CONVERSION OF CAROTENOIDS INTO VITAMIN A 



thus disproving With's^"^ suggestion that cryptoxanthin exerts its 

 vitamin A activity per se in the chick (see p. 265). Patel et al.^^^ also 

 showed that chickens with sterile intestinal lumena (sterilized by 

 treatment with sulphasuccidine) were able to carry out the conversion, 

 thus finally ruling out the intestinal flora as the possible agent responsi- 

 ble for the conversion. 



Thus we see that in normal animals the conversion of ^-carotene 

 into vitamin A has been clearly established to take place in the intes- 

 tinal wall. Although it has not been proved conclusively that the hver 

 cannot effect the conversion, it is highly probable that it cannot.*^ 

 Recently, however, it has been claimed that cow liver can accomplish 

 the conversion, ^ ^ ^ but this has been denied. ^ ° * Injection of carotene 

 into the portal vein of dogs is stated to result in the accumulation of 

 vitamin A in the liver. ^^' Bieri^^'^ has found that intramuscular 

 injections of carotene into eviscerated rats, produces in the blood 

 plasma a material with an absorption spectrum similar to that of vita- 

 min A. 



Factors Controlling the Conversion of Carotene into Vitamin A 



Apart from the role of the tocopherols in preventing the oxidation 

 of carotenoids in the intestinal lumen (see p. 252), not a great deal is 

 known of other factors which may control the conversion of carotene 

 into vitamin A. Considerable attention has been focused on the thyroid 

 gland in this connection. Kunde^''^ in 1926 noted the appearance of 

 vitamin A deficiency in thyroidectomised rabbits which were fed 

 carotene, and somewhat later in 1932-3 Fellenberg and Greuter, ^ ^ ° 

 and Fasold and Heidemann ^ ^ ^ claimed that carotene appeared in the 

 milk of thyroidectomised goats. Abelin^^^ ^t the same time noted 

 that in guinea pigs the administration of thyroxin adversely affected 

 the metabolism of carotene and vitamin A equally. The clinical 

 aspects of the subject have been reviewed by Drill. ^ ^ ^ 



After Abelin's work there was a considerable gap before the subject 

 was again investigated. Drill and Truant, ^ ^ * using the remission of 

 xerophthalmia in rabbits as the criterion of vitamin A production, 

 failed to demonstrate its formation from carotene in thyroidectomised 

 animals. A considerable objection to this work is that the carotene 

 was injected and, as stated on p. 277, there are serious doubts whether 

 injected carotene is utilized to any great extent. Canadell and Val- 

 descas ^ ^ * appear to confirm Drill and Truant's work, but Remington, 

 Harris and Smith^^^ state that eye symptoms are cured by the oral 

 administration of carotene to thyroidectomised animals. Di Bella^^'"^^* 

 found carotene effective, but with reduced efficiency. Carotene is also 



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