The Chemistry of Vitamin A and Vision

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Abstract
Vision is initiated in the retina by the photochemical isomerization of an 11‐cis‐retinal Schiff base to its all‐trans counterpart in the photosensitive protein rhodopsin. All‐trans ‐retinal is released, reduced to form all‐trans ‐retinol (vitamin A), and finally esterified. In order for vision to proceed an 11‐cis ‐retinoid must be regenerated in the eye. Kühne observed in the 1870s that the pigment epithelium must be in place behind the retina in order for rhodopsin to form. And indeed it was in the pigment epithelium that an isomerase system was recently discovered that can produce 11‐cis ‐retinol from added vitamin A. Pauling predicted in the 1930s that certain cis double bonds in carotenoids and retinoids, including 11‐cis ‐retinoids, should be quite unstable with respect to their all‐trans congeners. This prediction is borne out by experiment, but leaves open the crucial question as to where the energy comes from to drive the isomerization process in the eye. This question was answered by the demonstration that the energy is found in the membrane phospholipids. The fact that membranes may serve as an energy source reveals a previously unsuspected biological function of membranes. The chemistry of the novel energy‐linked isomerization process is explored in this review.