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because it acts as a 1 catalyst maintaining an equilibrium ratio between 

 the aldehyde retinene and the corresponding alcohol, vitamin A. This 

 equilibrium is in the direction of much greater concentration of the 

 alcohol. If, however, the aldehyde concentration is sufficiently low, 

 the enzyme catalyzes the aldehyde production from the alcohol. To 

 function, alcohol dehydrogenase needs a coenzyme, diphosphopyridine 

 nucleotide (DPN + ) to accept the hydrogen atoms removed from the 

 aldehyde (see Chapter 18). In the living retina, as in all other tissues, 

 there is an abundant supply of DPN + and DPNH. 



Figure 3. The 11-12 mono-cis isomer of vitamin A x . The plane projection is 

 distorted so that it does not show the steric hindrance. This is indicated by the 

 broken line between the methyl group at 13 and the hydiogen attached to 

 position 10. 



The vitamin A produced by the action of alcohol dehydrogenase and 

 DPNH can pass through the rod membrane and into the blood stream. 

 The vitamin A in the retina is in equilibrium with that in the blood 

 under steady-state light conditions. The vitamin A in the blood stream 

 is, in turn, maintained at a more or less constant concentration by the 

 liver, which stores any excess of vitamin A. 



The over-all process is then a complex cycle, which is shown in 

 diagrammatic form in Figure 4. In the dark, the cycle is stopped by 

 all the opsin being bound in the form of rhodopsin. In the light, an 

 equilibrium must be established with a steady-state concentration of 

 rhodopsin. The concentration may be very close to zero, but a small 

 part of the opsin should always exist in the form of rhodopsin. 



From the point of view of the alcohol dehydrogenase, one may regard 

 the opsin as a trapping reagent which effectively shifts the equilibrium 

 so that the alcohol (vitamin A x ) is converted to the aldehyde (retinene-L). 

 The reaction of retinene! with opsin is exothermic and goes spontaneously. 

 From the point of view of the retinene, the opsin acts as an enzyme, con- 

 verting the retinene with the help of the photon hv from the less probable 

 cis form to the more probable all-trans form. One may describe the 

 protein opsin as a photo-isomerase. 



An additional feature of this reaction is that it tends to stabilize the 

 reactants. Opsin and retinene! are both relatively unstable. For 



