Rhodopsin vitamin

A, retinol Fat-Soluble Vitamins Rhodopsin, visual cycle, night vision... 

Two molecules of vitamin A are formed from one molecule of -carotene. Vitamin A crystallizes in pale yellow needles m.p. 64 C. It is optically inactive. It is unstable in solution when heated in air, but comparatively stable without aeration. Vitamin A is manufactured by extraction from fish-liver oils and by synthesis from / -ionone. The role of vitamin A in vision seems to be different from its systemic function. See also relincne and rhodopsin. 

Bleaching is reversed in the dark and the red-purple color of rhodopsin returns. This is thought to occur by the reduction of all-Pms-retinal to vitamin Ai (retinal), which diffuses from the rod into the pigment epithelium, where it is converted enzymatically to the 1 l-c isomer of vitamin At. The enzymatic isomerization is followed by diffusion back into the rod, oxidation to 11 -rfr-retinal, and combination with opsin to form rhodopsin. This process is shown schematically in Figure 12.5.[Pg.289]

The visual pigment present in rods has been termed rhodopsin and consists of 11-m-retinal, a derivative of vitamin A1( and a lipoprotein called opsin. Recent evidence(43) suggests that in native rhodopsin the retinal chromo-phore is covalently bonded to a phosphatidylethanolamine residue of the lipid portion of opsin. The structure of 1 l-cis-retinal is as follows ... 

A deficiency of vitamin A leads to vision defects, including a visual impairment at low light levels, termed night blindness. For the processes of vision, retinol needs to be converted first by oxidation into the aldehyde retinal, and then by enzymic isomerization to cw-retinal. c -Retinal is then bound to the protein opsin in the retina via an imine linkage (see Section 7.7.1) to give the red visual pigment rhodopsin. 

Vitamin A is essential for proper functioning of the retina, for the integrity of epithelial tissue, for growth and bone development and for reproduction. For vision the active vitamin appears to be retinal as the chromophore of both rods and cones is 11-cis-retinal which, in combination with the protein opsin, forms the photoreceptor rhodopsin. Retinoic acid is the active form associated with growth, differentiation, and transformation. Both all-trans and 9-cis retinoic acid act as a steroid hormone to affect cellular differentiation, especially for morphogenesis, reproduction and for immune responses. At... 

A stack of about 1 000 disks in each rod cell contains the lightsensing protein rhodopsin,10 in which the chromophore 11-o i-retinal (from vitamin A) is attached to the protein opsin. When light is absorbed by rhodopsin, a series of rapid transformations releases all-frans-retinal. At this stage, the pigment is bleached (loses all color) and cannot respond to more light until retinal isomerizes back to the 11 -cis form and recombines with the protein. 

Vitamin A Precursor to rhodopsin, a chemical used for vision assists in inhibiting bacterial and viral infections Night blindness (retinol)... 

Vitamin A is necessary for growth and reproduction, resistance to infection, maintenance and differentiation of epithelial tissues, stability and integrity of membrane structures, and the process of vision. In terms of the last function, vitamin A is a component of rhodopsin or visual purple, a photosensitive pigment in the eye that is needed for vision in dim light. An early mild clinical symptom of vitamin A deficiency is night blindness a severe deficiency of this fat-soluble vitamin results in xerophthalmia, an eye condition leading to blindness. 

A number of geometric isomers of retinol exist because of the possible cis-trans configurations around the double bonds in the side chain. Fish liver oils contain mixtures of the stereoisomers synthetic retinol is the all-trans isomer. Interconversion between isomers readily takes place in the body. In the visual cycle, the reaction between retinal (vitamin A aldehyde) and opsin to form rhodopsin only occurs with the 11 -cis isomer. 

Wald, et. al97,98. performed a set of experiments during the 1940 s that purported to demonstrate the formation of rhodopsin from either retinene, (now known as retinal) or Vitamin A, and a native protein. While their work involved materials showing a peak absorption at 500 nm, this is the wavelength of peak isotropic absorption of a large number of dipolar retinoids. Such a peak is not exclusive to the chromophoies of vision. Neither is it relevant to the anisotropic absorption spectrum of the chromophores of vision. 

Partridge, J. De Grip, W. (1991) A new template for rhodopsin (vitamin A, based) visual pigments Vision Renvoi. 31, pp 619-630... 

Vitamin A, or retinol, is one of the major fat-soluble vitamins. It is present in many foods the best natural sources are liver, butter, margarine, egg yolk, carrots, spinach, and sweet potatoes. Vitamin A is the precursor of retinal, the essential component of the visual pigment rhodopsin. 

When rhodopsin absorbs light in the vision process, the cis double bond between carbons 11 and 12 isomerizes to a trans double bond. This isomerization triggers a nerve impulse telling the brain that light has been absorbed by the eye. The imine of the isomerized product is unstable and is hydrolyzed to opsin and the all-trans form of retinal (also known as vitamin A aldehyde). All-trans retinal is converted back to 11 -c/s-retinal by enzymes so that it can be used again in rhodopsin formation. 

Figure 29-5. The visual cycle of vitamin A is central to vision. In the retina, light stimulates the conversion of 1 l-c/.s-retinal, part of rhodopsin (Rho), to all-iraws-retinal and activates rhodopsin (Rho ).This initiates the first step of the signal transduction cascade that results in the transmission of the visual signal to the brain.The visual cycle involves biochemical and metabolic events in both the photoreceptors (rods) and the retinal pigment epithelium.

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