Biochemical aspects of the visual process. XXVII. Stereospecificity of ocular retinol dehydrogenases and the visual cycle.
TL;DR: It is unlikely that the 11-cis retinol dehydrogenase from pigment epithelium is directly involved in providing 11- cis retinaldehyde from rhodopsin regeneration, but it may serve to make available 11-Cis retinehyde from Rhodopdsin, digested in phagocytized rod sacs, for the synthesis of visual pigment by the visual cells.
About: This article is published in Biochimica et Biophysica Acta.The article was published on 1970-03-01. It has received 153 citations till now. The article focuses on the topics: Retinol dehydrogenase & Retinaldehyde.
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TL;DR: 'Channelling' of retinoids between their corresponding binding protein and a particular protein target thus seems to be a general theme through which some retinoid-binding proteins exert their effects.
Abstract: Active vitamin A metabolites, known as retinoids, are essential for multiple physiological processes, ranging from vision to embryonic development. These small hydrophobic compounds associate in vivo with soluble proteins that are present in a variety of cells and in particular extracellular compartments, and which bind different types of retinoids with high selectivity and affinity. Traditionally, retinoid-binding proteins were viewed as transport proteins that act by solubilizing and protecting their labile ligands in aqueous spaces. It is becoming increasingly clear, however, that, in addition to this general role, retinoid-binding proteins have diverse and specific functions in regulating the disposition, metabolism and activities of retinoids. Some retinoid-binding proteins appear to act by sequestering their ligands, thereby generating concentration gradients that allow cells to take up retinoids from extracellular pools and metabolic steps to proceed in energetically unfavourable directions. Other retinoid-binding proteins regulate the metabolic fates of their ligands by protecting them from some enzymes while allowing metabolism by others. In these cases, delivery of a bound retinoid from the binding protein to the 'correct' enzyme is likely to be mediated by direct and specific interactions between the two proteins. One retinoid-binding protein was reported to enhance the ability of its ligand to regulate gene transcription by directly delivering this retinoid to the transcription factor that is activated by it. 'Channelling' of retinoids between their corresponding binding protein and a particular protein target thus seems to be a general theme through which some retinoid-binding proteins exert their effects.
394 citations
TL;DR: Three novel enzymatic activities in cone-dominant ground-squirrel and chicken retinas comprise a novel pathway that regenerates opsin photopigments at a rate 20-fold faster than the known visual cycle, and it is suggested that this pathway is responsible for sustained daylight vision in vertebrates.
391 citations
TL;DR: Recent developments in current understanding of the retinoid cycle at the molecular level are summarized, and the relevance of these reactions to phototransduction is examined.
373 citations
Journal Article•
TL;DR: This cell line retains many of the metabolic and morphologic characteristics of RPE cells in vivo although there are some differences, particularly the loss of some enzymatic activities and cytoskeletal polarization.
Abstract: Purpose A spontaneously arising, apparently transformed, cell line has been cloned from a primary culture of human retinal pigment epithelial (RPE) cells and has been subcultured more than 200 times. The similarities of these cells to human RPE cells in vivo have been determined. Methods The structure of the transformed cells has been determined by light and electron microscopy and by immunocytochemistry using antibodies that detect cytoskeletal and other proteins. The ability of the cell line to bind and phagocytose photoreceptor material has also been assessed by fluorescence and electron microscopy. The metabolism of all-trans-retinol has been investigated by incubation of the cells with 3H-all-trans-retinol and analysis of the metabolic products by high-performance liquid chromatography. Results The transformed cells possess an epithelial cobblestone morphology with intercellular junctional complexes containing N-cadherin. The cytoskeleton of these cells comprises cytokeratins that are characteristic of epithelial cells, together with actin, spectrin, and vimentin. The keratins expressed are those typical of RPE cells. The cells also express cellular retinaldehyde binding protein and retinol dehydrogenase activity but do not express retinoid isomerase or lecithin retinol acyl transferase activities. These cells also exhibit phagocytic activity. Conclusions This cell line retains many of the metabolic and morphologic characteristics of RPE cells in vivo although there are some differences, particularly the loss of some enzymatic activities and cytoskeletal polarization. These cells should be useful in further studies of RPE cell metabolism and other functions.
268 citations
TL;DR: These findings illuminate the physical principles of membrane protein association due to chemically nonspecific interactions in fluid lipid bilayers and yield a conceptual framework for understanding how the tightly regulated lipid compositions of cellular membranes influence their protein-mediated functions.
262 citations
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