IMPORTANCE IN VISION 575 



cis-carotenoids are fed to rats and to chickens, some a\\-trans forms can be 

 isolated from their tissues. Neo-a vitamin A, studied by Robeson and 

 Baxter,"* and by Hubbard and Wald,^^^ and 5-ci6-retinene, prepared by 

 Graham and co-workers,- '^ yield bioassays comparable with those obtained 

 with the corresponding all-trans compounds. This fact has been cited as 

 evidence of the biologic transformation of the cis and trans forms. ^^^ More- 

 over, when rats were fed neovitamin A, characteristic mixtures of all-^rans 

 and 3-cis retinene were deposited in the liver.'^^'*'^'^^ Although the speed 

 of isomerization of vitamin A in the body is not known, it does occur at a 

 rate sufficiently rapid to allow effective growth. Wald-^* points out that, 

 inasmuch as a temperature as low as 60°C. is known to cause steieoisom- 

 erization in vitro Avithin one to two hours, the temperature of the animal 

 (37°C.) might catalyze stereoisomeric changes at a slower rate, possibly 

 over a period of a day. To explain the role of the retinal tissues in effecting 

 stereoisomerism in vivo, Wald-^^ proposed the hypothesis that enzymes may 

 cause stereoisomerization. 



(3) Species Variations in Biochemical 

 Requirements for Vision 



Wald^^* presented evidence that plants as well as animals employ carote- 

 noids in the primary processes of photoreception. Light, which stimu- 

 lates photokinetic responses in many different plant structures, is absorbed 

 by the carotenes and xanthophylls in their tissues. Astaxanthin, which is 

 ordinarily found only in animal tissues, also appears in the intermediate 

 forms of the unicellular zoophytes (Protista), for example in the green 

 euglena (Euglena viridis), in other species of Euglena, and in the red, fresh- 

 water protozoan (Haematococcus spp.), where it is concentrated in the eye- 

 spots. Although the chemical mechanism of photoreception in inverte- 

 brates is not well understood, vitamin Ai and retinene: occur in the image- 

 forming eyes of the molluscs and arthropods. Vitamin Ai and retinenei 

 continue to play the dominant role among the retinal pigments of the higher 

 species, except that, in the case of freshwater species of fishes, rhodopsin is 

 replaced by porphyropsin, and vitamin Aj and retinenej are replaced by 

 vitamin A2 and retinene2. The salmon and eels which inhabit both fresh 

 water and salt water have both the porphyropsin and the rhodopsin sys- 

 tems concomitantly. Both systems likewise occur in some amphibia. In 

 marine fishes, as well as in land vertebrates, vitamin Ai and retinenei ap- 

 pear alone in the retinal pigments. Astaxanthin is present not only in the 

 eye-spots of the Protista, but also in the eyes (and integument) of most of 

 the crustaceans, and in the cones of certain birds and turtles, where it is 



