CONVERSION OF CAROTENOIDS INTO VITAMIN A 



neither in England ^ ^ 5 nor in America ^ ^ * has thyroidectomy resulted 

 in the appearance of carotene in the blood and milk of goats. Good- 

 win ^^^ and Cama and Goodwin ^ ^' ^ ** could never demonstrate the 

 presence of carotene in the systemic blood of rabbits fed a carotene- 

 rich diet together with large doses of thiouracil. These results appear 

 to rule out the first possibility and there is good reason also to reject 

 the second possibility, for Cama and Goodwin ^^e j^^ye shown that 

 thiouracil has no effect on carotene stability m vitro. A group of 

 Italian workers, however, found that thyroxin retarded the destruction 

 of colloidal solutions of ^-carotene when the yellow colour of the 

 solution was taken as criterion but accelerated it when the colour with 

 SbClg was measured. ^2 7 Cama and Goodwin did not find this to be 

 so. 1 2 6 That thiouracil exerts its action by reducing the absorption of 

 carotene from the gut wall (possibility C) has been made highly pro- 

 bable by the results of a recent investigation by Cama and Goodwin. ^ ' 

 Under controlled dietary conditions rats treated with thiouracil 

 excrete a greater percentage of a given dose of j3-carotene than do 

 control rats ; desiccated thyroid on the other hand increased absorp- 

 tion. (See Table 48). This has recently been confirmed in cows and 

 goats by Owen and his co-workers, ^ ^ s ^yho also found that the ratio 

 vitamin A : carotene in the milk was increased by thyroxine and 

 decreased by thiouracil. One can now offer an explanation of the 

 apparently opposite results of Johnson and Baumann ^ ' and Wiese 

 et al.^^^ In the latter experiments very small doses were used and the 



Table 48 



Illustrating the Effect of Thiouracil and Desiccated Thyroid on the 

 excretion of ^-carotene by Rats 



From Cama, H. R., and Goodwin, T. W. (1949), Biochem, J., 45, 236. 



281 



