Effect of Chloride Ion on the Thermal Decay Process of the Batho Intermediate of Iodopsin at Low Temperature

The photochemical and the subsequent thermal behaviors or iodopsin (Cl^--bound form) and N-iodopsin (iodopsin whose Cl^- was replaced by NO₃^-) in CHAPS-phosphatidylcholine (PC) were studied by low-temperature spectrophotometry. Irradiation of the iodopsin preparation at -185 °C produced a photo-steady-state mixture composed of iodopsin, bathoiodopsin, and isoiodopsin. Bathoiodopsin was thermally reverted to the original iodopsin. These results were almost the same as those reported previously [Yoshizawa, T., & Wald, G.(1967) Nature 214, 566-571] in which iodopsin was extracted with 2% digitonin. Therefore, photochemical and subsequent thermal behaviors or iodopsin were independent of the detergent to solubilize iodopsin. Irradiation of N-iodopsin at -185 °C produced the similar photo-steady-state mixture. However, N-bathoiodopsi was thermally converted to the next intermediate, presumably N-lumiiodopsin. These results suggest that the batho-lumi transition of iodopsin at low temperature is likely to be inhibited by the Cl^- bound to the protein moiety of iodopsin, while at room temperature the Cl^- bound to iodopsin could be released on the conversion process of batho- to lumiiodopsin.