Lipid to Protein Chromophore Transfer in the Photolysis of Visual Pigments

Abstract

VERTEBRATE visual pigments in their native state are the principal components of the membranes of rod and cone outer distal segments¹. Like all membrane structures they have a predominantly lipoprotein composition^2,3. The traditional picture of the native form of these pigments, developed largely from their preponderant protein properties⁴, is one of a polyene chromophore bound to a protein through a Schiff base linkage⁵ and, indeed, the recent demonstration of Bownds⁶ and Aktar⁷ of the NaBH₄ reductive affixation of the retinylidene chromophore to the ε-amino group of a lysine residue in irradiated bovine rhodopsin seems to support this notion. But to assume that this is the binding site in native rhodopsin discounts the possibility of migration of the chromophore in the early stages of the physiological process, and further implies that the chromophore could not have migrated to this particular ε-amino group of lysine from some other site in the near neutral pH conditions of the NaBH₄ reduction^6,7 where Schiff bases are known to hydrolyse readily and undergo inline exchange⁸.