Vitamin A as an Activator and Sensitizing Chromophore for Rhodopsin

Abstract

Absorption of light by rhodopsin isomerizes its 11-cis retinal chromophore to the all-trans conformation. The rhodopsin then activates a biochemical cascade that produces an electrical response by the photoreceptor. Eventually, all-trans retinal dissociates from the opsin and is reduced to vitamin A. The truncated retinal analog, beta-ionone, can pharmacologically activate some types of visual pigment, mimicking the effects of light. Beta-ionone is not normally found in the retina, however, vitamin A is present within the photoreceptor and can reach millimolar concentrations after exposure to bright light. Can vitamin A activate rhodopsin? In suction electrode recordings from isolated green-sensitive rods of salamander, exogenous vitamin A decreased circulating current and flash sensitivity, and accelerated flash response kinetics, changes that can also be seen during exposure to background light. Microspectrophotometric measurements showed that vitamin A accumulated in the outer segments, and an in vitro binding assay confirmed the binding of vitamin A to rhodopsin. These results suggested that vitamin A activated rhodopsin. In addition, suction electrode recordings showed that vitamin A improved the relative sensitivity of rods to UV light and in an in vitro bleaching assay, vitamin A enhanced the bleaching of rhodopsin by UV light. Presumably, both effects involved Forster resonance energy transfer (FRET) from vitamin A to the 11-cis chromophore of rhodopsin. Next, we tested whether FRET could be induced by endogenous vitamin A. After bleaching a large fraction of the rhodopsin in green-sensitive rods to generate vitamin A, relative sensitivity to UV wavelengths did indeed increase. Therefore, vitamin A can bind rhodopsin, activate it and also serve as a sensitizing chromophore.