The molecular biology of retinoid metabolism : Identification of an 11-cis retinol dehydrogenase in the retinal pigment epithelium

Vitamin A derivatives (retinoids) are essential molecules in the course of embryonic development, postnatal growth and adult physiology. The physiological functions of retinoids are thought to be mediated mainly by retinoic acid which regulates the expression of various genes, and 11-cis retinaldehyde, which is the common chromophore of visual pigments. The present study primarily focuses on the retinal pigment epithelial (RPE) cells of the eye, with the aim to gain better understanding of the molecular mechanisms responsible for cellular uptake and subsequent intracellular metabolic steps generating the active retinoids. The RPE provides a useful model system to study these events, since it accumulates large amounts of retinoids and is the location of 11-cis retinaldehyde biosynthesis. A 32 kDa membrane-bound protein, abundantly expressed in bovine RPE, was identified as an enzyme specitically oxidizing 11-cis retinol to 11-cis retinaldehyde, and was shown to be a member of the short-chain alcohol dehydrogenase/reductase superfamily (SDR). This protein has been termed 11-cis retinol dehydrogenase ( 11-cis RDH) and has been postulated to be responsible for the (re)generation of the chromophore. This protein is, to date, the first cloned enzyme which might be involved in the visual cycle. 11-cis RDH interacts directly or indirectly with the RPE-specitic p63, which has been implicated in the cellular uptake of all-trans retinol. Cell biological and biochemical characterization of 11-cis RDH revealed that the catalytic domain of the enzyme is localized to lumenal aspects of various intracellular membranes. These data suggest the existence of a metabolic pathway, where generation of 11-cis retinaldehyde is compartmentalized and coupled to the cellular uptake of all-trans retinol. To explore a potential link between mutation(s) in the 11-cis RDH gene and hereditary eye diseases, a human 11-cis RDH gene was isolated and its chromosomal localization and genomic structure determined. The gene is localized on chromosome 12ql3-ql4, but no hereditary eye disease has yet been mapped to this locus so far. The primary structure of bovine 11-cis RDH provided the basis of screening for conserved, homologous enzymes, expressed in non-ocular tissues. The screening procedure resulted in the molecular cloning of a related enzyme, which might act as a 9-cis retinol dehydrogenase (9-cis RDH) in various adult and embryonic mouse tissues. The existence of a 9-cis RDH in several cell types suggests a novel pathway in the biosynthesis of 9-cis retinoic acid from 9-cis retinol via a 9-cis retinaldehyde intermediate, involving two oxidation reactions. The characterization of these novel enzymes denotes important physiological functions to members of the SDR family in the biosynthesis of active retinoids