Alternate Cone Pathway
It is believed that as well as this pathway of chromophore recycling described above, that cone photoreceptors have an alternate pathway of chromophore recycling which uses Muller's cells within the retina to hasten chromophore recycling. In this pathway, cones reduce all-trans retinal to all-trans retinol via all-trans Retinol Dehydrogenase, then transport all-trans retinol to Muller's cells. There, it is transformed into 11-cis retinol by all-trans retinol isomerase, and can either be stored as retinyl esters within Muller's cells, or transported back to the cone photoreceptors, where it is transformed from 11-cis retinol to 11-cis retinal by 11-cis Retinal Dehydrogenase. This pathway helps explain the rapid dark adaptation in the cone system, and the presence of 11-cis Retinal Dehydrogenase in cone photoreceptors, as it is not found in rods, only in the RPE.
Read more about this topic: Bleach And Recycle
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