Absorption of a photon by rhodopsin causes photoisomerization of the chromophore, 11 -as-retinylidene to all-ira/is-retinylidene, and concomitant changes in the apoprotein opsin. Ultimately, the Schiff base linkage is hydro-lyzed and the chromophore is released from the binding pocket of opsin through a mechanism that has not been elucidated on the mechanistical level (Figure 7, reaction a). In one of the models, the chromophore is released into the intradiskal space, and then pumped out by a photoreceptor-specific ATP-dependent exchanger, ABCR protein . This model may require further revisions, because the mice and humans lacking the functional ABCR protein are still able to regenerate rhodopsin at rates that are only slightly lower compared to normal controls. Therefore, the possible function of ABCR is to drive out the smallest amounts of all-/m«s-retinal to prevent formation of toxic
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