310 IV. DIGESTION OF LIPIDS OTHER THAN FATS 



depleted rats stored less vitamin A after carotene than did similar animals 

 winch had been treated with thiouracil. Moreover, the preformed stores 

 of vitamin A were found to be more slowly depleted in the hypothyroid 

 rats than in their normal controls. After ten weeks on a vitamin A-free 

 diet the controls retained 5% of the original vitamin A, while 17 and 14% 

 were retained in hypothyroid rats and in pair-weighed controls. 



The finding of Kaplanskii and Balaba 253 that carotene can be converted 

 to vitamin A by in vitro incubation of colloidal solutions of carotene with 

 aqueous solutions of either thyroglobulin or iodinated casein has also been 

 considered to offer support for the supposition that the thyroid hormone 

 stimulates the conversion of carotene to vitamin A. However, neither 

 Cama and Goodwin, 400 Lowry and Lowry, 255 nor Wiese. 401 have been able 

 to confirm this observation. Although the bulk of evidence would seem 

 to support the hypothesis that the thyroid secretion regulates the carotene 

 -*■ vitamin A reaction, more proof is needed before this theory can be 

 accepted unequivocally. Drill 402 reviewed this subject in 1943. 



8. The Digestion and Absorption of Phytofluene 

 in the Gastrointestinal Tract 



Phytofluene, a polyene closely related to the carotenoids, was first 

 discovered by Zechmeister and Polgar. 403 It has been found to be widely 

 distributed in plant organs which also produce carotenoid pigments. 404 

 Sandoval et a/. 405 found that this pigment was absorbed to some extent 

 by rabbits, and was deposited in the liver. However, the major portion 

 of the phytofluene fed was destroyed; it has not been determined whether 

 or not this destruction occurs in the gastrointestinal tract. 



9. The Digestion, Absorption, and Transformations of the Fat- 

 Soluble Vitamins 



(1) Vitamin A 



a. The Absorption of Vitamin A from the Intestine. Vitamin A is 

 much more readily absorbed from the gastrointestinal tract than is its 



400 H. R. Cama and T. W. Goodwin, Biochem. J., 43, xliv (1948). 



401 C. E. Wiese, The Site of Conversion of Carotene to Vitamin A in the Rat. The Effect 

 of Hypothyroidism on this Conversion. Thesis, Univ. So. Calif., Dept. Biochem. Nutrit., 

 June, 1948. 



402 V. A. Drill, Physiol. Revs., 23, 355-379 (1943). 



403 L. Zechmeister and A. Polgar, Science, 100, 317-318 (1944). 



404 L. Zechmeister and A. Sandoval, J. Am. Chem. Soc, 68, 197-201 (1946). 



406 A. Sandoval, E. R. Meserve, H. J. Deuel, Jr., and L. Zechmeister, Arch. Biochem., 

 11, 373-375 (1946). 



