Biochemical properties of purified human retinol dehydrogenase 12 (RDH12):: Catalytic efficiency toward Retinoids and C9 aldehydes and effects of cellular retinol-binding protein type I (CRBPI) and cellular retinaldehyde-binding protein (CRALBP) on the oxidation and reduction of retinoids

Retinol dehydrogenase 12 (RDH12) is a novel member of the short-chain dehydrogenase/ reductase superfamily of proteins that was recently linked to Leber's congenital amaurosis 3 (LCA). We report the first biochemical characterization of purified human RDH12 and analysis of its expression in human tissues. RDH12 exhibits similar to 2000-fold lower Km values for NADP+ and NADPH than for NAD(+) and NADH and recognizes both retinoids and lipid peroxidation products (C-9 aldehydes) as substrates. The k(cat) values of RDH12 for retinaldehydes and C-9 aldehydes are similar, but the K-m values are, in general, lower for retinoids. The enzyme exhibits the highest catalytic efficiency for all-trans-retinal (k(cat)/ K-m similar to 900 min(-1) mu M-1), followed by 11-cis-retinal (450 min(-1) mM(-1)) and 9-cis-retinal (100 min(-1) mM(-1)). Analysis of RDH12 activity toward retinoids in the presence of cellular retinol-binding protein (CRBP) type I or cellular retinaldehyde-binding protein (CRALBP) suggests that RDH12 utilizes the unbound forms of all-trans- and 11-cis-retinoids. As a result, the widely expressed CRBPI, which binds all-transretinol with much higher affinity than all-trans-retinaldehyde, restricts the oxidation of all-trans-retinol by RDH12, but has little effect on the reduction of all-trans-retinaldehyde, and CRALBP inhibits the reduction of 11-cis-retinal stronger than the oxidation of 11-cis-retinol, in accord with its higher affinity for 11-cis-retinal. Together, the tissue distribution of RDH12 and its catalytic properties suggest that, in most tissues, RDH12 primarily contributes to the reduction of all-trans-retinaldehyde; however, at saturating concentrations of peroxidic aldehydes in the cells undergoing oxidative stress, for example, photoreceptors, RDH12 might also play a role in detoxification of lipid peroxidation products.