CAROTENOIDS 



fraction of the ingested lutein (xanthophyll) and to store the altered 

 products in the feathers and the skin respectively. However, it is only 

 when we come to invertebrates, in particular the marine invertebrates, 

 that the ability to produce highly oxygenated carotenoids is very marked, 

 but even in this class there are some exceptions. 



It is perhaps not for the biochemist to speculate on the evolutionary 

 significance of the variations which are played on the carotenoid 

 theme ; his primary aim should be to enquire into the mechanisms 

 whereby carotenoids are altered and into the functions in the animal 

 economy of the resulting products. 



Investigations of the carotenoids on such specifically biochemical 

 lines are woefully few and, except in the case of mammals, virtually 

 nothing is known of the ways in which carotenoids are modified or of 

 the factors controlling the processes. It is natural that the energies of 

 biochemists interested in carotenoids should recently have been 

 canalized to elucidate the conversion of carotene into vitamin A in 

 mammals. Advance in general biochemical knowledge of the caro- 

 tenoids has undoubtedly been hampered by this narrowing of the scope 

 of the investigations and it is hoped that now the fundamental features 

 of the carotene — > vitamin A conversion are reasonably well under- 

 stood, investigators will look further afield. However, studies on 

 carotenoids in mammals have sometimes had some bearing on the 

 wider problems of comparative carotenoid biochemistry. We know 

 why mammalian species differ in the degree to which they store 

 carotene in their adipose tissues ; the deciding factor is the efficiency 

 of the animals' intestinal " carotenase " in converting carotene into 

 vitamin A. The fatty tissues of a goat, for example, are free from 

 carotene because of the extreme efficiency of its intestinal enzyme 

 system which does not allow any pigment to spill over into the blood ; 

 humans and cows, on the other hand, being inefficient converters, 

 allow this overspill. There is as yet no evidence that such variations 

 between species can be fitted into a functional pattern. 



To turn away from mammals, not a single recorded fact throws light 

 on the mechanism of, for example, astaxanthin formation in some 

 insects and in lobsters, or of pectenoxanthin production in the scallop ; 

 many other similar cases could be cited. Although there are no hard 

 facts there are plausible ideas as to how such processes could occur. 

 However, when the biochemical problems are posed in questions such 

 as : " What is the special need of the lobster which calls for astaxan- 

 thin ? " or " What is the function of pectenoxanthin in the scallop ? " 

 it then emerges that we have neither facts nor ideas — so far these 

 problems baffle us. 



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