Photoreception 



411 



Wald^^*' has revived the name rhodopsin, which will be used because it 

 serves to differentiate this pigment from other photolabile pigments asso- 

 ciated with rod vision in fresh-water fishes and tadpoles (see page 412). 



Rhodopsin is a conjugated protein. It possesses a characteristic color and 

 other properties by virtue of its carotenoid prosthetic group. Extraction (in 

 the dark) of dark-adapted retinae, rich in rhodopsin, produces a low yield 

 of carotenoid. Rhodopsin exposed to light bleaches rapidly and gives rise to 

 orange or yellow photoproducts. Extraction of retinae shortly after subjec- 

 tion to high illumination yields a stable yellow carotenoid identified as retin- 

 enci, having a characteristic absorption spectrum. Extraction of retinae a 

 long time after onset of illumination results in a high yield of a carotenoid, 

 identified as vitamin Aj, which also has a characteristic absorption spectrum. 

 These results indicate that retinenei and vitamin Ai enter into the photo- 

 reception process. Broda and Goodeve^** demonstrated that light incident on 

 a solution of rhodopsin in 75 per cent glycerin and at — 73° C. results in the 

 appearance of an orange decomposition product. At higher temperatures this 

 orange product changes rapidly into the familiar visual yellow. ^^' ^* This 

 material has been called transient orange and is believed to be the immediate 

 result of the bleaching of rhodopsin. At normal temperatures this orange 

 product has only a fleeting existence, which accounts for the infrequency 

 of its discovery. The incorporation of transient orange into the photochemical 

 cycle gives the scheme of Figure 119. 



Rhodopsin, extracted in aqueous solution, has an absorption spectrum with 

 a maximum of 500 ^. This absorption spectrum agrees well with the 



VISUAL PURPLE 



(Circulatory 

 system) - 



VISUAL WHITE - 



- TRANSIENT ORANGE 

 2 



VISUAL YELLOW 



Fig. 119. The photochemical cycle including transient orange and illustrating the 

 relationships of the various reactions. From Jahn."* 



spectral sensitivity curve of human rod vision (Fig. 118) and with the 

 spectral sensitivity curves of other animals. 



A photosensitive pigment associated with cone vision has been success- 

 fully extracted from chicken retinae by Wald.'^'-*- ^^- This pigment is sensi- 

 tive to deep red light, and its absorption characteristics are similar to the spec- 

 tral sensitivity of cone vision in chickens.^^" Since this pigment is violet in 

 color, Wald suggested the name iodopsin. Although the chemical nature of 

 iodopsin is yet unknown, the similar spectral properties of rhodopsin and 

 of iodopsin imply a chemical similarity. 



Despite incompleteness of the data, the indications are that carotenoids 

 are perhaps universally distributed among light-sensitive organisms, and 

 that they are exclusively found in the photoreceptor or photosensitive areas 



