Specific photoisomerization of retinal in squid rhodopsin and metarhodopsin

The composition of retinal isomers in the photosteady-state mixtures formed from squid rhodopsin and metarhodopsin was determined by high-pressure liquid chromatography. A large amount of 9-cis-retinal was obtained at liquid N2 temperature when rhodopsin was irradiated with orange light, but only small quantities of 9-cis-retinal were obtained at 15 degrees C. Scarcely any 9-cis-retinal was produced from metarhodopsin by irradiation at liquid N2 temperature. A large quantity of 7-cis-retinal was found in the photoproduct of rhodopsin irradiated at solid carbon dioxide temperature, but not at 15 degrees C and liquid N2 temperature. 7-cis-Retinal was not produced from metarhodopsin at any temperatures. These results indicate that the photoisomerization of retinal is regulated by the structure of the retinal-binding site of this protein. The formation of 9-cis- and 7-cis-retinals is forbidden in the metarhodopsin protein.