12 FORMATION IN PLANTS II 



Steenbock and his collaborators were the first to suggest, about 30 years 

 ago, that a connection exists between the yellow plant pigments (carotene) 

 and vitamin A.^^ During the following years, the question of the growth- 

 promoting properties of carotene was investigated by different workers. The 

 results of these investigations were very contradictory and a definite solution 

 of this problem was only achieved in 1929 by B. v. Euler, H. v. Euler and 

 Karrer^^. The results of their investigations definitely proved that carotene 

 possesses qualitatively the same biological effect (resumption of arrested 

 growth) as vitamin A, and was therefore probably related to the latter. This 

 result at first appeared difficult to understand as carotene is a deeply coloured 

 crystalline substance, quite different from the pale yellow vitamin A*. Later 

 experiments by Karrer and collaborators elucidated the constitution of 

 vitamin A^'^ and of jS-carotene^^ and the relationship between the two compounds. 

 The chemical structure of j3-carotene is such that by the uptake of water it 

 can be converted into two molecules of vitamin A, and the growth-promoting 

 properties of ^-carotene thus find their explanation. 



CH3 CHs CH, CH, 



C CH3 CH3 CH3 CH3 c 



/\ I I I I /\ 



CH2 C-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CH=CH-C CH,j 



CHj C-CHg I HgC-C CHj 



\/ 2 H.0 \/ 



CHo CH, 



C CHo CHo 



/\ I I 



CHa C-CH=CH-C=CHCH=CH-C=CH-CHoOH 



•I II 



CHo C" CHo 



Vitamin A 

 CHa 



In agreement with this view, experiments reported by T. Moore showed that 

 rats kept on a vitamin A-free diet, and whose livers contained practically no 

 vitamin A, again accumulated vitamin A in the liver after being given 

 /S-carotene^^. y-Carotene, which contains only cme j8-ionone ring in the molecule, 

 and can therefore only form one molecule of vitamin A by the addition of 

 water, exhibits much weaker growth-promoting properties than jS-carotene^". 



Very little is yet known about the mechanism by which ^-carotene and 

 other pro- vitamins A are converted into the vitamin. It has been assumed that 

 this process depends on a ferment, carotenase. It is very probable that the 

 reaction occurs in the liver^i or in the intestine^^. In animals deficient of 



Cristalline vitamin A was not yet known at the time. 

 References p. ly-ig. 



