Effects of chloride on chicken iodopsin and the chromophore transfer reactions from iodopsin to scotopsin and B-photopsin.

Spectroscopic properties of chicken iodopsin were investigated in correlation with the concentration of chloride in digitonin extracts. When chloride in the extract was depleted by extensive dialysis, chloride-depleted iodopsin (absorption maximum, 512 nm) was formed. It was converted to chloride-bound iodopsin (absorption maximum, 562 nm) by the addition of chloride in the extract. There existed an equilibrium between two forms of iodopsin with a dissociation constant of 0.8 mM chloride. The chromophore-transfer reaction from iodopsin to scotopsin or B-photopsin, the protein moiety of chicken rhodopsin or chicken blue-sensitive cone pigment, respectively, in digitonin extract was also investigated in correlation with the concentrations of chloride, other monovalent and divalent anions, and detergent. The chromophore of chloride-depleted iodopsin was easily transferred to scotopsin in the extract, resulting in formation of rhodopsin. On the other hand, chloride-bound iodopsin was fairly stable even in the presence of scotopsin, indicating that the reaction is inhibited by binding of chloride to iodopsin. The chromophore-transfer reaction to B-photopsin was also observed from chloride-depleted iodopsin but not from chloride-bound iodopsin. The reaction was observable in the 10% digitonin extract as well as in the 2% digitonin extract. The reaction was also observed when 25 mM Na2SO4 was present in the mixture instead of NaCl, but was not when 67 mM NaNO3 was present. All these facts suggest that the chloride binding site of iodopsin does not accept a divalent anion such as SO4(2+), but does accept a monovalent anion such as Cl- or NO3-, which causes inhibition of the chromophore transfer.