Visual cycle vitamin

Visual cycle, vitamin A and, 551-554 Vitamin A, 492,493, SS4-556,556 absorption and stoirage, 556-557 biochemistry, 557-558 cancer and, 59(1, 592 Cell differenttaticiri and, 559-550 hormonal efferts, S60-55I tebnol-binding protein (RBPJ, 557-558 toxicity, 555... 

Since alcohol dehydrogenase is required for the conversion of retinol to retinal, excessive and prolonged ethanol ingestion can impair the physiological function of vitamin A. The decreased conversion of retinol to retinal results from competitive use of the enzyme by ethanol. Night blindness may result, since the visual cycle is a retinol-dependent physiological process. 

Reproduction Retinol and retinal are essential for normal repno duction, supporting spermatogenesis in the male and preventing fetal resorption in the female. Retinoic acid is inactive in maintain ing reproduction and in the visual cycle, but promotes growth and differentiation of epithelial cells thus, animals given vitamin A only as retinoic acid from birth are blind and sterile. 

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. 

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.

The rate-liniituig step in initiation of the visual cycle is the regeneration of 11 -c/s-retinaldehyde. In vitamin A deficiency, when there is hide 11 -c/s-retinyl ester in the pigment epithelium, hoth the time taken to adapt to darkness and the ability to see in poor light will be impaired. 

Vitamin A Visual cycle and maintaining epithelial cells Fruits, vegetables, fish-liver oils Night blindness and eventually total blindness, anorexia (appetite loss), dermatitis, recurrent infections in children, cessation of skeletal growth and lesions in the central nervous system. 

The molecular mechanism through which vitamin A functions in visual system is known as Rhodopsin cycle or Wald s visual cycle for which George Wald was awarded Nobel Prize. 

It is important to note that all-trans-retinal is incompletely converted to 11-cis-retinal hence a constant supply of vitamin A is needed in the diet. Describe other functions of vitamin A, besides visual cycle. 

Two of the vitamins are converted to hormones. These are vitamin A and vitamin D. Vitamin A also functions as a cofactor in the visual cycle. 

Generally, vitamin A serves three classes of functions (1) support of the differentiation of epithelial cells, (2) support of the viability of the reproductive system (fetal growth and vitality of the testes), and (3) utilization in the visual cycle. Dietary retinoic acid can support only the first function. Animals raised on diets containing retinoic acid as the only source of vitamin lose their ability to see in dim light and become sterile. In males, sperm production ceases. In females, fetuses are resorbed. Retinoic acid cannot be stored in the liver, as it lacks the hydroxyl group needed for attachment of the fatty acid. Retinyl esters, retinol, and retinal are interconvertible. Retinal can be oxidized to form retinoic acid. All three functions of vitamin A can be supported by dietary retinyl esters, retinol, or retinal. Although these forms can be converted to retinoic acid, retinoic acid apparently cannot be reduced to form retinal. These relationships are summarized in Figure 9.44. 

The diagrams represent some of the changes that take place in the visual cycle involving the rhodopsin system in which the I l-mono-m isomer of vitamin A is functional in the aldehyde form. ... 

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

Vitamin A is the isoprenoid alcohol retinol, which is required for a number of body processes including vision, growth, maintenance of mucous membranes, reproduction and proper growth of the cartilage matrix upon which bone is deposited. Of these, the molecular basis of its role in the visual cycle is best understood. In addition to the alcohol retinol, both retinal and retinoic acid (Fig. 2.4) also act to restore some of the deficiency symptoms. In rod vision, retinol is oxidised to retinal. 11-ds-Retinal combines with the protein opsin to form rhodopsin. After the absorption of a photon, rhodopsin undergoes a series of changes, eventually dissociating to opsin and all-fraMS-retinal. This last compound is converted back to 11-ds-retinal by retinal isomerase (EC.5.2.1.3) Retinoic acid is able to replace retinol in all functions except the visual cycle and reproduction. 

Probably the best understood role of vitamin A is its participation in the visual cycle in rod cells. In a series of enzyme-catalyzed reactions retinol undergoes (1) a two-electron... 

What defects in vitamin A transport or utilization are likely to occur in the diseased eye The cytosol and IPM binding proteins and their possible receptors are clearly essential components of the visual system, as are the specific enzymes that are implicated at certain stages of the visual cycle (Table II). Failure to isomerize el -trans to Il-cu-retinoid is one obvious possibility, and a clinical trial was carried out by Chatzinoff et al. (1968) in an attempt to answer this question. The compound ll-c/5- vitamin A (the retinoid used was not reported) was injected intramuscularly into retinitis pigmentosa patients over a 3-year period. This group was then compared with a parallel group of patients that had received the all-trans isomer. No beneficial effect of 11-crr-retinoid was found. However, the ease with which the 11-cis isomer isomerizes back to all-trans when dispersed in tissue preparations (see Section III,G,4,b) casts doubt on the likelihood that in this study any 11-cM-retinol would have survived to be delivered to the RPE. 

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