In recent years, some important findings, which promoted understanding of the visual cycle, have been reported. For example, (1) the enzymatic process of retinoid isomerization was detected in the vertebrate retinal pigment epithelium cells, (2) finding of a novel retinal-binding protein completed the visual cycle in the squid photoreceptor cells, and (3) the temporary formation of 11-cis-3-OH-retinol on the light-dependent synthesis of visual pigment was demonstrated in the eye of Drosophila. Based on these new findings, matabolic processes of visual pigments both in vertebrates and invertebrates are discussed.
