CAROTENOIDS 



Sherman and Smith, ^ it was Moore ^ who unequivocally demonstrated 

 the conversion of ^-carotene into vitamin A in rats which was stored in 

 the liver. It was soon realized that not all carotenoids were vitamin A 

 precursors and when, as a result of the classic work of Karrer and 

 Kuhn and their collaborators, the structures of many carotenoids and 

 of vitamin A were established, the relation between structure and 

 vitamin A activity became clear. 



P-Carotene is twice as active as is either y- or a-carotene, '' ' and 

 can be considered, for the moment at least, to be converted into 

 vitamin A by hydrolytic fission, as shown on the previous page. 



a-and y-carotenes being about one half as active as ^-carotene as 

 vitamin precursors <*- ^ and lycopene being inactive, it is obvious that 

 a [3-ionone residue is a first essential for activity. The inactivity of 

 zeaxanthin and of lutein (xanthophyll) indicates that the [3-ionone 

 residue has to be unsubstituted. Unilateral oxidative degradation of 

 the p-carotene molecule with the production of apocarotenes ^ ^' ^ ^ 

 does not destroy activity as long as the vitamin A side chain remains 

 intact. For example, p-apo-8'-carotenal and p-apo-12'-carotenal 

 are still as active as a-carotene. 



Me He Me 

 ^ / Me Me Me 



^vV' - -- 



-/-Me 

 Me 



p-apo-8'-caroten|l p-apo- 1 2'-carotenal 



The apo-a-carotenals are inactive because only an a-ionone residue 

 remains, e.g., apo-8-carotenal ^ ^ : — 



a-apo-8-carotenal 



Oxidation which opens the (i-ionone ring without reducing the num- 

 ber of carbon atoms destroys activity, for example, p-carotenone ^ ^ 

 is inactive but [3-hydroxycarotene ^ * and ^-semicarotenone ^ ^ have 

 activities of the same order as a-carotene because they still contain 

 one unsubstituted ^-ionone residue, Deliydrogenation of p-carotene to 

 isocarotene 1 ^ (dehydro-[3-carotene) removes its activity because there 

 also occurs a rearrangement of double bonds with the production of 



270 



