Squid m-retinochrome. Two forms of metaretinochrome.

When retinochrome absorbs light, it bleaches to m-retinochrome, which may act as a direct supplier of 11-cis-retinal to protein opsin to form rhodopsin. The present experiments were aimed at further elucidating the molecular state of m-retinochrome. Retinochrome and m-retinochome were mixed each with sodium borohydride, subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the resultant fluorescence bands were examined. The reduced retinochrome showed only one band due to N-retinyl protein, whereas the reduced m-retinochrome had two bands. When extracted with successive volumes of n-hexane, m-retinochrome released the 11-cis-retinal chromophore in a time-course consisting of fast and slow phases. These findings indicated that m-retinochrome exists in two forms, with loose and tight coupling of the chromophore to the protein moiety. Those forms were usually balanced in a molar ratio of about 1:2, and the proportion of the tight form of m-retinochrome was increased in the presence of excess 11-cis-retinal. Based upon these findings, the role of m-retinochrome in the visual cells is discussed.