Halide control of color of the chicken cone pigment iodopsin

Abstract

The chicken retina contains rhodopsin, a rod cell visual pigment and iodopsin, which is believed to be a cone pigment. The spectrum of iodopsin is controlled by the binding of a halide ion. At physiological salt concentrations iodopsin is in the long wavelength form (562 nm); if chloride is removed rigorously the spectrum shifts to 520 nm. At intermediate chloride concentrations the two forms exist in equilibrium. the chloride-depleted form can be titrated back to the original long wavelength species with half saturation at 1·7 m m-chloride. The spectral shift is observed either in detergent solubilized preparations or in the native membrane, sonicated to reduce light scattering. Bromide can replace chloride, titrating iodopsin to a very similar long wavelength form with a very similar apparent binding constant. Fluoride and iodide cannot shift iodopsin to the long wavelength form. Changes of cation seem to have no effect. In the same system there are no changes of the rhodopsin spectrum with changes of halide concentration. Both the long wavelength form and the short wavelength form of iodopsin regenerate rapidly after bleaching and subsequent addition of 11- cis retinal. Chloride binding protects the iodopsin chromophore from hydroxylamine; while both forms are attacked by hydroxylamine, the chloride-free form is attacked much more rapidly. This fact and the large spectral shift are consistent with but do not prove an anion binding site very close to the retinal chromophore.