September 2016
Volume 57, Issue 12
Open Access
ARVO Annual Meeting Abstract  |   September 2016
The Role of Free Radicals in Thermal, Light-driven and Enzyme-catalyzed Isomerization of Retinoids
Author Affiliations & Notes
  • Tongzhou Xu
    Stein Eye Institute, University of California, Los Angeles School of Medicine, Los Angeles, California, United States
  • Quan Yuan
    Stein Eye Institute, University of California, Los Angeles School of Medicine, Los Angeles, California, United States
  • Joanna J Kaylor
    Stein Eye Institute, University of California, Los Angeles School of Medicine, Los Angeles, California, United States
  • Avian Tsan
    Stein Eye Institute, University of California, Los Angeles School of Medicine, Los Angeles, California, United States
  • Gabriel H Travis
    Stein Eye Institute, University of California, Los Angeles School of Medicine, Los Angeles, California, United States
  • Footnotes
    Commercial Relationships   Tongzhou Xu, None; Quan Yuan, None; Joanna Kaylor, None; Avian Tsan, None; Gabriel Travis, None
  • Footnotes
    Support  None
Investigative Ophthalmology & Visual Science September 2016, Vol.57, 1722. doi:
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      Tongzhou Xu, Quan Yuan, Joanna J Kaylor, Avian Tsan, Gabriel H Travis; The Role of Free Radicals in Thermal, Light-driven and Enzyme-catalyzed Isomerization of Retinoids. Invest. Ophthalmol. Vis. Sci. 2016;57(12):1722.

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      © 2017 Association for Research in Vision and Ophthalmology.

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Abstract

Purpose : The visual chromophore for most opsin pigments is 11-cis-retinaldehyde (11-cis-RAL), which is isomerized to all-trans-retinaldehyde (all-trans-RAL) upon absorption of a photon. After briefly stimulating visual transduction, vertebrate opsins dissociate to yield free all-trans-RAL. Restoration of light sensitivity follows re-isomerization of all-trans-RAL to 11-cis-RAL by enzymes of the visual cycle, which include the isomerases, Rpe65 and DES1. Retinoid isomerization also occurs in the absence of proteins upon exposure to light or molecular iodine. In this study, we tested the effects of spin trap and spin probe reagents on several modes of retinoid isomerization, attempting to understand their mechanisms. Retinoids contain a system of conjugated double bonds that stabilize carbocation or radical-cation intermediates through electron delocalization. If retinoid isomerization involves a free-radical mechanism, the intermediate should be stabilized by spin trap and spin probe reagents, which would inhibit the isomerization.

Methods : The effects of 4-hydroxy-TEMPO, DMPO, DMPIO, PBN and PTMIO reagents on DES1 and Rpe65 retinoid isomerase activities were tested in homogenates of HEK-293T cells that stably express these proteins. We also tested tissue homogenates of chicken retinas (for DES1) and RPE cells (for Rpe65). Inhibition of all-trans-retinol or all-trans-RAL isomerization catalyzed by iodine was tested in n-heptane. Inhibition of all-trans-RAL photoisomerization was tested in methanol using 380-nm monochromatic light, in the presence of the same spin traps and probes.

Results : DES1-catalyzed retinol isomerization was strongly inhibited by 4-hydroxy-TEMPO and PTMIO, while Rpe65-catalyzed isomerization was inhibited by DMPIO, PBN and PTMIO. Iodine-catalyzed isomerization of retinol and retinaldehyde were inhibited by 4-hydroxy-TEMPO and PTMIO. In contrast, photoisomerization of retinaldehyde was not significantly inhibited by any reagents tested.

Conclusions : Rpe65-, DES1-, and iodine-catalyzed isomerization of retinol and retinaldehyde appear to involve free-radical intermediate(s). Photoisomerization of retinaldehyde appears not to involve a free-radical intermediate.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

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