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Review
. 2010 Dec;91(6):788-92.
doi: 10.1016/j.exer.2010.08.013. Epub 2010 Aug 27.

Retinol Dehydrogenases (RDHs) in the Visual Cycle

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Free PMC article
Review

Retinol Dehydrogenases (RDHs) in the Visual Cycle

Ryan O Parker et al. Exp Eye Res. .
Free PMC article

Abstract

The isomerization of 11-cis retinal to all-trans retinal in photoreceptors is the first step in vision. For photoreceptors to function in constant light, the all-trans retinal must be converted back to 11-cis retinal via the enzymatic steps of the visual cycle. Within this cycle, all-trans retinal is reduced to all-trans retinol in photoreceptors and transported to the retinal pigment epithelium (RPE). In the RPE, all-trans retinol is converted to 11-cis retinol, and in the final enzymatic step, 11-cis retinol is oxidized to 11-cis retinal. The first and last steps of the classical visual cycle are reduction and oxidation reactions, respectively, that utilize retinol dehydrogenase (RDH) enzymes. The visual cycle RDHs have been extensively studied, but because multiple RDHs are capable of catalyzing each step, the exact RDHs responsible for each reaction remain unknown. Within rods, RDH8 is largely responsible for the reduction of all-trans retinal with possible assistance from RDH12. retSDR1 is thought to reduce all-trans retinal in cones. In the RPE, the oxidation of 11-cis retinol is carried out by RDH5 with possible help from RDH11 and RDH10. Here, we review the characteristics of each RDH in vitro and the findings from knockout models that suggest the roles for each in the visual cycle.

Figures

Fig. 1
Fig. 1
Oxidation and Reduction Reactions of the Visual Cycle. The classical visual cycle involves reduction and oxidation reactions carried out in the photoreceptors and retinal pigment epithelium (RPE), respectively. RDH enzymes are responsible for both reactions. In the photoreceptor OS, RDHs carry out the NADPH-dependent reduction of all-trans retinal (atRAL) to all-trans retinol (atROL). In the RPE, RDHs are responsible for the NAD+-dependent oxidation of 11-cis retinol (11cROL) to 11-cis retinal (11cRAL).
Fig. 2
Fig. 2
Distribution of Visual Cycle RDHs in the Eye. Multiple RDHs found in the eye contribute to retinoid metabolism. The proposed role of many RDHs is based on their location in the retinal pigment epithelium (RDH5, RDH11, RDH10), photoreceptor outer segments (RDH8, retSDR1), photoreceptor inner segments (RDH11, RDH12), and Müller cells (RDH10).

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