CHAPTER IX 



MAMMALIAN CAROTENOIDS 



All mammalian carotenoids are of alimentary origin and there is no 

 instance of any mammal manufacturing a ** specific " carotenoid 

 either de novo or from ingested carotenoids. Interest, then, centres 

 mainly on how mammals metabolize the carotenoids which they eat. 

 They have a general ability to convert certain carotenoids into vitamin 

 A, but this aspect deserves a chapter to itself (Chapter XI.) and will not 

 be considered further here. Apart from this common feature of caro- 

 tenoid metabolism, mammals can be divided into three groups accord- 

 ing to whether they accumulate in the fatty tissues of their body : 



{a) carotenes and xanthophylls (carotenoids) ; 



{b) primarily carotenes. 



{c) no carotenoids. 

 ■f Apart from a genetic variant of the rabbit {see p. 248), no mammal 

 has yet been observed which preferentially stores xanthophylls. 



These groups will be considered separately and emphasis will be laid 

 on the carotenoid distribution in certain special structures {e.g., adrenals, 

 retina), for it is in the apparently similar distribution which one finds in 

 these special structures in animals of all the groups, that a function of 

 carotenoid per se in mammals may eventually be recognized. 



It has just been stated that the problem of the conversion of a caroten- 

 oid with vitamin A activity into the vitamin will be discussed later, but 

 at the moment it should be borne in mind that these carotenoids are 

 converted to vitamin A in the intestinal wall ^' 2. 3. ^' ^ and not in the 

 liver as had been assumed until recently ; this necessitates a reorienta- 

 tion when considering the older literature and also affords a simple 

 explanation of a number of facts which were difficult to reconcile with 

 the old liver-conversion theory. 



CAROTENOID ACCUMULATORS 

 (i) Humans 



The only mammal which absorbs its carotenoids unselectively and 

 which has been extensively studied is man. This statement may require 

 slight modification in the light of the unconfirmed claim of Karrer and 



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