CHANGES IN THE INTESTINAL WALL 445 



tion in hypothyroidism was obtained when the condition was produced by 

 the use of the thyroid antagonist, thiouracil, rather than by thyroidectomy. 

 Thus, following treatment with this drug, rats were found to have a lower 

 than normal content of vitamin A in the liver after the administration of 

 carotene. ^^'^^^ Moreover, carotene was found to be ineffective in removing 

 the ocular symptoms caused by avitaminosis A under these conditions. '^^ 

 The administration of desiccated thyroid powder corrected these abnormali- 

 ties in carotene metabolism. 



The level of plasma vitamin A ester has been used as a measure of the 

 effect of the thyroid hormone upon the conversion of carotene to vitamin A. 

 In normal blood, the vitamin A alcohol is believed to originate in the liver, 

 and it usually has a fairly constant value ;^^'^ in contradistinction to this, 

 the ester fraction is believed to represent the small amount of vitamin A 

 en route from the intestinal wall to the liver, where it is stored as such until 

 liberated into the blood stream as free vitamin A alcohol. ^^^-^^^ Thus, the 

 observation of Chanda and Owen^^* that the amount of vitamin A ester in 

 the blood was increased in cows fed carotene when they were treated with 

 either thyroxine or a thyrotropin preparation has been interpreted as 

 evidence that the extra vitamin A originates from the carotene in the intes- 

 tine. 



A number of reports have failed to confirm the thyroid effect upon the 

 carotene -^ vitamin A reaction. Thus, Wiese and her collaborators ^^^•^'^^ 

 were unable to demonstrate variations in carotene metabolism in rats 

 rendered hypothyroid by means of thiouracil. Morgan and Arnrich^^ 

 are of the opinion that the normally functioning thyroid gland is not 

 essential for carotene utilization in rats and dogs. Finally, a number of 

 investigators^^^"^^" have been unable to confirm the results of Kaplanskii 



i8» B. KeUey and H. G. Day, /. Biol. Chem., 175, 863-866 (1948). 



181 J. M. Canadell and F. G. Valdecasas, Experientia, 3, 35-36 (1947). 



'82 J. Glover, T. W. Goodwin, and R. A. Morton, Biochem. J., 43, 109-114 (1948). 



183 R. Chanda, H. M. Clapham, and E. C. Owen, Biochem. J., 60, 391-399 (1955). 



'8* R. Chanda and E. C. Owen, UnpubUshed results, 1954; cited by T. W. Goodwin, 

 The Chemical Pathology of Carotenoids, in R. T. Williams, The Chemical Pathology of 

 Animal Pigments, Biochem. Soc. Symposia, No. 12, Cambridge Univ. Press, 71-84, 

 (1954), p. 76. 



'85 C. E. Wiese, H. J. Deuel, Jr., and J. W. Mehl, Proc. Soc. Exptl. Biol. Med., 66, 

 213-214 (1947). 



'8« C. E. Wiese, J. W. Mehl, and H. J. Deuel, Jr., J. Biol. Chem., 176, 21-28 (1948). 



187 A. F. Morgan and L. Arnrich, Federation Proc, 12, 424-425 (1953). 



188 H. R. Cama and T. W. Goodwin, Biochem. J., 43, .xhv (1948). 



189 C. W. Lowry and J. R. Lowry, Arch. Biochem., 26, 287-290 (1950). 



'^ C. E. Wiese, The Site of Conversion of Carotene to Vitamin A in the Rat, Thesis, 

 Univ. So. Calif., Dept. Biochem. Nutrit., June, 1948. 



