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GH Travis, RA Radu, J Lee, NL Mata; Identification of a New Pathway for Retinol Isomerization in Cone-Dominant Ground Squirrel and Chicken Retinas . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3608.
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Purpose: Several lines of evidence suggest that visual pigments in rod and cone photoreceptors regenerate by different mechanisms. For example, cones regenerate visual pigment in retinas separated from the RPE and can utilize 11-cis-retinol (11cROL) to regenerate visual pigment, while rods can do neither. In the current study, we investigated the mechanism of retinoid regeneration in cones by studying retinas from two cone-dominant species. Methods: Subcellular fractions were prepared from bovine, mouse, chicken, and ground squirrel retinas and RPE. These fractions were used to quantify endogenous retinoids, and in enzymatic assays for retinyl-ester synthase and retinol-isomerase activities. In some experiments, inhibitors of the ester synthase, lecithin retinol acyltransferase (LRAT) were added to the assay systems. Results: As previously reported, we observed an isomerase activity that converts all-trans-retinyl esters (atRE) to 11cROL in membranes from rod-dominant mouse and bovine RPE. No isomerase activity was detectable in membranes from mouse or bovine retinas. In contrast, we observed an isomerase activity in cone-dominant ground squirrel and chicken retinas that directly converted all-trans-retinol (atROL) to 11cROL with no requirement for atRE. This activity was stimulated by addition of palmitoyl-coenzyme A but not by ATP. 11cROL formed by these cone-dominant retinal membranes was rapidly converted to 11-cis-retinyl esters (11cRE). This ester-synthase activity was completely insensitive to inhibitors of LRAT. Conclusion: These observations suggest the existence of a novel pathway for retinoid regeneration in cone photoreceptors. This pathway appears to involve an isomerase that converts atROL directly to 11cROL using hydrolysis of fatty acyl-coenzyme A as an energy source. The pathway also includes a new 11cRE synthase, distinct from LRAT. In an accompanying abstract by Mata et al., we demonstrate a novel 11cROL dehydrogenase in cone-dominant retinas, which represents the third step in a new pathway for cone-pigment regeneration.
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