Kinetics of Rhodopsin Regeneration in the Eye of the Frog

Abstract

RHODOPSIN, the red photosensitive pigment of rod vision, consists of the colourless protein, rod opsin, carrying 11-cisretinal (formerly retinene) as chromophore¹. In the light, rhodopsin bleaches through orange-red intermediates (lumi- and meta-rhodopsin) into the yellow mixture of free all-trans retinal and opsin². The enzyme alcohol dehydrogenase and DPNH₂ in the rod outer segments reduce the retinal to retinol (formerly vitamin A)³. The all-trans retinol migrates from the rod outer segments into the pigment epithelium, where, most likely, an energy-utilizing process isomerizes the all-trans retinol into 11-cis retinol⁴. The exact material for the isomerization is, however, uncertain. This process supplies the pigment epithelium both in light- and dark-adapted frogs' eyes with a relatively great amount of 11-cis retinol. About 95 per cent of the retinol in the eye of the frog (Rana pipiens) is in the form of fatty acid-esters⁴.