CONVERSION OF CAROTENOIDS INTO VITAMIN A 



who found that with 1-0 [ig. of tocopherol /day, no biological 

 difference was apparent between 1 -0 (xg. of vitamin A and 1 -0 [ig. 

 of p-carotene. Johnson and Baumann/' however, could not 

 detect any visible affect of added tocopherol when the amount 

 of vitamin A stored in the liver after a dose of carotene was 

 measured ; it should be noted that compared with Koehn, 

 Johnson and Baumann used much higher doses of carotene. 



(c) Factors such as thyroxin {see p. 279) may have a much greater 

 effect on carotene than on vitamin A metabolism. 



According to Johnson and Baumann ^^ more vitamin A is 

 formed from a given amount of carotene in hyperthyroid than 

 in normal rats, and Cama and Goodwin ^ ^> ^ ° have shown that 

 this is due primarily to the action of the thyroid on the 

 absorption of carotene. Thiouracil reduces and desiccated 

 thyroid increases carotene absorption (see p. 281). This has been 

 recently confirmed in cows and goats. ^''^ 



Implicit in the assumption of a symmetrical fission is the fact that 

 the activity of any ^-carotene derivative in which one (S-ionone residue 

 is intact should be the same as that of a-carotene. This is not always 

 the case, e.g., semi-^-Carotenone, although allowance has never been 

 made for variations in absorption. Apart from differences which may 

 result from the biological methods of assay in different laboratories 

 and which are often spurious, real differences in activity may exist. 

 These are probably due to one or a number of the factors just discussed 

 rather than to some inherent property of the molecules. The problem 

 is, however, by no means solved and recent important work by Johnson 

 and Baumann 5 s, e i j^^s opened up new possibilities ; they found 

 that cryptoxanthin is as active as [3 -carotene when assayed by the 

 vitamin A liver storage test and twice as active when assayed by the 

 growth method. When a-carotene is compared with [3-carotene the 

 situation is reversed. 



It is very likely that the breakdown of ^-carotene into vitamin A 

 takes place in two stages ; the first stage involves an oxidative scission 

 with the production of vitamin A aldehyde (retinene) and the second 

 a rapid conversion of retinene into vitamin A. ® ^ 



Site of Conversion of Carotene into Vitamin A 



Moore 2 in 1929 first unequivocally demonstrated that in mammals 

 carotene was converted into vitamin A which was then stored in the 

 liver. Since then it has, until recently, been tacitly assumed that the 

 conversion takes place in the liver. In vitro experiments undertaken 



275 



