Elucidation of the Visual Cycle by Study of Two Retinoid-Binding Proteins

  • John C. Saari
  • Ann H. Bunt-Milam
  • Vinod Gaur
Part of the Cell and Developmental Biology of the Eye book series (EYE)


The visual cycle (Wald, 1968) includes all metabolic reactions of vitamin A known to occur in retina, with the exception of those that generate retinolc acid, a retinoid with no known role in the visual process. A modern version of the visual cycle, shown in Figure 1, is very similar to the cycle outlined by Wald in 1968 except for a few details. Many of the questions of interest at that time remain with us today. The nature of the generation of the 11-cis-configuration in the dark remains unclear. Is the process enzymatic? Is it the aldehyde, alcohol or retinyl ester that is isomerized? In which cell type(s) of the retina does the isomerization take place? How many dehydrogenases function in the visual cycle? How are these reactions controlled? How do retinoids traverse the extracellular space that separates the retinal pigment epithelium (RPE) and neural retina? The answers to these and other questions are not yet known, partly because of the difficulties inherent in dealing with water insoluble substrates and membrane associated enzymes. Until recently, analytical methods available for retinoid separations, especially separation of geometrical isomers, were tedious and not well suited to the task. Currently there is renewed interest in the reactions of the visual cycle, stimulated in part by development of rapid and efficient methods for the separation of retinoids by HPLC.


Retinal Pigment Epithelium Retinal Pigment Epithelium Cell Outer Segment Visual Pigment Neural Retina 
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© Springer-Verlag New York Inc. 1987

Authors and Affiliations

  • John C. Saari
  • Ann H. Bunt-Milam
  • Vinod Gaur

There are no affiliations available

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