CHANGES IN THE INTESTINAL WALL 439 



b. Provitamins A Other Than /^-Carotene. Provitamins A other than 

 /3-carotene must also be broken down to vitamin A in the gut wall; other- 

 wise they would appear in the lymph and blood of the rat and other animals 

 having a similar carotenoid metabolism. Since a- and 7-carotenes do not 

 appear as such in the liver of rats after they have been fed, but only in the 

 form of increased levels of vitamin A, it is natural to postulate a metabolic 

 fate similar to that of /3-carotene. Ganguly ei al}'^^ proved that rats have 

 the ability to transform a-carotene to vitamin A in the gut wall. More- 

 over, since practically no chromogen is deposited in the liver of hens after 

 the ingestion of pro-a-carotene, which is a potent provitamin A,^"*^ one may 

 conclude that this carotenoid can also be metabolized in the intestinal wall. 

 Cryptoxanthin, also, was found by Patel et al.^'^ to undergo transformation 

 to vitamin A in the small intestine of rats, while Ganguly and DeueP^ 

 observed a similar phenomenon in the case of chickens. 



c. Carotenoids Other Than Provitamins A. Little is known about the 

 metabolic fate of carotenoids other than the provitamins A, since no end- 

 products comparable to vitamin A have been identified. One can differ- 

 entiate between the behavior of the carotenoids in animals which have j8- 

 carotene in the blood and their action in animals whose blood does not con- 

 tain this component. In the case of man and cattle, which represent the 

 first class of animals in respect to their carotenoid metabolism, the presence 

 of characteristic provitamins A, including a- and 7-carotenes, pro-7-caro- 

 tene, and cryptoxanthin, in the tissues and in metabolic products such as 

 milk and eggs, speaks for the absorption of these compounds in unchanged 

 form. Moreover, the widespread distribution of carotenoids not conver- 

 tible to vitamin A, as for instance zeaxanthin, lutein, lycopene, and cap- 

 santhin, in the tissues of man and of cattle and, in some cases, in those of 

 chickens, indicates that these carotenoids, also, are readily absorbed 

 unchanged. 



Since /3-carotene can be converted to vitamin A in the intestinal wall not 

 only by animals in Group C but also by those in Group A and even by 

 chickens (Group D), some explanation for the variations in metabolism 

 other than the capacity for conversion in the intestinal wall is necessary. 

 Goodwin^" suggested that the differences can be explained on the basis of 

 differences in efficiency of the enzyme systems in the several species. Thus, 

 Group C animals would have the most efficient carotenase system, "be- 

 cause no carotene overspills into the general circulation." In this group, 



'^ J. Ganguly, N. Krinsky, and H. J. Deuel, Jr., Unpublished experiments, 1955. 

 '^' H. J. Deuel, Jr., J. Ganguly, B. K. Koe, and L. Zechmeister, Arch. Biochem. Bio- 

 phys.,33, 143-149 (1951). 



