Reduction of retinal

A research direction based on a hypothesis over 200 years old, but only recently starting to emerge, proposes to evaluate the role of the MP for optimal visual performance, thus investigating lutein s and zeaxanthin s effects beyond risk reduction of retinal diseases. 

Cathodic reduction of retinal leads to a regioselective coupling in the presence of malonic ester to produce the corresponding pinacol (Fig. 26) [129]. Chromium(III) ions facilitate the reduction and favor the regioselective coupling of conjugated dienones to pinacols. A Cr(III)-carbonyl compound complex is evidenced as the reason for the selectivity [130]. 

Figure 15.11 The biochemical reactions that result in the conversion of trans-retinal to ds-retinal, to continue the detection of light To continue the process, trans-retinal must be converted back to c/s-retinal. This is achieved in three reactions a dehydrogenase converts trans-retinal to trans-retinol an isomerase converts the trans-retinol to c/s-retinol and another dehydrogenase converts c/s-retinol to c/s-retinal. To ensure the process proceeds in a clockwise direction (i.e. the process does not reverse) the two dehydrogenases are separated. The trans-retinal dehydrogenase is present in the photoreceptor cell where it catalyses the conversion of trans-retinal to trans-retinol which is released into the interstitial space, from where it is taken up by an epithelial cell. Here it is isomerised to c/s-retinol and the same dehydrogenase catalyses its conversion back to c/s-retinal. This is released by the epithelial cell into the interstitial space from where it is taken up by the photoreceptor cell. This c/s-retinal then associates with the protein opsin to produce the light-sensitive rhodopsin to initiate another cycle. The division of labour between the two cells may be necessary to provide different NADH/NAD concentration ratios in the two cells. A high ratio is necessary in the photoreceptor cell to favour reduction of retinal and a low ration in the epithelial cell for the oxidation reaction (Appendix 9.7).

Therefore it appears that we can achieve and maintain intraocular levels of either PEDF or K1K3 angiostatin in neonatal and adult mice sufficient to expect significant reduction of retinal NV. In the ischemic mouse model the level of retinal NV is measured quantitatively by enumerating the endothelial cells above the inner limiting membrane (ILM) of the retina (see later). Such an analysis showed that PEDF treated eyes had 74% fewer endothelial cells above the ILM compared to paired controls and 78% fewer compared to paired controls for K1K3 treated eyes (Raisler et al., 2002). 

The cleavage of p-carotenc to form retinal, followed by the reduction of retinal to retinol, is shovk n in Figure 9.41, The refinoJ is converted to the retinyl ester, packaged in chylomicrons, and exported in the lymphatic system. 

Figure 7.6. Reduction of retinal to retinol in gut and formation of retinyl esters.

A better method was available. It consisted of the reduction of retinal-11- H with lithium borohydride at ambient temperature, followed by acetylation of the obtained retinol-11- H with pyridine and acetyl chloride to give all-rran -reti-nyl-ll- H acetate in good yield (Kaegi et al., 1982a). This easily executed reduction proceeds smoothly, avoiding all the complications of the more usual reduction of the methyl ester of retinoic acid with lithium aluminum hydride at low temperature. Reduction of retinal on a 0.1-mmol scale always furnished retinyl-11- H acetate in yields better than 60% after recrystallization from methanol at low temperature. 

TLC (4) IS very useful for quickly examining product formation dunng a chemical reaction, e.g., reduction of retinal to retinol as described in Subheading 3.2.1. 

Transfer a few crystals (or powder or oil) of retinal, 1-10 mg as required, into a test tube. Dissolve the retinal in about 1 mL of methanol or ethanol (the reduction reaction does not occur efficiently in nonhydroxylic solvents such as hexane or acetonitrile). Dilute this solution (for example, touch the retinal solution with a Pasteur pipet, and then immerse the pipet in about 1 mL of methanol) and scan its absorption spectrum, diluting or concentrating this solution as necessary to obtain a representative spectrum (A, ax 370-380 nm, Fig. 6). Add a small quantity (typically 10-25 mg, need not be exact) of sodium borohydnde to the concentrated-retinal solution, and shake the solution gently. The reaction should be rapid. Note the change in color from bright yellow to pale yellow and the rapid evolution of H2 bubbles Record the absorption spectrum. If the absorbance maximum is at 325 nm and the spectrum is characteristic of retinol as shown m Fig. 6, the reduction of retinal to retinol is complete. 

More than 300 papers have been published on the pharmacological properties of this extract [23]. Experimentally documented activity includes increased tolerance to hypoxia in brain tissue, improved learning capacity and memory, inhibition of platelet activator factor (PAF) [24], improved cerebral and peripheral circulation, neuroprotective effects [25], and reduction of retinal edema. Although all the constituents of the ginkgo extract are considered to contribute to the therapeutic effects, the ginkgo flavonoids are assumed to play an important role that is due to their free radical scavenging capacity. 

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]

Moreover, efficient rhodopsin regeneration may precede enzymatic reduction of all-fran.v-retinal to all-trans-retinol in the aged retina (Figure 15.2c) (Schadel et al., 2003). Upon rhodopsin regeneration, all-trans-retinal is released from the exit site of the protein into the lipid membrane (Figure 15.2c) (Schadel et al., 2003). From here the removal of all-tnms-retinal to the outer leaflet of the disc membrane is dependent on activity of ATP-binding cassette trasporter A4 (ABCA4) present in the rim of photoreceptor disc, known also as ABCR protein. 

This photoaffinity labelling analogue of all-fraws-retinal, 95b, has been tritium labelled80 by reduction of unlabelled aldehyde 95a with [3H]-NaBH4 and subsequent oxidation of the obtained tritium-labelled retinol with activated manganese dioxide. The product 95b (specific activity 38.3 mCimmol-1) has been isolated by preparative TLC (equation 36). 

Foster In view of the concentration of chromophore in the visual system, a 500-fold reduction of chromophore in the eye may have very httle effect on the inner retinal photoreceptors. It is striking that Drosophila carotenoid-depletion experiments only reduce the visual ERG but did not abolish it. 

In a comparable approach, Valla et al. [73] described the synthesis of 9-methylene analogues of retinol, retinal, retinonitrile and retinoic acid, using the p-methylenealdehyde derived from P-ionone. Homer-Emmons condensation with ethyl 4-(diethoxyphosphoryl)-3-methylbut-2-enoate carbanion afforded the ester in 55% yield, as a mixture of 13E/13Z isomers (50/50). This ethyl 9-methylene-retinoate was saponified with ethanolic NaOH to give the corresponding 9-methylene-retinoic acid in 55% yield (13 /13Z 50/50). The retinol analogue was obtained by DIBAL-H reduction of the ethyl ester (75%, 132T/13Z isomers 65/35). 

In the same way, the anion of ethyl 3-cyano-2-methylprop-2-enyl-phosphonate was reacted with the p-methylenealdehyde to give the 9-methylene-retinonitrile in 50% yield, as a mixture of 13E/13Z isomers (65/35). DIBAL-H reduction of the latter compound provided the related retinal (70%, 13E/13Z 65/35). Alternatively, MnCh oxidation of the... 

The visual chromophores. Rhodopsin has been an object of scientific interest for over 100 years.462 Wald and associates469 470 established that rhodopsin contains 11-ds-retinal bound to the opsin in Schiff base linkage (Eq. 23-36). When native rhodopsin is treated with sodium borohydride, little reduction is observed. However, after the protein is bleached by light, reduction of the Schiff base linkage becomes rapid, and the retinal is incorporated into a secondary amine, which was identified as arising from Lys 296. 

Fig. 1. Representative fluorescent photomicrographs of retinal whole mounts showing the loss of Fluorogold (FG)-labeled RGCs in ischemic retina of rat. RGCs were retrogradely labeled with the fluorescent dye FG injected, under stereotaxic guidance, bilaterally into the superior colliculus of a rat 4 days after 50 min ischemia and sacrificed after additional 4 days. Obvious reduction of FG-labeled RGCs is evident in the retina undergone ischemia/reperfusion (panel B) as compared to the contralateral, nonischemic, retina (panel A). Photomicrographs were obtained from the peripheral area of the superior quadrant of the retina. Scale bar 50 fim.

Miscellaneous Physical Chemistry. A kinetic study has been made of the electrochemical reduction of /8-carotene. The photoelectron quantum yield spectrum and photoelectron microscopy of /3-carotene have been described. Second-order rate constants for electron-transfer reactions of radical cations and anions of six carotenoids have been determined. Electronic energy transfer from O2 to carotenoids, e.g. canthaxanthin [/8,/3-carotene-4,4 -dione (192)], has been demonstrated. Several aspects of the physical chemistry of retinal and related compounds have been reported, including studies of electrochemical reduction, the properties of symmetric and asymmetric retinal bilayers, retinal as a source of 02, and the fluorescence lifetimes of retinal. Calculations have been made of photoisomerization quantum yields for 11-cis-retinal and analogues and of the conversion of even-7r-orbital into odd-TT-orbital systems related to retinylidene Schiff bases. ... 

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