204 VITAMINS 



Vitamin A plays a vital role in connection with vision. The rods in 

 the retina of the eye contain a rose-colored pigment, rhodopsin, which 

 is a conjugated chromoprotein. When light falls on the retina, rhodopsin 

 is broken down into a simple protein, opsin, plus trans-retinene, a yellow 

 substance now known to be vitamin A aldehyde. Associated with this 

 chemical change is the production of a stimulus which, being imparted 

 to the optic nerve, results in vision. These changes are very rapid and 

 are probably completed within a few hundredths of a second. 



Subsequently, a slower process, not dependent on light, brings about 

 the regeneration of rhodopsin so that an adequate supply for normal 

 vision is continuously present in the retina. Rhodopsin is formed from 

 opsin plus cts-retinene, a geometric isomer of the ^rans-retinene released 

 by the action of light on rhodopsin.^ The m-retinene needed may be 

 obtained by isomcrization of the trans form, or from cis-vitamin A through 

 the action of alcohol dehydrogenase and DPN (p. 275) : 



alcohol 



C19H27CH2OH + DPN <^ ^ , '• CnH^vCHO + DPN-Hj 



dehydrogenase 



CIS-Vitamin A cis-Retinene 



Since the breakdown and resynthesis of rhodopsin are probably not per- 

 fectly efficient processes, some retinene derived from vitamin A no doubt 

 must be continually supplied. 



The retinas of fresh-water fish do not contain rhodopsin but a closely 

 similar, light-sensitive pigment, porphyropsin. This substance fulfills the 

 same functions for the vision of these fish as rhodopsin does for higher 

 animals and man. The outstanding difference lies in the retinene and 

 vitamin A associated with it, which are different from those derived from 

 rhodopsin. To distinguish the two varieties of the vitamin, that from 

 mammals is designated Ai and the other, A2. The corresponding retinenes 

 are similarly designated retmenei and retinene2. Fresh-water fish have 

 vitamin A2, not only in the retina, but also in the liver and other organs. 



Since vitamin A is needed for rhodopsin formation, it would be expected 

 that persons deficient in this vitamin would have subnormal amounts of 

 rhodopsin in the "retina. In any event, it has been observed that such 

 individuals cannot see as well in dim light or adapt themselves to changes 

 in light intensity as readily as those whose supplies of vitamin A are 

 ample for immediate resynthesis of the hght-sensitive pigment. By means 

 of the biophotometer it is possible to measure accurately one's ability 

 to adapt to darkness after the eyes are exposed to a bright light for 

 a few minutes. Adaptation is determined by the amount of light neces- 

 sary for the individual to recognize the number of openings in a screen 



^ For a brief explanation of cis- and irans-isomers see p. 79. Since vitamin A and 

 retinene each contain five double bonds, theoretically (2)5 such isomers of each can 

 exist. It is not known which particular cis-isomer is involved in rhodopsin formation. 



