304 IV. DIGESTION OF LIPIDS OTHER THAN FATS 



include the transformation of vitamin A aldehyde (retinene) to vitamin 

 A, and the esterification of the vitamin A formed from carotene. 



a. The Transformation of Carolenoids Into Vitamin A. Until quite 

 recently, it has been generally assumed that the liver is the organ in which 

 /3-carotene is changed to vitamin A. 360 An enzyme, referred to as "caro- 

 tenase," has been credited with bringing about the cleavage. 



However, Sexton et aZ. 361 were the first to suggest that the intestine is 

 the site of the transformation of carotene to vitamin A. They state that 

 "the conversion of carotene to vitamin A may be an extrahepatic function 

 in the rat. The wall of the intestine is suggested as a possible site of such 

 transformation." There are several arguments in favor of this concept. 

 In the first place, it was shown that no appreciable amounts of /3-carotene 

 are present in the livers of rats, regardless of how large an amount of 

 carotene is given in the diet. Moreover, it was found that carotene, in- 

 jected intrasplenically, could not be utilized as a source of vitamin A. 

 Rats died, presenting typical symptoms of avitaminosis A, in spite of the 

 fact that large amounts of carotene were present in the livers of the ani- 

 mals at the time of death. No intrinsic impairment of carotene metabo- 

 lism could be demonstrated in these vitamin A-deficient rats, since caro- 

 tene given orally promptly relieved the symptoms of avitaminosis A. 

 Lease et a/. 362 likewise demonstrated that injected carotene was ineffective 

 as a source of vitamin A. 



Mattson, Mehl, and Deuel 257 demonstrated that vitamin A could be 

 detected in the intestinal walls of vitamin A-deficient rats shortly after 

 the feeding of /3-carotene; vitamin A appeared in the intestinal wall 

 earlier than in the liver, and remained at higher levels there for four hours. 

 Moreover, it was shown that, when normal rats were fed large doses of 

 vitamin A and then kept on a vitamin A-free diet for approximately two 

 weeks, no vitamin A could be detected in the intestinal wall, in spite of 

 the fact that a high concentration of vitamin A was present in the liver. 

 This experiment indicates that the level of intestinal vitamin A is not a 

 reflection of the quantity stored in the liver, and that the intestinal level 

 may be entirely independent of that in the liver. 



The results of Mattson et a/. 257 were presented almost simultaneously 

 with a note by Thompson, Ganguly, and Kon, 258 which was later published 

 in extenso. 2 ™ The intestinal wall has since been shown to be the site of 

 transformation of /3-carotene to vitamin A, not only in the rat, 260-262 



360 T. Moore, Biochem. J., 25, 275-286 (1931). 



361 E. L. Sexton. J. W. Mehl. and H. J. Deuel, Jr., ./. Nutrition, 81, 299-320 (1946). 

 382 J. G. Lease, E. J. Lease, II. Steenbock, and C. A. Baumann, ./. Lab. Clin. ?ifed., 27, 



502-510(1941-1942). 



