THE VISUAL PIGMENTS 



ineffective also. However, both the all- trans and neo-a isomers could 

 be made as effective as the liver oil concentrate by exposing them to 

 light in the presence of iodine (see Fig. 5.3). 



These experiments demonstrated that the differences in behaviour 

 of the various vitamin A^ preparations resulted from geometrical 



1 I 1 1 1 1 1 1 ] 



Opsin*Alcohol dehydrogenase *D?N * Vitamin A (30/ug) 



• crystalline aU-'iroAS 

 o fiver coneentraf e 



0.4 



O 1 2 3 400 500 600 



Time in dark — hours Wave len^th-^m^ 



Fig. 5.2. Synthesis of rhodopsin (visual purple) from a.\\-trans vitamin Aj 

 and from the vitamin A of fish liver oil. Cattle opsin, horse liver alcohol 

 dehydrogenase and cozymase (DPN) were incubated in darkness with 

 the vitamins A. Curves on the left show the rising extinction at 500 m/i 

 as the synthesis proceeded, Hydroxylamine (NH2OH) was then added 

 to destroy spurious retinene complexes. The synthesized pigments were 

 then bleached by light (A to B, C to D). Difference spectra for the 

 bleachings are shown on the right. 

 (Hubbard and Wald, 1952) 



isomerism and indicated that the synthesis of rhodopsin requires a 

 specific form of vitamin A^ and one which is neither the dXL-trans nor 

 the neo-a isomer. 



The site of isomer specificity. In the synthesis of rhodopsin from 

 vitamin A^, two proteins are involved. The first of these is the enzyme 

 protein, alcohol dehydrogenase which catalyses the oxidation of 

 vitamin A^ to retinene^ ; the second is opsin, the protein of rhodopsin 

 with which retinene combines. Either or both of these proteins could 

 require a specific isomer ; the first, of vitamin A^ and the second, of 



136 



