Interactions of retinol-binding protein with various chromophores and with thyroxine-binding protein. A model for visual pigments

Abstract

The properties of retinol-binding protein from human serum were compared with those of rhodopsin in an attempt to learn more about the role of protein in visual pigments. In the serum, retinol-binding protein (mol wt. 21 000) tightly binds one molecule of all- trans-retinol and one molecule of the tetrameric protein prealbumin (mol. wt. of tetramer, 54 000). Apo-retinol binding protein was found to be completely dissociated from prealbumin under conditions in which native retinol-retinol-binding protein and reconstituted all- trans-retinol-retinol-binding protein were tightly bound. This difference in binding to prealbumin between apo- and holo-retinol-binding protein is due most probably to different conformations of the apo- and holo-protein. By analogy, it is suggested that the incorporation of the retinal chromophore into apo-visual pigment might act as recognition marker for the incorporation of visual pigment into the photoreceptor disc membrane. Apo-retinol binding protein was shown to form stable 1 : 1 molar complexes with retinols, retinals, retinoic acid, retinyl acetate and retinyl oxime, but not with retinyl palmitate. Only the retinol isomer and retinoic acid-retinol-binding protein complexes were bound to prealbumin at physiological ionic strength, while the other chromophores-retinol·binding protein complexes were not. The various chromophores were all bound to the same site on the retinol-binding chromophore, the binding was noncovalent and irreversible under normal physiological conditions. All the chromophores-retinol-binding protein complexes showed a large induced optical activity of the chromophore absorption band upon binding to the protein. The rotatory strength of the circular dichroism bands of the various chromophore retinol-binding protein complexes was of the same order of magnitude as that of rhodopsin. The binding of retinol and retinal to bovine serum albumin did not produce optical activity in the chromophore absorption band.