ISOMERISM AND THE VISUAL PIGMENTS 



retinenci. However, hubbard and wald found that aXl-trans 

 vitamin A^ and liver vitamin A were both readily oxidized to retinene 

 by alcohol dehydrogenase and cozymase, in fact the 3.\\-trans isomer 

 (inactive in the synthesis of rhodopsin) was the more reactive. 



o.^o.. 



15. 



o 0.10 



i^ 0.05. 



0.05.. 



—I 1 r 1 1 



opsin* Alcohol dehydrogenase t-D PN* I^/jlc^. 



vitamin A: 

 o all -trans 



e tsom. all-trans 



o isom neo-a 



o Liver concentrate 



400 



450 



500 



550 



COO 



Wave Length -'m^ 



Fig. 5.3. Synthesis of rhodopsin from vitamin Ai of five different types. 



Curves show the difference spectra obtained on bleaching the pigments 



synthesized from equal amounts of each form of vitamin Aj. 



{Hubbard and Wald, 1952) 



This experiment strongly suggests that it is in the combination of 

 opsin with retinene that a particular stereoisomer is essential. 



ISOMERS OF RETINENEi 



Opsin and retinenej will react directly to form rhodopsin without 

 the intervention of the coenzyme and enzyme necessary when vita- 

 min Ai is the starting material. For this reason, and also because it 

 is an easier matter to prepare and separate pure specimens of the 

 stereoisomers of retinenci, than of vitamin Aj, the problem of deter- 

 mining the stereo-requirements of opsin was undertaken by hubbard, 

 GREGERMAN and WALD using retinenci instead of vitamin A^. 



137 



