Highly efficient retinal metabolism in cones

Abstract

After bleaching of visual pigment in vertebrate photoreceptors, all-trans retinal is reduced to all-trans retinol by retinol dehydrogenases (RDHs). We investigated this reaction in purified carp rods and cones, and we found that the reducing activity toward alltrans retinal in the outer segment (OS) of cones is >30 times higher than that of rods. The high activity of RDHs was attributed to high content of RDH8 in cones. In the inner segment (IS) in both rods and cones, RDH8L2 and RDH13 were found to be the major enzymes among RDH family proteins. We further found a previously undescribed and effective pathway to convert 11-cis retinol to 11-cis retinal in cones: this oxidative conversion did not require NADP ؉ and instead was coupled with reduction of all-trans retinal to all-trans retinol. The activity was >50 times effective than the oxidizing activity of RDHs that require NADP ؉ . These highly effective reactions of removal of all-trans retinal by RDH8 and production of 11-cis retinal by the coupling reaction are probably the underlying mechanisms that ensure effective visual pigment regeneration in cones that function under much brighter light conditions than rods.11-cis retinal ͉ 11-cis retinol ͉ all-trans retinal ͉ retinol dehydrogenase ͉ visual cycle L ight detection in vertebrates is mediated by rods and cones. Both rods and cones consist of two parts, the outer segment (OS) and the inner segment (IS). The OS contains the machinery responsible for conversion of a light signal to an electrical signal. The light-absorbing molecule, the visual pigment, is present in the OS and is composed of a chromophore, 11-cis retinal, and a protein moiety, opsin. Light isomerizes 11-cis retinal to all-trans retinal, which induces a conformational change in opsin to lead to activation of an enzymatic cascade to evoke a light response (1, 2).All-trans retinal, the product formed after light absorption, then dissociates from opsin and is reduced to all-trans retinol. This form of retinol is transported to the retinal pigment epithelium (RPE), isomerized, and oxidized to 11-cis retinal with multistep reactions. The newly synthesized 11-cis retinal is sent back to photoreceptors to regenerate visual pigment. This pathway of retinal processing (visual cycle) has been mainly studied in rods or rod-dominant retinas (3, 4). Because cones function under much brighter light conditions, the visual cycle for cones could differ from that for rods in a qualitative and/or quantitative way, as is seen in the phototransduction mechanism in rods and cones (5, 6).The reduction of all-trans retinal to all-trans retinol is attained by RDHs within a photoreceptor cell. This reduction could be the rate-limiting step of the visual cycle (7), and the rate of the reduction has been reported to be 10-40 times higher in cones than in rods under in vivo conditions (8). The supply of 11-cis retinal to regenerate visual pigment could be faster in cones. Several lines of evidence suggest that cone pigment is regenerated in an RPE-independent manner. Fo...

RDH13L, an Enzyme Responsible for the Aldehyde-Alcohol Redox Coupling Reaction (AL-OL Coupling Reaction) to Supply 11-cis Retinal in the Carp Cone Retinoid Cycle

“…Cloning of Carp and Mouse RDH Genes-Carp RDH13L (KM377625) and carp RDH14 (KM377626) were cloned from a carp retinal cDNA library (21) based on partial sequences obtained in our previous study (16 WT proteins of carp RDH13L, carp RDH8 (AB439579), carp RDH8L2 (AB439580), and carp RDH13 (AB439581) were expressed in Sf9 cells using Bac-to-Bac Baculovirus Expression System (Invitrogen). Sf9 cells (5 ϫ 10 6 cells) were infected and incubated according to the method described in a manufacturer's protocol.…”

“…In our previous studies (16,17), we reported that carp cone inner segment membranes show a very efficient 11-cis retinol oxidation activity that does not require the addition of NADP ϩ or NAD ϩ usually required in the oxidation of 11-cis retinol by retinol dehydrogenases (RDHs). Instead, some hydrophobic aldehydes possibly produced in the inner segment supported the reaction: 11-cis retinol was oxidized with concomitant reduction of an aldehyde at a 1:1 molar ratio (aldehyde-alcohol redox-coupling reaction, AL-OL coupling reaction; Ref.…”

“…17). We estimated that, at its maximum rate, the AL-OL coupling supplies 11-cis retinal at a 240 times higher rate than the RPE retinoid cycle (16). This reaction could also contribute to eliminate toxic aldehydes produced in mitochondrion in the inner segment.…”

Parallel visual cycles in the zebrafish retina

“…In contrast to the data from chicken and ground squirrel, the oxidation of 11-cis ROL into 11-cis RAL by RDHs was not effective. The authors speculated that the co-factor might be all-trans RAL rather than NADP+ (Miyazono et al, 2008).…”

“…Novel enzymatic activities unique to the intra-retinal visual cycle (in red arrows) include an isomerase (Mata et al, 2005), a RE hydrolase (Bustamante et al, 1995), a retinylester synthase (Mata et al, 2002;Muniz et al, 2006), and a retinol dehydrogenase (Mata et al, 2002;Miyazono et al, 2008). For experimental details see (Schonthaler et al, 2007) and (Fleisch et al, 2008) Figure 4 down.…”