III. BIOCHEMICAL SYSTEMS 



61 



is called porphyropsiii. It engages in a system of reactions exactly parallel 

 with the rhodopsin cycle, but involving other carotenoids.^ In the porphy- 

 ropsin system, retinene is replaced by retinene2, and vitamin A by vitamin 

 Ao (Fig. 13). The enzymes of both systems are identical, however, and 

 opsin itself appears to be interchangeable in both systems. In virtually 

 every detail of its behavior the porphyropsin system mimics rhodopsin. 



The first light-sensitive pigment of cone vision was discovered in the 

 chicken retina in lOS?."* It has lately been shown to take part in a system 

 of chemical reactions which differs from the rhodopsin system only in that 

 it involves a new protein. The carotenoids of both systems are identical.^ 

 As in the case of the heme proteins, in which the linkage of a single pros- 



10 

 i 08 



^ 06 



X 



w 

 0.4 



0.Z 



300 



400 



600 



500 

 Wavelength - m/^i 



Fig. 13. Components of the rhodopsin and porphyropsin systems. Absorption 

 spectra of crude preparations from retinas of the fresh-water calico bass (solid lines) 

 and the marine scup (broken lines). Rhodopsin and porphyropsin are dissolved in 

 1% aqueous digitonin, the retinenes and vitamins A in chloroform. All maxima have 

 been brought to the same height to facilitate comparison. (From G. Wald.^) 



thetic group, iron protoporphyrin, to different proteins determines whether 

 the end result is hemoglobin, catalase, or peroxidase, so here it is opsin 

 that decides whether one obtains rhodopsin or iodopsin. Actually either rod 

 or cone opsin — as indeed hemoglobin — is not a single protein, but a family 

 of proteins, the individuals of which vary from species to species. The 

 family of rod proteins, since they have to do with so-called scotopic vision, 

 may be called scotopsins; those of cone vision, phot opsins. 



The rhodopsin and porphyropsin systems, therefore, differ only in their 

 carotenoids, the rhodopsin and iodopsin systems only in their proteins. 

 The visual systems are otherwise remarkably homogeneous in structure 

 and content; what is said of one of them very nearly applies to all. The 



3 G. Wald, ./. Gen. Phifsiol. 22, 775 (1938-1939). 



' G. Wald, Xature 140, 545 (1937j. 



5 G. Wald, P. K. Brown, and P. H. Smith, Federation Proc. 11, 304 (1952). 



