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Shinya Sato, Sadaharu Miyazono, Shuji Tachibanaki, Satoru Kawamura; 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. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5514.
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© ARVO (1962-2015); The Authors (2016-present)
Cones, unlike rods, have 11-cis retinol oxidation activity that can promote regeneration of cone visual pigments and thus dark adaptation of cones. However, the responsible enzyme(s) for this reaction is still not clear. We previously reported a novel, very efficient 11-cis retinol oxidation activity in carp cone inner segments (aldehyde-alcohol redox coupling; AL-OL coupling) that utilizes hydrophobic aldehydes, not NADP+ or NAD+, as oxidants. In this study, we tried to identify and characterize an enzyme responsible for the AL-OL coupling using biochemical methods.
Cones were purified from common carp retina (Cyprinus carpio) by using Percoll density gradient centrifugation. For purification of the enzyme, proteins in cones were fractionated by FPLC and native-PAGE. A 37 kDa protein in the sample was identified as retinol dehydrogenase 13-like (RDH13L) by mass spectrometry. The AL-OL coupling activity of RDH13L was studied using recombinant RDH13L proteins. NADP(H) bound to recombinant RDH13L was quantified by HPLC. Subcellular localization of RDH13L in carp cones was studied by immunocytochemistry. The AL-OL coupling activity in mouse homologs of carp RDH13L was examined using recombinant mouse RDH proteins.
RDH13L was identified as a candidate protein responsible for the AL-OL coupling. Recombinant RDH13L showed AL-OL coupling activity. Substrate specificity and subcellular localization of RDH13L were similar to those previously observed for the AL-OL coupling activity of carp cones. Recombinant RDH13L contained tightly bound NADP+. Mouse RDH14 showed AL-OL coupling activity. In addition to the AL-OL coupling, carp RDH13L and mouse RDH14 also exhibited conventional RDH activity, i.e., NADP+-dependent 11-cis retinol oxidation activity. However, temperature dependences of these two reactions were different.
We identified proteins having a dual mechanism of 11-cis retinol oxidation from both carp and mice. These proteins might contribute to an efficient cone pigment regeneration. However, because our study is limited to biochemical analyses, it is still not clear how much these proteins have an impact on cone function in living animals. Physiological studies will be needed for further analysis.
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