CAROTENOIDS 



There remains the possibiUty that only the all-/r«w5-forms have 

 vitamin A activity and that other stereoisomers are active only in so 

 far as they are rearranged, probably in the digestive tract, to all trans- 

 P-carotene. Kemmerer and Fraps^^ have produced evidence that, in 

 the case of neo- ^-carotene U, this rearrangement does in fact take 

 place, and recently it has been shown that in chickens pro-y-carotene 

 and lycopene undergo considerable stereoisomerization in their passage 

 down the intestinal tract. 



For example, 71 per cent, of the recovered y-carotene had been 

 isomerized into a number of pigments including the dW-trans form. '" ^ 

 Cw-isomers of vitamin A also vary in their biological activity {see 

 Goodwin ^ ^). 



The Conversion of Carotene into Vitamin A 



The mechanism of the conversion of carotene into vitamin A is 

 obscure. Biological assays generally reveal that the relative molar 

 potencies of [3-carotene, a-carotene, and vitamin A are 2 : 1 : 2. If 

 the 2 : 1 activity of [^-carotene and a-carotene is explained by the fact 

 that conversion occurs by fission at tlie central double bond, then one 

 molecule of [^-carotene should give rise to 2 molecules of vitamin A 

 and on a molar basis, ^-carotene should be twice as active as vitamin A. 

 This has only been reported twice {vide infra)^^'^^ compared with 

 numerous reports of lesser activity. 



One is, however, loath to discard the theory of symmetrical fission 

 in favour of fission at points other than the central until all the factors, 

 considered below, have been fully investigated. The failure of |3- 

 carotene normally to be as effective as a symmetrical fission would 

 suggest, is probably due to three main reasons : 



{a) the poor efliciency of absorption of carotene from the gut com- 

 pared with vitamin A {see p. 250). Even at very low doses 

 (1-2 (jig./day) up to 20 per cent, can be lost in the faeces. ^^ 



{b) the stability of [3-carotene (and other carotenoids) in the intes- 

 tinal lumen may be much less than that of vitamin A. Destruc- 

 tion is probably due to non-specific oxidation which can be 

 reduced in the presence of anti-oxidants such as the tocopherols 

 {see p. 252). In fact, Koehn^'* has recently conducted experi- 

 ments using " optimum " amounts of a-tocopherol, in which 

 he increased the relative activity of (3-carotene to a value ap- 

 proaching that required by symmetrical fission and this has 

 recently been confirmed by Burns, Hauge and Quackenbush, ^ ^ 



274 



