Retina isomers

Both MS and NMR coupling to HPLC have been employed for the analysis of p-carotene isomers and determination of lutein and zeaxanthin isomers in spinach, sweet com, and in retina. Capillary high performance hquid chromatography with stop flow connected to NMR (600 MHz) was used for stracture elucidation of all-trans deoxylutein 11 and its isomers.Efforts are in progress to eliminate the remaining major drawbacks such as obligatory use of deuterated solvents in the mobile phase, poor sensitivity, and low throughput of HPLC-NMR analyses. 

There ensues a series of dark reactions or conformational changes that have the effect of greatly activating the imine linkage of the all-frans-rhodopsin towards hydrolysis. On hydrolysis, all-frawj-retina] is released and is unable to recombine with opsin until it is reconverted to the 11-cis isomer. The trans-to-cis rearrangement is a thermal rather than a photochemical reaction and is catalyzed by the enzyme retinal isomerase. The cycle of reactions is summarized in Figure 28-13. 

In 1933 G. Wald discovered the presence of vitamin A in the retina [3]. Subsequently, it was also found that it is the 11-c/s isomer of vitamin A aldehyde,... 

Rhodopsin is a light-sensitive compound located in the membrane of the rod cells in the retina of the eye. Rhodopsin contains the protein opsin bonded to 11-c/s-retinai via an imine iinkage. When light strikes this molecule, the crowded 11-cis double bond isomerizes to the 11 -trans isomer, and a nerve impulse is transmitted to the brain by the optic nerve. 

The fluorenone-sensitized irradiation of the all rrawj-triene (140) gives a photostationary mixture composed of the trans-cis-trans, cis-trans-trans and cis-cis-trans isomers. The potential energy surfaces of the ground and excited state of the triene (140) have been mapped,and a study of the fluorescence from this triene in lipid bilayers and isotropic solvents has been carried out. The photophysical properties of the dithienylpolyenes (141) and (142) have been measured. The photoisomerism of all-tranj-retinal to the 11-m isomer brought about by honeybee retina enzyme photoisomerase has been described. ... 

Recently Inoue and coworkers also reported ab initio study of shieldings for linear jr-conjugated systems. A photoreceptive protein such as rhodopsin (Rh) or bacte-riorhodopsin (bR) possesses a retinal isomer bound to a lysine residue via the protonated Schiff base linkage. Rh exists in the rod cell of the retina of vertebrate and possesses 11 -c/s-rcli iial (Figure 2), which is isomerized into the al l-Zraw.v form by the absorption of photons, finally leading to signal transduction. 

Dehydroretinal, 3,4-Didehydroretinat 3,7-di-methyl-9-(2 6 6-triniethyi-l,3-cyclohexadien 1 -yl)-2,4,6,8-nonatetraena) retinal 2 retinene 2 vitamin A2 aldehyde. c hm° mo1 wt 282.42. C 85.06%. H 9.28%, O 5,66%. Carotenoid component of the visual pigments of fresh -water and migratory fish, lampreys and certain amphibians- Of the 16 possible stereoisomers 7 have been synthesized. The 11 -ei-s.isomer is the chromophore of porphyropsin and cyanopsin, q.q. y. Isoln from retinas of fresh-water fish G. Wald, Nature 139, 1017 (1937). Recognition as vitamin A2 aldehyde R. A. Morton, ibid. 153, 69(1944). Prepn by the... 

The conversion of a cis isomer to the trms isomer (or vice versa) is a process called cis-trans isomerization. Such conversions are crucial in the vision process. The molecules in the retina that respond to light are rhodopsin, which has two components called 11-cw-retinal and opsin. Retinal is the light-sensitive component, and opsin is a protein molecule. Upon receiving a photon in the visible region, 11-cw-retinal isomer-izes to all-trans retinal by breaking a carbon-carbon tt bond. With the TT bond broken, the atoms connected by the carbon-... 

Cis-trans isomerism piays a key role in the process of vision. The rod ceiis in the retina of the eye contain a red, light-sensitive pigment called rhodopsin. This pigment consists of the protein opsin combined at its active site with 1 1 -c/s-retinal. When visible light with the appropriate energy is absorbed by rhodopsin, the complexed c/s-retinal is isomerized to the trans isomer. This process is fantastically fast, occurring in only... 

In the retinal cells of the eye, vitamin A (all-trans-retinol) is converted into the 11-ds-isomer, which is then oxidised to 11-cts-retinaldehde. In the dark the latter then combines with the protein opsin to form rhodopsin (visual purple), which is the photoreceptor for vision at low light intensities. When light falls on the retina, the czs-retinaldehyde molecule is converted back into the aW-trans form and is released from the opsin. This conversion results in the transmission of an impulse up the optic nerve. The all-frans-retinaldehyde is converted to all-trans-retinol, which re-enters the cycle, thus continually renewing the light sensitivity of the retina (Rg. 5.2). 

All-tra/w-retinol, 11-cw-retinol (A2) and 11-m-retinaldehyde are found in the 1PM (Liou et al., 1982c). At present, it is not clear which retinoid, or which isomer, is in transit from the RPE to the ROS. With regard to carrier proteins, the evidence suggests that CRBP and CRAIBP are present in the 1PM (Liou et al., 1982c), but it is probable that they have leached from the RPE and retina cytosols where both proteins occur. On the other hand, interstitial retinol-binding protein is believed to be a true IPM protein because it is absent from the RPE and retina cytosols. This conclusion has been confirmed by immunocytochemical observations in the author s laboratory (Fong et al., 1984) and by Bunt-Milam and Saari (1983). Although IRBP binds the all-tranj-retinol that is released when rhodop-sin is bleached, it also carries some 11-cis isomer, and it has not been established whether it may be implicated in the two-way transport of retinoids. 

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