CHAPTER XI 



CONVERSION OF CAROTENOIDS INTO VITAMIN A 



The occurrence of vitamin A is, as far as is known at present, 

 confined to mammals, birds, fish and Crustacea. In the case of mam- 

 mals, birds, and Crustacea, if vitamin A is not eaten preformed it is 

 produced by the in vivo conversion of " biologically active " caro- 

 tenoids into vitamin A. In fish, although conversion of carotenoids 

 into vitamin A can take place, there is a growing belief that fish vitamin 

 A is provided preformed in Crustacea. Almost certainly fish do not 

 synthesize vitamin A de novo. 



At this point it may be useful to condemn the phrases ** biologically 

 inactive " and " biologically active " as usually applied to carotenoids ; 

 these terms are generally used in the sense that the pigment under 

 discussion is either capable or incapable of being converted into 

 vitamin A. This implies too narrow a concept of carotenoid function ; 

 ** biologically inactive " carotenoids are ** biologically active ", for 

 example, in such a function as chromatophore response in fish (see 

 p. 196). When discussing the conversion of carotenoids into vitamin 

 A the terms ** vitamin A precursors " or, less»good, " provitamins A," 

 should be used. 



After a considerable amount of pioneer work by various investigators 

 which has been ably summarized by numerous authors including 



Me Me Me 



269 



