pigment epithelial cells. The fluorescent label was confined to the choroidal surface of the cell. The hemicellular labelling pattern was used for a vectorial orientation of isolated cells, using the melanin pigment as an internal filter for fluorescence. Fluorescent labelled hexokinase, an unrelated protein, did not show specific labelling of pigment epithelial cells. Prealbumin, which circulates in plasma as a tight complex with retinol-binding protein, did not bind to pigment epithelial cells. Retinol-binding protein can act as a bridge between the pigment epithelial cell surface receptor and fluorescein tagged prealbumin. This suggests that the topological sites of the retinol-binding protein molecule for binding to the cell surface receptor and to prealbumin are different.