78 



VITAMINS A AND CAROTENES 



isomer of a carotenoid can be isomerized to a mixture of all possible isomers 

 by exposure to light in the presence of a trace of iodine. When this is done 

 to crystalline all-^rans vitamin A and neovitamin Aa, they become as 

 effective precursors of rhodopsin as fish liver oil concentrate (Fig. 22). 



o.ao.. 



0.15.. 



2 0.10.. 



i, 0.05. 



o ._ 



-0.05. 



I I 1 1 \ 



Opsin ^Alcohoi dehydropenase^DPN-t- 14 r 



vitamin A: 

 e all-trans 

 <s> neo- a 

 e Lsom all -trans 

 • isom. neo- a 

 Oliver concentrate 



400 450 500 550 



600 



Fig. 22. The S3'nthesis of rhodopsin from vitamin A of live different origins: crys- 

 talline sW-trans, crystalline neovitamin Aa, isomerized aW-trans, isomerized neo- 

 vitamin-a and fish liver oil concentrate. The figure shows the difference spectra of 

 rhodopsin obtained by incubating equal amounts of each of these preparations with 

 opsin, alcohol dehydrogenase, and cozymase. Only traces of rhodopsin were synthe- 

 sized from all-/rans or neovitamin Aa; l)ut after these substances had been isomerized 

 with light in the presence of iodine, thej^ were as effective as liver oil vitamin A in 

 forming rhodopsin. (From R. Hubbard and G. Wald.^'") 



It is apparent, thei'eforc, that we are dealing here with a remarkable in- 

 stance of stereochemical specificity. What it involves ])riniarily is the shape 

 of the vitamin A molecule. k\\4rans vitamin A, as shown in Fig. 21, has a 

 relatively straight side chain. All the other isomers are bent b}^ their cis 

 linkages in various ways. Such differences in shape may be unimportant in 



"" R. Hubbard and G. Wald, ./. Gen. Physiol. 36, 269 (1952-1953). 



