Effect of chloride on the thermal reverse reaction of intermediates of iodopsin.
Among the intermediates in the bleaching process of iodopsin, a chicken red-sensitive cone visual pigment, the batho and meta I intermediates (batho and meta I) formed at low temperatures revert to the original iodopsin by thermal reactions [Yoshizawa & Wald (1967) Nature 214, 566-571; Imamoto, Imai, Yoshizawa, & Shichida (1994) FEBS Lett. 354, 165-168]. In order to elucidate the relationship between Cl- binding to iodopsin and these reverse reactions, we have prepared a sample of iodopsin whose Cl(-)-binding site is vacant (anion-unbound iodopsin) and compared the thermal reactions of its batho and meta I intermediates with those of Cl(-)-bound (native) and nitrate-bound iodopsins. The reverse reaction from batho is observed in both Cl(-)-bound and anion-unbound iodopsins, while the reaction from meta I is observed only in Cl(-)-bound iodopsin. These results indicate that Cl- binding is indispensable for the reverse reaction from meta I, but not from batho. The reverse reaction from meta I has been further investigated as a function of Cl- concentration, and the dissociation constant of Cl- in meta I is estimated to be approximately 20 mM. This value is about 200 times larger than that of iodopsin (0.1 mM), and close to the physiological Cl- concentration in photoreceptor cells, suggesting that Cl- could be released from the protein moiety during the bleaching of iodopsin.
Tachibanaki
S
S
Department of Biophysics, Faculty of Science, Kyoto University, Japan.
Imamoto
Y
Y
Imai
H
H
Shichida
Y
Y
D000818:Animals; D002645:Chickens; D002712:Chlorides / Q000737:chemistry*; D066298:In Vitro Techniques; D010777:Photochemistry; D012168:Retinal Pigments / Q000737:chemistry*; D017299:Rod Opsins; D013057:Spectrum Analysis; D013696:Temperature
D002712:Chlorides; D012168:Retinal Pigments; D017299:Rod Opsins; C034941:iodopsin