IMPORTANCE IN VISION 569 



crimination. The modulator action spectra fall into groups located in three 

 spectral areas. This type of mechanism is particularly efficient for color 

 vision. It is believed that, under suitable conditions, vitamin A or reti- 

 nene might give rise to "halochromic" molecules which resemble the modula- 

 tors. Ball and Morton^24 reviewed Granit's theory. Wald^^^ gave an ex- 

 cellent discussion of the biochemical systems involv^ed in vision. In an 

 earlier review, Wald-^^ discussed the biochemistry of visual systems and 

 their distribution among animals. 



(1) Biochemical Systems Involved in Vision 

 in Higher Animals 



a. Retinal Pigments. Two kinds of organs occur in the retinae of most 

 vertebrates, namely the rods (dim-light vision) and the cones (color and 

 bright-light vision). In the case of the rods, the light-sensitive pigments 

 rhodopsin and porphyropsin are present, or at least one of them, while in 

 the cones the single active pigment is called iodopsin. All these substances 

 belong to the category of carotenoid proteins, i.e., proteins to which is 

 attached a prosthetic group related to vitamin A. 



(a) Rhodopsin. Rhodopsin is a rose-colored carotenoid protein Avhich, 

 in aqueous solution, has a single broad absorption band with a maximum 

 at about 500 m/x. When exposed to light, it bleaches to an orange inter- 

 mediate ("transient orange"), then to a j^ellow product ("visual yellow" 

 or "indicator yellow") and finally to a colorless product known as "visual 

 white." Rhodopsin is found chiefly in the rods of marine fishes and of 

 land vertebrates. Wald^^^-^^^ postulated that rhodopsin undergoes a series 

 of reactions in light and dark. 



Under ordinary conditions, in aqueous solutions, retinene and opsin are 

 the final products when light acts upon rhodopsin. However, a reductase 

 is present in the retina w^hich can convert retinene into vitamin A alcohol. 



(6) Porphyropsin. Porphyropsin is a purple carotenoid protein, which 

 has been reported in the retinae of freshwater fishes and of some anadro- 

 mous fishes, of lamprey eels, and of some amphibians. Wald"^ identified 

 it in white perch {M or one americana), yellow perch {Perca flavescens) , and 

 estern pickerel {Esox reticulatus) . According to Wald,^-^ porphyropsin 

 undergoes reactions which are identical with those in Avhich rhodopsin 

 participates; in the case of porphyropsin, retinene2 is the first product, and 

 this is converted into vitamin A2. The protein portion of the carotenoid 



"^ S. Ball and R. A. Morton, Biochem. J., 45, 298-304 (1949). 

 9« G. Wald, J. Gen. Physiol, 21, 795-832 (1937-1938). 

 9^« G. Wald, J. Gen. Physiol., 22, 775-794 (1938-1939). 



