Regulation of the Visual Cycle: Retinol Dehydrogenase and Retinol Fluorescence Measurements In Vertebrate Retina

The initial and only light-activated step in vision is the photoisomerization of the ligand of the visual pigment,11-cis retinal to all-trans retinal, which occurs while being covalently bound to a lysine deep in the membrane region of the visual pigment. This event leads to the activation of the g-protein, transducin, which in turn activates c-gmp phosphodiesterase causing the destruction of c-gmp, the closure of cation channels in the plasma membrane of the photoreceptor, and eventually to the reduction in release of synaptic transmitter to other retinal neurons in the visual pathway. However, the visual pigment containing retinal in its all-trans form is now incapable of absorbing photons in the visual region of the spectrum and can no longer activate the transduction cascade. In order to do so, it must be regenerated into a form that contains 1 1-cis retinal. The biochemical reactions by which this occurs are collectively called the visual cycle. This complex series of reactions is initiated in the photoreceptors themselves, by the reduction of all-trans retinal to all-trans retinol by all-trans retinol dehydrogenase (rdh) and the cofactor nadph. All the subsequent steps in the regeneration of 11-cis retinal take place in the retinal pigment epithelium. Regenerated 11-cis retinal is then transported from the retinal pigment epithelium through the extracellular matrix back to the photoreceptor cells where it condenses with opsin to reform visual pigment.