544 IX. CAROTENOIDS AND VITAMINS A 



depleted rats after the test substance was fed in doses of 10 to 80 ng. per 

 day, Johnson and Baumann^^^ reported the following biopotencies of several 

 stereoisomeric forms (all-fraws-/3-carotene = 100%) : neo-/3-carotene U, 

 33; neo-|8-carotene B, 48; all-^rans-a-carotene, 25. In the case of vitamin 

 A-depleted chicks, Johnson, Swick, and Baumann^*' reported that neo-/3- 

 carotene B is somewhat more than one-half as active as all-^rans-/8-carotene 

 when the test is based upon the level of vitamin A in the liver after the 

 feeding of test substances. It is of interest that a synthetic cis isomer, 

 namely 15,15'-monocis-/3-carotene (central monoc?s-j8-carotene), likewise 

 has about 50% of the bipotency of its all-trans isomer, as determined by 

 growth tests, when it is fed to depleted rats and to chickens at a low daily 

 level. "^ This isomer cannot be efficiently utilized when fed in large 

 amounts, as the expected higher groAvth responses are not observed under 

 these conditions. 



It has not been decided whether the stereoisomeric forms of the pro- 

 vitamins A are themselves directly active as a source of vitamin A, or 

 whether their biopotency is in proportion to the extent to which they are 

 reconverted to the all-^rans form of the isomer. There is some evidence that 

 a biologic transformation from one stereoisomeric form of a carotenoid to 

 another is possible. Thus, Kemmerer and Fraps^^^ observed that neo-;S- 

 carotene U was isomerized to all-/rans-/3-carotene in rats. Deuel et al. ^^^ ob- 

 served the presence of considerable amounts of all-^rans-7-carotene, as well as 

 of neo-7-carotenes and of a poly-c2s-7-carotene, not only in the feces but also 

 in the carotenoids obtained from the tissues and gut washings of chicks after 

 pro-7-carotene had been fed. Moreover, a similar isomerization occurred 

 in the case of mature hens when 15 mg. of prolycopene was administered 

 over a period of three days. In the latter tests, the following pigments 

 (in mg.) were recovered from the feces: unchanged prolycopene, 0.445; 

 all-^rans-lycopene, 0.712; neolycopenes, 0.562; polym-lycopenes, 0.611; 

 unidentified, 0.050; total recovered, 2.380. A similar distribution of isom- 

 erization products was noted in the material recovered from the tissues 

 and gut washings. When 15 mg. of lycopenes were fed during three days 

 to carotenoid-depleted hens, neolycopenes but no polycts-lycopenes were 

 islated from the feces. In the case of the synthetically -prepared 15,15'- 

 monocis-((3-carotene, Zechmeister and co-workers^'^^ demonstrated that a 

 considerable transformation of this monocis isomer to the all-^rans form 

 had taken place in the gastrointestinal tract, as determined by the composi- 

 tion of the fecal carotenoids. 



"* L. Zechmeister, H. J. Deuel, Jr., H. H. Inhoffen, J. Leeman, S. M. Greenberg, 

 and J. Ganguly, Arch. Biochern., 36, 80-88 (1952). 



"« A. R. Kemmerer and G. S. Fraps, J. Biol. Chem., 161, 305-309 (1945). 



