Absorption Spectrum of Rhodopsin denatured with Acid

Abstract

THE visual pigment rhodopsin is composed of the chromophore, retinal, and the protein moiety, opsin^1,2. Retinal and opsin are considered to be bound to each other primarily by a Schiff base linkage, and secondarily by a stereospecific link^3–5. An ε-amino group, probably of a lysine residue in opsin, may participate with the Schiff base binding^6,7. The nature of the secondary link is still obscure, but its existence is suggested by: (1) rhodopsin can be regenerated from opsin and 11-cis retinal beyond other isomers^1,2; (2) a bathochromic shift of absorption spectrum of retinal in rhodopsin cannot be simply explained by the Schiff base with amino compounds⁸; (3) Schiff base linkages are usually unstable at pHs at which rhodopsin is stable^8,9; (4) when rhodopsin is attacked by denaturants of protein or by heat, bleaching results^10,11,16; (5) retinal in rhodopsin has been found to be fixed asymmetrically on the protein^12–15.