Interactions of plasma retinol-binding protein with its receptor. Specific binding of bovine and human retinol-binding protein to pigment epithelium cells from bovine eyes.

Bovine and human retinol-retinol-binding protein (RBP) were iodinated to high specific activity. At least 34% of the iodinated material was native and retained its retinol chromophore as judged by its characteristic ability to bind to plasma prealbumin. Bovine and human retinol-125I-RBP were found to bind specifically to intact isolated pigment epithelium cells from bovine eyes. The specific binding was complete in about 1 min with an estimated second order rate constant of about 10-8 M-minus 1 S-minus 1 (at 22 degrees). The specific binding was temperature dependent with the binding at 0 degrees being some 7-fold slower than at 22 degrees. The specific binding of both human and bovine retinol-125I-RBP was a linear function of the number of binding sites (number of cells) and was saturable with respect to retinol-125I-RBP. Bound iodinated retinol-RBP was rapidly displaced by the addition of unlabeled retinol-RBP, indicating that the specific binding process was a surface phenomenon and was not due to endocytosis. At saturation about 3.7 to 5.2 times 10-4 molecules of either bovine or human retinol-125I-RBP bound to one bovine pigment epithelium cell. The dissociation constant for the binding between retinol-125I-RBP and pigment epithelium receptor was estimated to be about 5 times 10-minus 12 M. Addition of human prealbumin (thyroxine-binding prealbumin) did not affect the binding of either human or bovine retinol-125I-RBP to pigment epithelium cells. Retinol-125I-RBP did not bind specifically to isolated bovine rod photoreceptor outer segments. Human apo-RBP was less effective in displacing bound retinol-125I-RBP than either native or reconstituted human retinol-RBP. These results suggest a mechanism whereby, after delivering its retinol to the cell, apo-RBP is displaced from the specific receptor on pigment epithelium cell by another retinol-RBP molecule. This postulated mechanism makes it possible to control the delivery of retinol to the target cell by the relative plasma concentrations of apo- and retinol-RBP and their relative affinities for the specific receptor binding site.