126 



VERTEBRATE PHOTORECEPTORS 



phyropsin is peculiarly characteristic of fresh-water fishes 

 alone. The absorption spectra of rhodopsin from the rabbit, 

 bullfrog, four marine fishes, and a typical porphyropsin 

 absorption spectrum from the white perch are given in 

 Figure 83 (Wald, 1938). 



Porphyropsin in aqueous solution possesses also the prop- 

 erties of a protein. Its spectrum consists of a broad band, 

 maximal at 522 m^u. Under illumination it bleaches in a 

 succession of light and Mark' (thermal) reactions to a 



400 



500 



600 



700 



Wavelength - mju 



Fig. 83. Spectra of rhodopsin preparations from the rabbit, bullfrog, and 

 four marine fishes; and of a typical porphyropsin preparation from the fresh- 

 water white perch. (From Wald, 1938. /. Gen. Physiol., v. 21.) 



russet color, due to a carotenoid pigment. This colored 

 product has been termed by Wald, retinene2, which in the 

 retina is transformed further to a new pale yellow carotenoid 

 yielding with antimony chloride a band maximal at 696 m^. 

 This has been designated vitamin A 2 in contrast with the 

 vitamin Ai of the rhodopsin system which gives a band at 

 615-620 m/x in antimony chloride. Porphyropsin was found 

 to participate in a retinal cycle identical in form with that 

 of rhodopsin but in which a 706 m/i chromogen (retinene2) 

 replaces retinene and a 696 m^u — chromogen (vitamin 

 A2) replaces vitamin Ai (Wald, 1937a, 1939a, 19396). 



