Isomerization of 11-cis- Retinoids to All-trans-retinoids in Vitro and in Vivo

Abstract

The regeneration of 11-cis-retinal, the universal chromophore of the vertebrate retina, is a complex process involving photoreceptors and adjacent retinal pigment epithelial cells (RPE). 11-cis-Retinal is coupled to opsins in both rod and cone photoreceptor cells and is photoisomerized to all-trans-retinal by light. Here, we show that RPE microsomes can catalyze the reverse isomerization of 11-cis-retinol to all-trans-retinol (and 13-cisretinol), and membrane exposure to UV light further enhances the rate of this reaction. This conversion is inhibited when 11-cis-retinol is in a complex with cellular retinaldehyde-binding protein (CRALBP), providing a clear demonstration of the protective effect of retinoid-binding proteins in retinoid processes in the eye, a function that has been long suspected but never proven. The reverse isomerization is nonenzymatic and specific to alcohol forms of retinoids, and it displays stereospecific preference for 11-cis-retinol and 13-cis-retinol but is much less efficient for 9-cis-retinol. The mechanism of reverse isomerization was investigated using stable isotope-labeled retinoids and radioactive tracers to show that this reaction occurs with the retention of configuration of the C-15 carbon of retinol through a mechanism that does not eliminate the hydroxyl group, in contrast to the enzymatic all-trans-retinol to 11-cis-retinol reaction. The activation energy for the conversion of 11-cis-retinol to all-trans-retinol is 19.5 kcal/mol, and 20.1 kcal/mol for isomerization of 13-cis-retinol to alltrans-retinol. We also demonstrate that the reverse isomerization occurs in vivo using exogenous 11-cis-retinol injected into the intravitreal space of wild type and Rpe65؊/؊ mice, which have defective forward isomerization. This study demonstrates an uncharacterized activity of RPE microsomes that could be important in the normal flow of retinoids in the eye in vivo during dark adaptation.The regeneration of 11-cis-retinal is critical for sustaining vision in vertebrates (for a review, see Ref. 1). The visual pigments of rod and cone photoreceptors require 11-cis-retinal, as their chromophores complex via Schiff base to opsin molecules. Upon absorption of light, 11-cis-retinal photoisomerizes to all-trans-retinal, triggering the phototransduction process through a G-protein cascade, ultimately leading to neuronal signaling (2-4). Eventually, all-trans-retinal is released by opsin and converted back to 11-cis-retinal through a series of enzymatic steps, termed the retinoid cycle, occurring in both photoreceptor cells and adjacent RPE 1 (1). In the currently described model, the chromophore undergoes several chemical transformations. First, all-trans-retinal is released from the opsins by hydrolysis of the protonated Schiff base. It is then transported out of the rod outer segment (ROS) disks by either an ATP-binding cassette transporter (5, 6) or by simple diffusion and finally is reduced by all-transretinol dehydrogenase (7-10) in the reaction that appears to be the rate-limiting step in...

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

“…Surprisingly, this value is much higher than the photoactivation energy at visible wavelengths (60 kcal/mol at 500 nm). E a is also much higher than the reaction enthalpy change (32)(33)(34)(35) kcal/mol) (7,8) (Fig. 1B).…”

Pharmacological Chaperone-mediated in Vivo Folding and Stabilization of the P23H-opsin Mutant Associated with Autosomal Dominant Retinitis Pigmentosa

“…Moreover, these pigments also acquired a mature state of glycosylation. 2 11-cis-retinoids are unstable in vitro and in vivo and rapidly undergo "reverse isomerization" (33). Therefore, locked analogs were used throughout the rest of this study.…”

Microsomal Glutathione S-Transferase 1 in the Retinal Pigment Epithelium: Protection against Oxidative Stress and a Potential Role in Aging,

“…The pathways of retinal oxidative damage remain to be determined; however, evidence supports the involvement of highly oxidizable retinoids and lipids. UV light can induce aggregation and loss of activity of the retinal ATP-binding cassette transporter in the presence of all-transretinal in vitro (6) and enhances the rate of RPE microsomal-catalyzed reverse isomerization of 11-cis-retinol (and 13-cis-retinol) to all-trans-retinol (7). Dietary vitamin A-depleted rats as well as animals with impaired visual cycle proteins are more resistant to light damage than normal control animals (8)(9)(10).…”