A survey of the chemistry of visual pigments

Abstract

Visual pigments consist of a protein of the opsin family combined with either retinaldehyde or 3-dehydroretinaldehyde, which participate in a cis-trans isomerization cycle in the eye. The 11-cis form of the aldehyde is required to join with opsin, and light splits it from the protein by converting it to the all-trans isomer. Cattle opsin appears to be a fairly normal lipoprotein, but is easily denatured and loses its ability to form a pigment; it is not known why the absorption maximum of retinaldehyde moves from 385 mμ to the region of 440–560 mμ on union with an opsin. In the photochemical breakdown of rhodopsin to retinaldehyde and opsin, only the first step (rhodopsin→pre-lumirhodopsin) is photochemical, involving the isomerization of the retinaldehyde moiety. The subsequent stages of breakdown (pre-lumirhodopsin→lumirhodopsin→metarhodopsin I→ metarhodopsin II → retinaldehyde + opsin) are thermal reactions set in train by the initial photochemical reaction. The chemical nature of these intermediates is unknown other than that the changes probably involve alterations in the configuration of the opsin molecule and thus in the interaction of retinaldehyde and opsin. The only substance with a known chemical structure obtainable as an intermediate is N-retinylideneopsin, a Schiff's base formed by union of retinaldehyde and an amino group on opsin, which can be formed under certain conditions at the stage between metarhodopsin and retinaldehyde. Suggestions have been made that rhodopsin has a Schiff's base structure but the hypothesis that it is a protonated Schiff's base seems less probable since the discovery that cattle metarhodopsin exhibits λmax at 478 mμ in alkali and 380 mμ in acid, behaviour hard to account for in a Schiff's base. Little work has been done on the structural requirements in the retinaldehyde molecule for pigment formation, but 4-oxoretinaldehyde has been shown to form a pigment (λmax 465–470 mμ) with opsin in vitro. When fed to rats maintained on retinoic acid, 4-oxoretinaldehyde failed to produce a visual pigment in vivo.