Retinoidzyklus

Retinoidzyklus, or the retinoid cycle, is the biochemical process that regenerates the visual pigment in retinal photoreceptors after light exposure. It links the phototransduction cascade in the outer retina with vitamin A–dependent metabolism in the retinal pigment epithelium (RPE) and supporting glial cells, ensuring a continuous supply of 11-cis-retinal for pigment regeneration.

In rod and cone photoreceptors, light converts 11-cis-retinal bound to opsin into all-trans-retinal, triggering the visual signal. All-trans-retinal is released and quickly reduced to all-trans-retinol, which is transported from the outer segment to the RPE by interphotoreceptor retinoid-binding protein (IRBP). In the RPE, all-trans-retinol is esterified by LRAT to all-trans-retinyl esters for storage and isomerized by the retinoid isomerase RPE65 to 11-cis-retinol. This isthen oxidized by 11-cis-retinol dehydrogenases to 11-cis-retinal, which returns to photoreceptors and recombines with opsin to reform the light-sensitive pigment (rhodopsin in rods, cone pigments in cones).

Cones utilize a parallel retinoid cycle that involves cone-specific processes and Müller glia, enabling faster pigment regeneration important for diurnal and color vision. The cycle is essential for dark adaptation and sustained visual function, and it depends on dietary vitamin A intake to supply the retinal pool. Disruptions in the cycle, such as mutations in RPE65 or LRAT, can lead to impaired vision and inherited retinal diseases, underscoring its physiological and clinical significance.