Localization of cellular retinal-binding protein in bovine retina and retinal pigment epithelium, with a consideration of the pigment epithelium isolation technique.

Bovine RPE was isolated by commonly used brushout procedures and analyzed by light and electron microscopy. The preparation was found to consist almost entirely of cells with retained organelles (mitochondria, pigment, and other granules) but with broken surface membranes and extracted cytoplasm. In keeping with this, the wash obtained by sedimenting these broken cells contained approximately 97 percent of the cellular retinol-binding protein present in the suspension. Cellular retinoic acid-binding protein, present in bovine retinal extracts, was found in low amounts in the wash from RPE. The cellular retinol-binding protein present in the RPE wash was of high specific activity and similar in properties to that obtained from bovine retina. Supernatant obtained from sonicated rod outer segments contained approximately 10 percent of the retinol-binding protein of the retina. No retinoic acid-binding protein was found. The relatively large amount of cellular retinol-binding protein present in the RPE (more than is found in the retina) is consistent with a functional role of this protein in uptake and transport of retinol by the RPE.