PHOTOCHEMISTRY OF RETINOCHROME

Abstract— Retinochrome is a photopigment found in the visual cells of cephalopods. It has been considered to act as a supplier of the 11‐cis‐retinal required for synthesis of rhodopsin, because its all‐trans chromophore is isomerized to 11‐cis form in the light. Light and thermal reactions of squid retinochrome were investigated by low‐temperature spectrophotometry.On irradiation with green light at liquid‐nitrogen temperature, retinochrome (λmax 496 nm, – 190°C) is converted mainly to an intermediate lumiretinochrome (λmax 475 nm, – 190°C), its chromophore being changed to 11‐cis‐retinal. On irradiation with blue light at ‐ 190°C, retinochrome is changed to a photosteady–state mixture (λmax 487 nm, – 190°C) composed mainly of retinochrome and lumiretinochrome, since lumiretinochrome is partially regenerated back to retinochrome. Similarly, irradiation of lumiretinochrome with blue light also results in the same photosteady‐state mixture, which can be completely reverted to lumiretinochrome on re‐irradiation with green light.Lumiretinochrome is stable at a wide range of temperatures from – 190°C to about – 20°C. Above – 20°C, it is further converted, thermally, into metaretinochrome (λmax 470 nm), which is the same bleached product as has been observed on irradiation of retinochrome at room temperatures. Thus, the light‐bleaching process of retinochrome is rather simple compared with that of rhodopsin.