Squid retinochrome. Configurational changes of the retinal chromophore

The configurations of the retinal chromophore in light and dark reactions of squid retinochrome were investigated by means of high-performance liquid chromatography. Orange light isomerized the chromophore of retinochrome, all-trans-retinal, mainly to the 11-cis configuration in metaretinochrome. Irradiation with shorter-wavelength lights not only accelerates the photoreversal of metaretinochrome to retinochrome but also leads to a slight production of isoretinochrome (13-cis-retinochrome), yielding a photoequilibrium mixture of three kinds of retinochrome. 13-cis- and 9-cis-retinochromes are photosensitive, and are converted into metaretinochrome upon irradiation with orange light. When steadily exposed to orange light in the presence of a trace of retinochrome-protein, all of the all-trans-, 13-cis-, and 9-cis-retinals are catalytically isomerized only to the 11-cis form, although the reaction rate is reduced in the order of the retinals listed above. In the dark, 9-cis-retinochrome, like retinochrome, remains unchanged, but both meta- and 13-cis-retinochromes slowly change to retinochrome. The chromophore of 13-cis-retinochrome changes directly to the all-trans form, whereas the 11-cis chromophore of metaretinochrome goes to all-trans mainly through the 13-cis form. The direct isomerization from 11-cis to all-trans hardly occurs at temperatures as low as 20 degrees C, and shows high values of the activation enthalpy and entropy changes. Based upon these findings, the role of retinochrome in the photoreception of the visual cells is discussed.