Formation of Isorhodopsin in the Frog's Eye during Continuous Illumination

Abstract

BOTH in solution and in the intact rods the rhodopsin of the frog, Rana temporaria, absorbs light maximally at the wave-length 502 mµ (λ_max 502 mµ.)¹. Like the other vertebrate rhodopsins it is composed of a colourless protein, opsin, combined with the 11-cis isomer of retinal (formerly retinene)². Light isomerizes the 11-cis retinal (the chromophore) to the all-trans configuration, thereby converting rhodopsin into all-trans lumi- and meta-rhodopsin. In the dark or in weak illumination the meta-rhodopsin (λ_max at about 480 mµ) completely hydrolyses to free opsin and pale yellow all-trans retinal, which reduces to colourless retinol (formerly vitamin A) (ref. 3). But it has been demonstrated that in solutions a bright light can isomerize the all-trans chromophores of lumi- or meta-rhodopsin into mixtures of stereoisomeric chromophores, all still attached to opsin. The 11-cis fraction of this mixture is rhodopsin, the 9-cis fraction isorhodopsin (a thermally stable but photosensitive rhodopsin analogue, λ_m486 mµ (refs. 3–5)). It has been demonstrated that this back-isomerization of coloured intermediates also takes place during flash-irradiation in suspensions of rods⁵ and in the living rat eye⁶, but the yield of isorhodopsin is less in these experiments in situ (about 20 per cent of the thermally stable pigment) than in the corresponding experiments with solutions (about 50 per cent)⁴.