Pentaenyl cations from the photolysis of retinyl acetate. Solvent effects on the leaving group ability and relative nucleophilicities: an unequivocal and quantitative demonstration of the importance of hydrogen bonding

Pienta, Norbert J.; Kessler, Robert J.

doi:10.1021/ja00033a017

Cited by 28 publications

(13 citation statements)

References 17 publications

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“…In conclusion, RPE65 may harness the ability of retinoids to form carbocations (55) or radical cations (56) to produce cis isomers of retinol. Although RPE65 can generate both 11-cis-and 13-cis-retinol, the innate selectivity of the visual cycle for 11-cis retinoids (35) ensures that only 11-cis-retinol is made under normal physiological conditions.…”

Section: Discussionmentioning

confidence: 99%

RPE65, Visual Cycle Retinol Isomerase, Is Not Inherently 11-cis-specific

Redmond

Poliakov

Kuo

et al. 2010

Journal of Biological Chemistry

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The mechanism of retinol isomerization in the vertebrate retina visual cycle remains controversial. Does the isomerase enzyme RPE65 operate via nucleophilic addition at C 11 of the all-trans substrate, or via a carbocation mechanism? To determine this, we modeled the RPE65 substrate cleft to identify residues interacting with substrate and/or intermediate. We find that wild-type RPE65 in vitro produces 13-cis and 11-cis isomers equally robustly. All Tyr-239 mutations abolish activity. Trp-331 mutations reduce activity (W331Y to ϳ75% of wild type, W331F to ϳ50%, and W331L and W331Q to 0%) establishing a requirement for aromaticity, consistent with cation-carbocation stabilization. Two cleft residues modulate isomerization specificity: Thr-147 is important, because replacement by Ser increases 11-cis relative to 13-cis by 40% compared with wild type. Phe-103 mutations are opposite in action: F103L and F103I dramatically reduce 11-cis synthesis relative to 13-cis synthesis compared with wild type. Thr-147 and Phe-103 thus may be pivotal in controlling RPE65 specificity. Also, mutations affecting RPE65 activity coordinately depress 11-cis and 13-cis isomer production but diverge as 11-cis decreases to zero, whereas 13-cis reaches a plateau consistent with thermal isomerization. Lastly, experiments using labeled retinol showed exchange at 13-cis-retinol C 15 oxygen, thus confirming enzymatic isomerization for both isomers. Thus, RPE65 is not inherently 11-cis-specific and can produce both 11-and 13-cis isomers, supporting a carbocation (or radical cation) mechanism for isomerization. Specific visual cycle selectivity for 11-cis isomers instead resides downstream, attributable to mass action by CRALBP, retinol dehydrogenase 5, and high affinity of opsin apoproteins for 11-cis-retinal.A sequence of metabolic events, termed the visual cycle (1, 2), keeps retinal visual pigments, such as rhodopsin, in a state capable of responding to light. In brief, 11-cis-retinal bound to rhodopsin is photo-isomerized to all-trans-retinal, activating rhodopsin. To regenerate rhodopsin, all-trans-retinal is released, reduced to all-trans-retinol that is transported to the retinal pigment epithelium (RPE), 3 and esterified to all-trans-retinyl esters, the substrate for the retinol isomerase (3). All-transretinyl esters are enzymatically isomerized to yield 11-cis-retinol that is oxidized to 11-cis-retinal and returned to the photoreceptors (3, 4). Recently, the RPE protein RPE65 (5) has been identified as the isomerase central to this cycle (6 -8). The importance of RPE65 in chromophore regeneration had been well established by Rpe65 knock-out mice, which display extreme chromophore starvation (no rhodopsin) in the photoreceptors concurrent with overaccumulation of the all-transretinyl ester substrate of RPE65 in the RPE (9). Consequently, Rpe65 Ϫ/Ϫ mice are extremely insensitive to light. Mutations in the human RPE65 gene cause Leber congenital amaurosis 2, a condition of severe early onset blindness (10 -13), which has been the subject...

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Section: Discussionmentioning

confidence: 99%

RPE65, Visual Cycle Retinol Isomerase, Is Not Inherently 11-cis-specific

Redmond

Poliakov

Kuo

et al. 2010

Journal of Biological Chemistry

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“…There is ample evidence for the existence of a carbocation intermediate as a transition state for retinoids. Earlier studies involving pump-probe spectroscopy using all-trans-retinyl acetate have shown that carbocations can exist under certain chemical conditions for up to 5-10 ns (52). The formation of anhydroretinol isomers from all-trans-retinol in the presence of H + also indicates that protonation leads to the elimination of water and a loss of bond-order along the polyene chain (49).…”

Section: Nucleophilic Addition To C 11 Versus Carbocation Formationmentioning

confidence: 98%

Isomerization of all-trans-Retinol to cis-Retinols in Bovine Retinal Pigment Epithelial Cells: Dependence on the Specificity of Retinoid-Binding Proteins

et al. 2000

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In the retinal rod and cone photoreceptors, light photoactivates rhodopsin or cone visual pigments by converting 11-cis-retinal to all-trans-retinal, the process that ultimately results in phototransduction and visual sensation. The production of 11-cis-retinal in adjacent retinal pigment epithelial (RPE) cells is a fundamental process that allows regeneration of the vertebrate visual system. Here, we present evidence that all-trans-retinol is unstable in the presence of H(+) and rearranges to anhydroretinol through a carbocation intermediate, which can be trapped by alcohols to form retro-retinyl ethers. This ability of all-trans-retinol to form a carbocation could be relevant for isomerization. The calculated activation energy of isomerization of all-trans-retinyl carbocation to the 11-cis-isomer was only approximately 18 kcal/mol, as compared to approximately 36 kcal/mol for all-trans-retinol. This activation energy is similar to approximately 17 kcal/mol obtained experimentally for the isomerization reaction in RPE microsomes. Mass spectrometric (MS) analysis of isotopically labeled retinoids showed that isomerization proceeds via alkyl cleavage mechanism, but the product of isomerization depended on the specificity of the retinoid-binding protein(s) as evidenced by the production of 13-cis-retinol in the presence of cellular retinoid-binding protein (CRBP). To test the influence of an electron-withdrawing group on the polyene chain, which would inhibit carbocation formation, 11-fluoro-all-trans-retinol was used in the isomerization assay and was shown to be inactive. Together, these results strengthen the idea that the isomerization reaction is driven by mass action and may occur via carbocation intermediate.

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“…Many studies have been made to investigate the transient phenomena in the laser flash photolysis and pulse radiolysis studies of retinoids (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28). Previous studies on ATRA in methanol showed that 347 nm laser flash photolysis (LFP) of ATRA led to the production of triplet state of ATRA ( 3 ATRA*) via photoexcitation and retinoic acid cation radicals (ATRA •+ ) via photoionization (16).…”

Section: Introductionmentioning

confidence: 99%

Interaction of Retinoic Acid Radical Cation with Lysozyme and Antioxidants: Laser Flash Photolysis Study in Microemulsion

Wang

et al. 2013

Photochem & Photobiology

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All-trans retinoic acid (ATRA) plays essential roles in the normal biological processes and the treatment of cancer and skin diseases. Considering its photosensitive property, many studies have been focused on the photochemistry of ATRA. In this study, we investigated the transient phenomena in the laser flash photolysis (LFP) of ATRA in microemulsion to further understand the photochemistry of ATRA. Results show that 355 nm LFP of ATRA in both acidic and alkaline conditions leads to the generation of retinoic acid cation radicals (ATRA(•+)) via biphotonic processes. The employment of microemulsion system allows us to investigate the reaction of hydrophobic ATRA(•+) with molecules of different polarity. Therefore, we studied the reaction activity of ATRA(•+) to many hydrophobic and hydrophilic molecules. Results show that ATRA(•+) can efficiently interact with lysozyme, tyrosine, tryptophan and many antioxidants, such as curcumin (Cur), vitamin C (VC) and gallic acid (GA). The apparent rate constants of these reactions were measured and compared. These findings suggest that ATRA(•+) is a reactive transient product which may pose damage to lysozyme, and antioxidants, such as Cur, VC and GA, may inactivate ATRA(•+) by efficient quenching reactions.

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Pentaenyl cations from the photolysis of retinyl acetate. Solvent effects on the leaving group ability and relative nucleophilicities: an unequivocal and quantitative demonstration of the importance of hydrogen bonding

Cited by 28 publications

References 17 publications

RPE65, Visual Cycle Retinol Isomerase, Is Not Inherently 11-cis-specific

RPE65, Visual Cycle Retinol Isomerase, Is Not Inherently 11-cis-specific

Isomerization of all-trans-Retinol to cis-Retinols in Bovine Retinal Pigment Epithelial Cells: Dependence on the Specificity of Retinoid-Binding Proteins

Interaction of Retinoic Acid Radical Cation with Lysozyme and Antioxidants: Laser Flash Photolysis Study in Microemulsion

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