CAROTENOIDS 



converted into vitamin A in thyroidectomizcd goats, ^ ^ " but with what 

 degree of efficiency is not known. Barrick, Andrews, Beeson and 

 Harper ^2" consider that very high doses of thiouracil inhibits the 

 conversion in feeder lambs and this appears also to be true with 

 sheep. ^ 2 ^ 



Johnson and Baumann, ^ ' using the liver storage of vitamin A as 

 criterion, found that the same dose of carotene produces less liver 

 vitamin A in thiouracil-treated animals than in controls, and that 

 rather surprisingly, controls stored less than did rats dosed with 

 desiccated thyroid. Administration of thyroxin and thiouracil together 

 produced normal liver storage ; this indicated that the action exerted 

 by the thiouracil, causing reduced vitamin A storage, was anti-thyroid 

 and that the effect was not due to another (unknown) pharmaco- 

 logical action. Kelley and Day, ^ ^ - using the same criterion have 

 confirmed Johnson and Baumann's observations. Wiese, Mehl and 

 Deuel ^2 3^ in an important contribution, emphasized that in assessing 

 the effect of thiouracil on carotene by means of biological assays 

 involving measurements of weight increases, allowance must be made 

 for the growth-inhibiting action of thiouracil itself. That this growth 

 inhibition was due to thiouracil per se and not to its action on carotene 

 metabolism was demonstrated by the fact that normal growth in 

 thiouracil-treated animals could be elicited by the addition of desiccated 

 thyroid but not by massive doses of vitamin A. Wiese et al. overcame 

 this difficulty by evaluating the amount of carotene required to produce 

 one-half the maximum growth attained in the control and thiouracil- 

 treated groups. In this way they found that [^-carotene w^as equally 

 as effective in treated animals as in controls. These results probably 

 explain the " reduced efficiency " of carotene in thyroidectomizcd 

 animals noted by Di Bella. 



Goodwin, ^ ^ * and Cama and Goodwin ^^^^^ acting on the assump- 

 tion that the thyroid does have an action on carotene metabolism, 

 pointed out that none of the work just discussed indicates a possible 

 mode of action of the hormone. They considered that three possi- 

 bilities existed : 



{a) the enzyme " carotenase " is inhibited ; 



{b) thiouracil reduces the intestinal stability of carotene ; 



(c) intestinal absorption of carotene is reduced. 



If the first possibility were correct the carotene should traverse 

 the gut wall and appear in the general circulation ; a similar situation 

 possibly occurred in the unconfirmed report that thyroidectomizcd 

 goats' milk is yellow, ^ ^ ^ although it appears that this is incorrect for 



280 



