548 IX. CAROTENOIDS AND VITAMINS A 



vitamin A) has been sho\Aai to occur naturally. Robeson and Baxter^^* 

 isolated it from the "non-crystallizable portion" of natural vitamin A 

 samples. According to these investigators, approximately 35% of the 

 vitamin A in the liver oil of cod (Gadiis 7norrhua), dogfish {Scyllium ca- 

 nicula), hahbut {Hippoglossus /izppo^/ossws), and California jewfish or black 

 bass (Stereolepis gigas) is neovitamin A. This agrees with the results of 

 Hayes and Petitpierre/^^ who reported that neovitamin A constitutes 11 

 to 39% of the total vitamm A in fish liver oils. Neovitamin A is also pres- 

 ent in the synthetic vitamin A.''^^ 



Harris and co-workers^^^ recorded the biopotency of neovitamin A alcohol 

 as 87.1 ± 3.25% of that of all-^rans vitamin A alcohol, while the neovitamin 

 A acetate was found to possess 83.2 ± 4.28% of the bioactivity of ordinary 

 vitamin A acetate. Both of the above assaj^s are based upon gain-in - 

 weight by the usual U.S. P. method. On the other hand, when the com- 

 parison of vitamin A potency was made by the liver storage procedure, the 

 biopotency of neovitamin A was estimated to be only 75.6% of that of the 

 aW-trans vitamin A.'^^^ Harris and co-workers"^ assigned a potency of 

 2,690,000 U.S.P. units/g. to neovitamin A. 



Rats are able to transform neovitamin A to the all-^ran.s form, and vice 

 versa. According to Harris et a/.,^^^ when either isomer is fed in the pure 

 state to rats, a mixture of 88% all-^rans vitamin A and 12% neovitamin A 

 is stored in the livers. This proportion varies with different species, since 

 it has been found to be 65 : 35 in fish livers. Robeson and Baxter"* demon- 

 strated that the interconversion of aW-trans vitamin A and neovitamin A 

 could be effected in either direction by treating their anthraquinone-/3-car- 

 boxylate esters with traces of iodine in benzene solution. Neovitamin A 

 ester was converted to vitamin Ai ester to the extent of 70% in two hours 

 at 25°C., while the reverse change was accomplished under similar condi- 

 tions to the extent of only 30%. 



It is possible that other stereoisomeric forms of vitamin A are physio- 

 logically important. While natural vitamin A concentrates can be con- 

 verted in vitro to rhodopsin when opsin and two enzymes, liver alcohol 

 dehydrogenase and cozymase, are present, the reaction fails to take place 

 when the natural vitamin A is replaced by synthetic vitamin A and neo- 

 vitamin A.'^^" On the other hand, when synthetic ^'itamin A is exposed to 

 light in the presence of traces of iodine, rhodopsin is formed. This is be- 



"" C. D. Robeson and J. G. Baxter, J. Am. Chem. Soc, 69, 136-141 (1947). 

 "5 E. Hayes and M. Petitpierre, /. Pharm. and Pharmacol, 4, 879-885 (1952). 

 "6 p. L. Harris, S. R. Ames, and J. H. Brinkman, /. Am. Chem. Soc, 73, 1252-1254 

 (1951). 



'" R. Hubbard and G. Wald, Science, 115, 60-63 (1952). 



