An enzymatic mechanism for generating the precursor of endogenous 13-cis retinoic acid in the brain

13-cis Retinoic acid (13cRA), a stereoisomeric form of retinoic acid, is naturally generated in the body and is also used clinically to treat acute promyelocytic leukaemia, some skin diseases, and cancer. Furthermore, it has been suggested that 13cRA modulates brain neurochemical systems, as increased 13cRA levels are correlated with depression and increased suicidal tendencies. However, the mechanism for the generation of endogenous 13cRA is not well understood. This study identified and characterized a novel enzyme in zebrafish brain, 13-cis isomerohydrolase (13cIMH), that exclusively generated 13-cis retinol, which can be oxidized to 13cRA. 13cIMH shares 74% amino acid sequence identity with human RPE65, an 11-cis isomerohydrolase in the visual cycle, and retains the key residues essential for RPE65’s isomerohydrolase activity. Similar to RPE65, 13cIMH is a membrane-associated protein, requires all-trans retinyl ester as its intrinsic substrate, and its enzymatic activity is dependent on iron. The purified 13cIMH converted atRE exclusively to 13-cis retinol with Km = 2.6 μM and kcat = 4.4 × 10−4s−1. RT-PCR, Western blot analysis and immunohistochemistry detected 13cIMH expression in the brain. These results suggest that 13cIMH may play a key role in the generation of 13cRA and in the modulation of neuronal functions in the brain.