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S. Miyazono, T. Isayama, C. L. Makino; Activation of Rhodopsin by Vitamin A. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2043. doi: https://doi.org/.
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Absorption of light by rhodopsin induces a conformational change in the opsin protein leading to activation of a phototransduction cascade that evokes an electrical response by the photoreceptor. The truncated retinal analog, beta-ionone, can activate some visual pigments without light , but beta-ionone is not normally found in the retina. A related molecule, vitamin A, can accumulate to sub-millimolar concentrations within photoreceptors after exposure to bright light, therefore we investigated its effect on the photoresponse of rods.
Flash responses of individual, dark adapted green-sensitive rods of salamander (A. tigrinum) were measured by suction electrode recording before and during treatment with vitamin A, and then after the vitamin A was washed away with Ringer’s containing albumin. The extent of vitamin A uptake by rods was determined microspectrophotometrically.
We found that low micromolar concentrations of vitamin A reduced the circulating current, reduced absolute sensitivity to flashes and accelerated dim flash response kinetics, mimicking the effect of background light. Interestingly, relative sensitivity of the rods to UV light improved during exposure to vitamin A. All effects reversed completely after the vitamin A was washed off.
The finding that vitamin A enhanced the relative sensitivity of green-sensitive rods to UV light strongly suggests that vitamin A bound to rhodopsin and served as an antenna pigment; it absorbed UV light and transferred the energy of the UV photons to the native chromophore. Thus the opsin apoprotein can bind two retinoids simultaneously. Vitamin A also weakly activated salamander green-sensitive rods in the absence of light, presumably by destabilizing the rhodopsin. This activation might affect the course of dark adaptation of photoreceptors, because vitamin A accumulates in abundance after exposure to bright light. Since over-stimulation of photoreceptors results in retinal degeneration, vitamin A could contribute to some forms of eye diseases.
 Isayama et al., Vis Neurosci. 26: 267-274. (2009).
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