METABOLISM OF CAROTENOIDS AND VITAMINS A 539 



which is not supported by any convincing experimental data, is that the 

 conversion of ,S-carotene to vitamin A involves a central fission of the mole- 

 cule (between C 15 and C 15'), with the resultant formation of two mole- 

 cules of vitamin A. In fact, one might also postulate that, from spatial 

 considerations, the position 15 : 15' should be the favored spot for the action 

 of the enzyme, carotenase, since it is the only position in the carotene mole- 

 cule where a double bond occurs in which the tAvo carbon atoms on each 

 side of it are free from methyl groups. If the enzyme molecule must be ad- 

 jacent to the substrate, there would be a better opportunity for close 

 approximation here than at any other position on the molecule. 



A second theory is that /3-carotene is oxidized in a random manner. 

 Since this type of oxidation should occur only relatively seldom at the cen- 

 tral position (15:15'), but would generally take place on one side or the 

 other, one portion of the ruptured molecule would have a shorter aliphatic 

 chain than occurs in vitamin A, and hence would be excluded as a potential 

 source of this vitamin. The second fragment obtained under such condi- 

 tions would have a longer side-chain than occurs on the vitamin A mole- 

 cule. The longer chain could be shortened by oxidation to yield vitamin 

 A. In the case of the provitamins A other than /3-carotene, the single in- 

 tact |8-ionone fragment would be left with an aliphatic chain too short for 

 its conversion into vitamin A, 50% of the time. If this reasoning is cor- 

 rect, then only one vitamin A molecule would arise from the metabolism of 

 each /3-carotene molecule, while an average of two provitamin A molecules 

 would be required in order to obtain one molecule of vitamin A. Glover 

 and Redfearn^^^ postulated that the transformation of /3-carotene into vita- 

 min A in vivo involves terminal oxidation rather than central fission. 

 Painter and Glover™ oxidized lycopene by the osmium tetroxide method 

 of Wendler et al.,"^^- and demonstrated the formation of apo-lycopenals 

 ha\ang 9, 7, and 5 ethylenic bonds with a terminal aldehyde group. The 

 compounds having 9 and 7 ethylenic bonds could have arisen only by ter- 

 minal oxidation or by oxidation at a site other than the central double 

 bond. 



The experimental data which indicate that /3-carotene is twice as potent 

 as a provitamin A as are the carotenoids in which one of the j8-ionone groups 

 is modified would support either of the above theories. Considerable data 

 are available in the literature indicating the superiority of vitamin A over 

 an equal quantity of /3-carotene in satisfying the vitamin A requirement. 

 A relatively close correspondence in biologic activity can be obtained from 



^" J. Glover and E. R. Redfearn, Biochem. J., 58, xv-xvi (1954). 

 ''^ R. H. Painter and J. Glover, Biochem. J., 60, xvi (1955). 



