VITAMINS 



353 



vitamin A^, die former should be readily converted to the latter by 

 breaking the middle double bond and forming primary alcohol groups. 

 The actual biological yield is still in doubt, but the /3-carotene ingested 

 is converted at least in part to vitamin Aj. 



Obviously ^S-carotene might exist in a number of isomeric forms, 

 and some of these have been studied. Moreover, similar compounds 

 not isomeric with /3-carotene also serve as pro-vitamin A substances. 

 Some of these correspond to vitamin A through half the molecule 

 while the other half differs. At least eight such biologically active 

 carotenoids supplement the /3-carotene to some extent. Most are of 

 quite limited occurrence. 



Function 



A severe deficiency of vitamin A leads to a wide variety of symptoms. 

 Hence this factor must have general metabolic effects, but none have 

 been discovered in spite of a great deal of effort. The overall features 

 of one specialized role, that involving vision, have been worked out. 



Eyes of higher animals contain light-sensitive elements in the 

 retinas especially adapted for seeing at low levels of illumination. 

 These rods in turn contain a red photosensitive pigment called rho- 

 dopsin, a complex protein split by light. The known transformations 

 are summarized in Figure 14-3. The intermediate designated retincne 

 differs from vitamin A only in oxidation of the primary alcohol group 

 to an aldehyde, and opsin (or scotopsin) is the name applied to the 

 protein moiety. 



Interconversion of the forms of vitamin A probably occurs in the 

 livers of mammals and depends on the blood for transfer to and from 

 the eye. Oxidation and reduction require an enzyme which is similar 

 or identical to alcohol dehydrogenase. A direct interconversion of the 

 two retinenes may occur. The combination of opsin and c25-retinene 



rhodopsin 



light 



opsin + as-retinene^ 



DPNH- 



cfs-vitamin A 



retinene 

 reductase 



irons - retinene ^-h opsin 

 --^DPNH 



retinene 

 reductase 



DPN + 

 trans -vitamin A^ 



FIGURE 14-3. The visual cycle. 



