THE PHOTORECEPTUR PROCESS IN VISION 



673 



CH, 



CH, 



B.C. 



H,C 



CH, CH;, 



H H j H H H I H H 

 -C=C— c:=c— C=C— C=C— c=o 



C— CH3 



C 

 H. 



Retincne 



The retinene formed Ijv the bleaching of rhodopsin 

 is reduced to vitamin A by the enzyme, alcohol 

 dehydrogenase, working together with the coenzyme, 

 DPN. This process is readily carried out in free 

 solution (fig. 2). It involves only the transfer of 

 hydrogen from reduced DPN to the aldehyde group 

 of retinene, reducing it to the alcohol group of 

 vitamin A (4, 64, 75): 



CsHjjCHO + DPN— H + H+ 

 retinene 



alcohol dehydrogenase 



C,,H„CH,OH + DPN+ 

 vitamin A 



0.4 - 



0.3 - 



C 



o 



^0.2 



Uj 



07 - 



- 



1 1 1 1 



/ taminAj-* — ref i nene j 



Frog 



apoemyme 

 + DPN-H 



300 



400 

 Wavelength - mjj 



FIG. 2. The reduction of retinene to vitamin .\. Retinene 

 was mixed in digitonin solution with the enzyme, alcohol 

 dehydrogenase, extracted from frog retinas, and with reduced 

 cozymase (DPN-H). A control mixture was also prepared 

 which differed only in that the enzyme had been kept at ioo°C 

 for 0.5 min. Both mixtures were incubated, then extracted 

 with hexane. The absorption spectra of the hexane extracts 

 are shown. The control mixture (^solid circles') contains unaltered 

 retinene; the mixture containing active enzyme (_open circles) 

 shows complete conversion to vitamin .\. [From VVald (64).] 



DPN introduces a second \itamin into the chem- 

 istry of vision. Its active principle is nicotinamide, 

 the antipellagra factor of the \itamin B complex. 

 In the retina it is in the curious position of helping 

 to regenerate vitamin A. 



This completes the degradative processes in vision. 

 Rhodopsin having been bleached by light to a mix- 

 ture of retinene and opsin, the retinene is reduced to 

 vitamin A. The problem now is to go back. Kiihne 

 already recognized this to be a dual problem (38). 

 He described a resynthesis of rhodopsiti from yellow 

 precursors (anagenesis) which was relatively rapid 

 and occurred not only in the intact eye but in the 

 isolated retina and even slightly in solution. In 

 addition there occurred a relatively slow synthesis 

 of rhodopsin from colorless precursors (neogenesis) 

 which Kiihne could ob.serve only in the intact eye 

 and which seemed to require the cooperation of the 

 pigment epithelium. These two processes can now 

 be identified with the synthesis of rhodopsin from 

 retinene and opsin, and from vitamin A and opsin. 



The synthesis of rhodopsin from retinene and opsin 

 is a spontaneous reaction. It requires neither an 

 enzyme nor, as do most syntheses, an external source 

 of energy. One has only to bring a mixture of these 

 two substances into the dark to form rhodopsin 

 (67). Like all spontaneous reactions, it is an energy- 

 yielding process, which can therefore do work. The 

 work it does in vision is to force the oxidation of 

 vitamin A. The equilibrium between vitamin A 

 and retinene lies far over toward the side of re- 

 duction — toward vitamin A. In the dark, however, 

 opsin "traps' retinene, removing it to form rhodopsin, 

 so displacing the equilibrium in the oxidative di- 

 rection. The basic mechanism of rhodopsin synthesis, 

 therefore, is the energy-demanding oxidation of 

 vitamin A to retinene, coupled with the energy- 

 yielding condensation of retinene and opsin to form 

 rhodopsin (33, 76). 



One important consequence of this arrangement 

 is that it is self-limiting. Vitamin A is oxidized to 

 retinene only as long as opsin is available to trap 

 the latter. Retinene therefore never accumulates. 

 When all the opsin in the visual receptors has been 

 converted to \isual pigments, the oxidation of vitamin 

 A automatically ceases. 



The rhodopsin system in more detail therefore 

 has the form shown in figure 3. Rhodopsin is con- 

 verted by light to the orange-red intermediate, 

 lumi-rhodopsin. .\t temperatures above — 20°C 

 this goes on to form meta-rhodopsin; and with access 



