OCCURRENCE OF THE VITAMINS A 681 



m/i) and l)y the position of the absoi^ption maximum of the antimony tri- 

 chloride reaction prochict at 615-620 m^i. 



A second type of retinal pigment closely related to rhodopsin occurs in 

 the fresh-water fishes. This has been called porphyropsin. Wald*^~** 

 succeeded in preparing a retinene from porphyropsin which differed from 

 the previously described retinene ; it was therefore referred to as retinene2, 

 while the original retinene isolated from illuminated rhodopsin is now known 

 as retinenei. Since the discovery of vitamin A2 in fresh-water fishes, it has 

 become evident that retinene2 bears the same relationship to vitamin A2 

 that retinenei does to vitamin Ai. Like retinenei, retinene2 reverts to the 

 conjugate protein (porphyropsin) in the retina; some vitamin A2 is also 

 formed as a by-product, just as vitamin Ai is a by-product in the formation 

 of rhodopsin. Retineneo has a maximum absorption of 390 nxfj. in petro- 

 leum ether and of 405 m^u in chloroform; these are distinctly higher than 

 those of retinenei. Moreover, the chromogenic product formed when it is 

 reacted with antimony trichloride has an absorption maximum somewhat 

 greater than 705 m^t. Finally, the vitamin A2 formed from retinene2 has 

 the typical absorption maxima of 345-350 m/i in cyclohexane, 355 mix in 

 chloroform, and 693 mpt for the reaction product with antimony trichloride. 



According to Wald,*'' rhodopsin or porphyropsin has never been found in 

 any tissue other than the retina. In the case of invertebrates, large 

 amounts of potential vitamin A may be stored in the retina and very little 

 or none at all in the liver. ^"^ Retinenei and vitamin Ai have been demon- 

 strated in the squid (Loligo peali). In a number of Crustacea, such as the 

 fiddler crab (Carcinus maenas and Uca pugnax) and the lobster {Homarus 

 spp.),*'^ vitamin Ai occurs in the retina, but no trace of retinenei has been 

 found. *^ In a fresh-water crustacean, the crayfish {Camharus virilis), three 

 components appear in the retina: vitamin Ai, astaxanthin, and retinene^ 



Although the retinas of land vertebrates contain vitamin Ai aldehyde ex- 

 clusively (except after feeding high concentrations of vitamin A2), the na- 

 ture of the pigment in the retinas of fishes is dependent upon their normal 

 habitat. Marine fishes have the rhodopsin- vitamin Ai system, while the 

 fresh-water fishes possess the porphyropsin-vitamin A2 system. Where 

 fishes live both in fresh- and in salt-water environs, the chief pigment in the 

 retinas is the type which is present when they are in the envirormient in 

 which they breed. Thus, the anadromous fishes, such as the salmon 

 (Salmo salar), which spawn in fresh water, possess both the rhodopsin and 

 the porphyropsin systems, but chiefly the latter, along with vitamin A2. 

 The retinas of three genera of this group, the brook or fountain trout 



" G. Wald, /. Gen. Physiol, 22, 391-415 (1938-1939). 



8^ G. Wald, Nature, I40, 545-546 (1937). 



8* G. Wald, J. Gen. Physiol., 22, 775-794 (1938-1939). 



86 G. Wald, Vitamins and Hormones, 1, 197-227 (1943). 



87 G. Wald, A7n. J. Physiol., 133, 479-480 (1941). 



