High-pH form of bovine rhodopsin

Since the 1930s, the spectrum of vertebrate rhodopsin has been considered to be independent of pH (Lythgoe, R.J. 1937. J. Physiol. 89:331-358; Wald, G. 1938. J. Gen. Physiol. 21:795-832). Here I report that the spectrum of bovine rhodopsin is pH dependent. At pHs greater than 9.0, there is a shift to shorter wavelengths of its 500-nm absorption band. This shift is accounted for by the existence of a high pH form of bovine rhodopsin, with absorption maximum at 494 nm and a slightly lower extinction coefficient. The high-pH form results from the low-pH form by the deprotonation of a single group with a pK of approximately 10.2 for rhodopsin in rod disk membranes in 4.0 M KCl. The shift is observed for sheep and chicken rhodopsins, but not for frog, toad, and octopus rhodopsins. This indicates a specific amino acid difference between these rhodopsins that is potentially relevant for the mechanism of color regulation.