CAROTENOIDS 



result of small variations in absorption would not be detected by the 

 biological assay. In Johnson and Baumann's experiments, on the 

 other hand, larger doses were used and a 10-15 per cent, loss would 

 easily be noted in the variations in the amount of vitamin A stored in 

 the liver. 



A further interesting point in connection with the relationship 

 between the thyroid gland and carotene metabolism is the claim that 

 thyroglobulin 1 2 ^' ^ ^ ^ and iodinated casein act as enzymes for the in 

 vitro conversion of carotene into vitamin A. Careful in\'estigations of 

 this claim have proved it to be in all probability incorrect. 1 2 e, 1 3 1, 1 3 ia 

 No evidence of an increased rate of conversion has been obtained in 

 vivo using hyperthyroidic calves, ^ ^ ^ although recently such evidence 

 has been submitted in the case of hyperthyroidic cows and goats. ^ ^^ 



A different aspect of the carotene-thyroid relationship was investi- 

 gated by Smith and Perman ; ^ ^ ^ they found that carotene inhibited 

 the action of thyroid extracts in increasing the oxygen consumption 

 of cats. It will be recalled that Rokhlina^^* claimed that carotene 

 antagonizes the thyrogenic stimulation of axolotl metamorphosis. 



Samaras and Hingerty^^s consider that blockage of the reticulo- 

 endothelial system in normal rats increases the efficiency of conversion 

 of carotene into vitamin A. Recently, Vavich and Kemmerer ^ ^ ^ have 

 reported an investigation which indicates that small rats utilize ^- 

 carotene better than do large rats. An unconfirmed report exists 

 claiming that the conversion of ^-carotene into vitamin A is stimulated 

 by insulin. ^ ^ ' Pregnant rats utilize carotene more efficiently than 

 non-pregnant ones. ^ ^ ^ 



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282 



