CAROTENOIDS 



retinene (vitamin A aldehyde), ^ ^ vitamin A ethers, 22-24 ^nd vitamin A 

 acid, 2 »- 2 ' are all active. The position of the hydrocarbon corresponding 

 to vitamin A (so called axerophthene) is not settled ; Karrer and Benz ' " 

 claim it is active whilst Meunier and his associates state that it is 

 inactive. 2 9, 3 



Me Me 



vAvAA, 



Mp 



Mututochrome 



Me 

 Me M^ Me Me-y^— ^ 



^Me Me Me Me 



Me 



Luteochrome 



Vitamin A 2 occupies a very interesting position from the point of 

 view of structural criteria for vitamin A activity. It is actiye per se 

 in the rat for it replaces vitamin A completely without in any degree 

 being converted into vitamin A. Its relative activity is, however, only 

 40 per cent, that of vitamin A. ^^ The structure of vitamin Aa is now 

 known, for Jones and his associates ^ ^ have shown by synthesis that it 

 is 3-dehydrovitamin A, thus confirming the suggestion of Morton, 

 1^^ ^^ ^^ Salah and Stubbs ^ ^ and refuting those of 



I I 1 CH OH Karrer, Geiger and Bretscher'^* and 



/yVVVV ' Shantz.35 This work on vitamin A^ 



\ / shows that vitamin A activity is still 



^ possible with a dehydro-p-ionone residue. 



The origin of vitamin A 2 in fish has 

 been considered on p. 174. 

 Anhydrovitamin A, ^ ^ obtained by treating vitamin A with alcoholic 

 HCl, has a double-bond system similar to that of iso-carotene {see p. 

 271) which is inactive. Shantz, ^ ' is, however, convinced that anhydro- 

 vitamin A has slight but definite activity. Most of it is stored in the 

 liver as a compound containing an hydroxyl group. This material, 

 named rehydrovitamin A, '^^ is found to be much more active than 

 anhydrovitamin A. 



The effect on biological activity of altering the side chain of vitamin A, 

 is not of major interest here because the compounds examined have been 



272 



