446 IX. CAROTENOIDS AND VITAMINS A 



and Balaba/^^ who noted an in vitro conversion of carotene into vitamin A 

 when thyroglobuhn or iodinated casein was added to the incubation mix- 

 ture. In spite of these negative reports, which may simply indicate that 

 the conditions were unfavorable for the functioning of the thyroid hormone, 

 one must conclude that the thyroid gland is active in increasing the avail- 

 abiUty of dietary carotene. The question still unanswered is whether this 

 effect occurs as a result of an increased absorption of carotene or whether 

 it should be ascribed to a more efficient conversion of carotene to vitamin A. 



Goodwin-** is inclined to beheve that the beneficial effect of the thyroid 

 hormone in reducing the loss of carotene in the feces is due to a better 

 absorption of carotene. This would explain the increased milk carotene 

 occurring in thyroid-treated cows'^'* as due to a greater ''overspill" from the 

 gut wall. Such a condition would not be expected if the increased removal 

 of carotene from the intestine resulted from a reinforced activity of carot- 

 enase. On the other hand, if the effect of the thyroid hormone were on 

 the carotenase, one might assume that a "carotene-saturated" intestinal 

 wall in a normal animal is rendered "unsaturated" after thyroid treatment 

 because of an accelerated removal of carotene due to conversion to vitamin 

 A. 2° This situation would allow for a greater absorption of carotene, but it 

 would not be expected to result in an increased carotene content in the 

 milk, in fact, the opposite situation should obtain. 



One fact which has been cited*^ against the increased absorption theory 

 and in favor of the carotenase activity hypothesis as an explanation for the 

 effect of the thyroid on carotene metabohsm is the findmg of Johnson and 

 Baumann^^ that the level of thyroid activity has no effect upon the amount 

 of vitamin A laid down in the liver and kidney following vitamin A feed- 

 ing. It should be recalled that the absorption of carotene and vitamin A 

 poses two different questions. Under usual conditions, only 50% of caro- 

 tene is absorbed, ^^- whereas vitamin A is almost completely utilized under 

 similar circumstances. One would not expect to improve the conditions 

 for absorption for a substance which is normally practically 100% absorbed, 

 but it would be reasonable to expect improvement in the absorption of a 

 poorly-utilized substance like carotene. 



Another fact which is difficult to reconcile with the carotene absorption 

 theory is that the thyroid has been known to stimulate absorption only in 

 the case of substances, such as sugars, which can be phosphorylated. On 



1" S. Kaplanskii and T. J. Balaba, Biokhimiija, 11, 327-331 (1946); Chem. Abst., 41, 

 507 (1947). 



192 -p. W. Goodwin, The Comparative Biochenustry of Carotenoids, Chapman & Hall, 

 London, 1952; Carotenoids, Their Comparative Biochet)iistry, Chem. Pub. Co., New York, 

 1954. 



