THE PHOTORECEPTOR PROCESS IN VISION 



679 



500 



700 



FIG. 10. Successive syntheses of iodopsin and rhodopsin 

 in solution. An extract of chicken retinas was wholly bleached 

 with an orange nonisomerizing light to a mixture of all-/ra«j 

 retinene and rod and cone opsins. To this mixture just enough 

 neo-A retinene was added to regenerate iodopsin alone. This 

 amount had been determined by preliminary trial. Iodopsin 

 forms so much more rapidly than rhodopsin that its synthesis 

 is complete when that of rhodopsin has scarcely begun (cf. 

 fig. 2i). The absorption spectrum of the product, formed within 

 a few minutes in the dark, is shown with ^olid circles. Then a 

 small excess of neo-b retinene was added, and the mixture was 

 reincubated in the dark for 30 min. This yielded rhodopsin 

 (_open circles). [From VVald el al. (72).] 



phyropsin. Cone opsin combines with neo-^ retinenci 

 to yield iodopsin. Clearly a fourth combination is 

 possible: cone opsin with neo-ft retinenej. 



This synthesis was recently performed in our labora- 

 tory. It yielded a blue photosensitive pigment called 

 cyanopsin which absorbs maximally in the orange- 

 red, at about 620 m/z (7i)- Always heretofore knowl- 

 edge of a visual pigment had developed in the 

 sequence: recognition, extraction, analysis, synthesis. 

 With cyanopsin this history was reversed. A pigment 

 was synthesized in solution which had never been 

 identified in a retina. Had it a place in vision? 



Where would one look? Obviously in retinas which 

 provide its ingredients: cones, hence photopsin; and 

 vitamin Ao. One might therefore look for cyanopsin 

 in a fresh-water fish possessing cones, or in the all- 

 cone retina of such a turtle as Pseudemys, which had 

 been shown to contain \itamin A2 CvO- 



Some years ago Granit measured electrophysiologi- 

 cally the spectral sensitivity of cone vision in a fresh- 

 water fish, the tench, and in the European tortoise, 

 Testudo graeca (15, 16). His measurements are shown 

 as the points in figure 12; the line is the main ab- 

 sorption band of cyanopsin. There is little doubt 



that cyanopsin is the pigment of cone vision in these 

 animals. 



Recapitulation 



This phase of the chemistry of visual excitation 

 ends on a very simple note. The visual systems which 

 have been studied involve the interaction of four 

 substances: a rod or cone opsin; the enzyme, alcohol 

 dehydrogenase; the coenzyme, cozyniase; and neo-6 

 Qii-cis) vitamin Ai or Aj. They can be summarized: 



light 



DPN+ I -\- rod opsin rhodopsin 



vitamin A, . retinene, , 'ight 



DNP-H [-f cone opsin . iodopsin 



(alcohol dehydrogenase) 



light 



DPN"*" [ +rod opsin . porphyropsin 



vitamin A2 retinene2 j light 



^ cyanopsin 



DPN-H 



[-\- cone opsm - 



In addition there are the four iso-pigments, the 

 carotenoid chromophores of which are stereoiso- 

 meric with those of the visual pigments. Since none 

 of the iso-pigments has yet been found in a retina, 

 they must for the present be regarded as artifacts. 

 How does the retina avoid forming them? Prelimi- 

 nary measurements indicate that traces of iso-a 

 vitamin A are present in liver oils, while in cattle 

 blood the iso-a isomer accounts for about 6 per cent 

 of the total vitamin A. No iso-a vitamin A has been 

 detected in the retina and pigment layers of the eye, 

 whereas the neo-b isomer is found only in the eye 

 (Wald, G., P. K. Brown & P. S, Brown, unpublished 

 observations). It is therefore likely that the eye actively 

 forms neo-6 vitamin A — presumably from the all- 

 trans isomer — and actively excludes iso-a vitamin A. 



Role of Opsin ill I'isuai Exeilation 



To this point the visual pigments have been dis- 

 cussed mainly from the point of view of their carot- 

 enoid components. Their properties, however, depend 

 greatly also upon the opsins. Though their color and 

 sensitivity to light are mediated principally through 

 the carotenoid prosthetic groups, almost everything 

 else derives from their character as proteins. Light 

 liberates retinene. Yet, like other carotenoids, 

 retinene is a bland, relatively inert substance, hardly 

 capable of initiating a nervous excitation. Further- 

 more, at physiological temperatures and pH it is 

 released relatively slowly as the last step in a chain 



