carotenoids 



Destruction in Lumen 



Little work has been carried out on the mode of action of destruction 

 of carotene in the intestinal tract, but Hove ^ ^ ' found that clear stomach 

 extracts in the presence of methyl-linoleate oxidize carotene very much 

 in the same way as does soya bean lipoxidase (Chapter III) ; extracts 

 of small intestine, however, are much less active. 



Much more work has been carried out on the practical aspects of 

 protecting carotenoids in the intestinal tract. Moore's 3*^® original 

 observation that a-tocopherol (vitamin E) when administered with 

 vitamin A or carotene to rats results in a greater storage of vitamin A 

 in the liver owing to improved stability of the vitamin A and carotene 

 in the gut, has been repeatedly confirmed using varying experimental 

 conditions. ^ ° ^" ^ ^ ^ Recently, however, using the plasma carotenoid 

 levels as criterion, it has been stated that a diet low in vitamin E does 

 not impair the utilization of carotene by cattle. ^ ^ * 



Hickman and his colleagues 309-312 have also found that substances 

 other than the tocopherols protect carotene and vitamin A against 

 intestinal destruction. Such stabilizers, which are termed co-vitamins, 

 are laurylhydroquinone, ascorbic acid, and palmityl ascorbic acid. 

 One result of such stabilization is that on a diet rich in covitamins a 

 marked increase occurs in the percentage of ingested carotene which 

 is excreted in the faeces. ^ ^ ^ This indicates that the absorption may not 

 be affected by covitamins. Although intestinal stabilization is probably 

 the greatest single locus of action of these covitamins, the work of Davis 

 and Moore ^ ^ ' suggests that a similar action may also be exerted in the 

 blood stream and tissues. It should be noted that recently Johnson 

 and Baumann ^ ^ ^ found that large amounts of tocopherol administered 

 to rats with rather high doses of carotene reduced the amount of 

 vitamin A stored in the liver ; when tocopherol was administered 

 eight hours after the carotene no effect was noted ; they could not find 

 any change in carotene excretion during tocopherol administration. 



It is difficult to decide whether the action of these covitamins is in 

 inhibiting a lipoxidase-type of' enzyme noted by Hove ^ "^ ' or in pro- 

 tecting the pigment against chemical oxidation, for a-tocopherol 

 inhibits both soya bean lipoxidase ^ ^ ^ and the atmospheric oxidation 

 of carotene solutions. 3 2 0, 3 2 1 



It has recently been stated that, in the presence of adequate amounts 

 of tocopherols, lutein (xanthophyll) tends to interfere with the utiliza- 

 tion of carotene in rats. ^^^ Confirmation of this has been 

 reported, 3 2 3, 3 2 4 by^ Sherman,3 25 j^as found that the presence of 

 lutein actually protects carotene from intestinal destruction. Johnson 

 and Baumann ' "^ • also failed to observe the interfering action of lutein. 



252 



