Yoshinori Shichida

Low‐temperature spectrophotometry of phoborhodopsin

Photoreactions of the fourth rhodopsin‐like pigment, phoborhodopsin (pR480), of Halobacterium halobium were studied by low‐temperature spectrophotometry. Upon irradiation of pR480 at ‐ 80 as well as ‐ 170°C with 436 nm light, a new intermediate having a difference absorption maximum at about 520 nm was produced. We designate this intermediate P520. Prolonged irradiation caused the formation of a photosteady‐state mixture composed of P520 and pR480, indicating that P520 was photoreversible to pR480 at ‐ 80°C. P520 was thermally converted back to pR480 through P350 and P530, both of which were detected previously [(1986) Biochem. Biophys. Res. Commun. 139, 389–395]. Based on these results, a new photochemical reaction cycle of pR is presented.

Biochemical and photochemical analyses of retinal proteins in chicken cone cells

Absorption of light by several types of cone cell with different spectral sensitivities initiates in vertebrates photopic vision which includes color discrimination. In order to understand the conversion mechanism of a photon signal in the cones, photochemical reactions of iodopsin, the red-sensitive cone visual pigment of chicken, and its interaction with a G-protein were investigated and compared with those of rhodopsin, the visual pigment in rod cells that are responsible for scotopic vision. The results obtained indicate that the photo- bleaching processes of both pigments and the signal transduction processes in the two types of the cell are similar to, but somewhat different from each other. This could partly explain the differences between the physiological responses of the different cells.