COMPARATIVE STUDY OF A MEMBRANE PROTEIN . CHARACTERIZATION OF BOVINE RAT AND FROG VISUAL PIGMENTS500

Rat and frog visual pigmentS500, which are membrane proteins found in the retinal rod outer segments, were purified and their properties compared with the previously isolated bovine visual pigment500 (Heller, J. (1968), Biochemistry 7, 2906, 2914). The rat and frog visual pigments500 are closely similar to the bovine pigment in all their physicochemical properties. These pigments have practically identical molecular weights and molar absorptivities at 280 and 500mμ. They contain only one retinal prosthetic group per molecule, as determined by the loss of lysine residues after reduction with sodium borohydride, have two disulfide bridges, and are all glycoproteins. In the native pigments retinal is bound to the protein through a bond which is not susceptible to reduction with sodium boroative hydride, and is thus most probably a substituted aldimine. On exposure to light the substituted aldimine is converted into a simple aldimine which can be reduced with sodium borohydride and concomitantly one sulfhydryl group per molecule becomes titrable. Similarly to the bovine pigment, rat and frog visual pigments undergo a conformational change on light exposure such that the light-exposed form is more expanded than the native molecule. The magnitude of this conformational change is identical in all these pigments. The only differences that were found among these pigments were small variations in their amino acid composition. On the basis of these observations it is suggested that visual pigment500, a membrane protein, from bovine, rat, and frog eyes forms a series of homologous proteins. © 1969, American Chemical Society. All rights reserved.