Cephalopod Retinochrome

Abstract

The visual cells of vertebrates and invertebrates, though anatomically quite different from each other, contain the same kinds of visual pigment composed of a protein, opsin, to which the 11-cis isomer of retinal (retinaldehyde, vitamin A aldehyde) is attached as chromophore. In cattle and squid rhodopsin, the two visual pigments whose chemical composition has been examined most thoroughly, retinaldehyde forms a Schiff base (an aldimine) with the ε-amino group of an internal lysine residue in opsin (Bownds, 1967; Hagins, 1973). As might be expected from their chemical similarities, all visual pigments have similar absorption spectra with a main α-band and a much smaller β-band, both owing to absorption by the chromophore, and a narrow γ-band near 280 nm owing to absorption by the aromatic amino acid residues in opsin. The precise wavelength position of the main band is different in the different visual pigments, but in general λ _max lies at longer wavelengths than 440 nm. Rhodopsin, the visual pigment in vertebrate rods and in the rhabdoms of invertebrates, usually has λ _max near 500 nm.