OCCURRENCE OF THE VITAMINS A 675 



dicated the possibility of 3 stereoisomers of all-^rans-vitamin Ai: S-cis- 

 vitamin A, 5-as-vitamin A, and 3,5-di-cis-vitamin A. Robeson and Bax- 

 ^gj.46,47 isolated a stereoisomeric form from some fish liver oils; this is prob- 

 ably the 5-cis form/^ 



2. Occurrence of the Vitamins A 



Vitamin A occurs only in the animal organism, and it is never found in 

 plants. This is in marked contrast to jS-carotene, which occurs primarily 

 in plants but which may be found in animal tissues under some conditions. 

 In the latter case, it is believed that the tissue carotenoids are derived from 

 the food; there is no absolute proof, however, that some animal tissues may 

 not be capable of synthesizing carotenoids from appropriate precursors. 

 The colored yeasts and other fungi are known to contain /3-carotene and 

 other carotenoids, but no vitamin A.^^'^^ Vitamin A is present in most 

 animals; the largest quantity is usually concentrated in the liver. 



(1 ) Vitamin A in Fishes 



Fish livers constitute the chief natural source of vitamin A, but extreme 

 differences in the concentrations are observed in various species. The dis- 

 tribution of a number of such oils is given in Table 2. The amount of vita- 

 min A present varies with the type of food consumed, with sex, the season 

 of the year, and with general living conditions. Schmidt-Nielsen and 

 Schmidt-Nielsen'" showed that the ratio of vitamin A in some fish liver oils, 

 as determined from the antimony trichloride blue values, was as follows: 



Halibut {Hippoglossus hippoglossus) 700 



Mackerel {Scomber scombrus) 570 



North Atlantic salmon {Salmo salar) 250 



Red perch or Norway haddock {Sebastes marinus) 70 



Porbeagle {Lainna cornubica) (mackerel shark) 6 



It was also noted by these workers that the liver oils which could be iso- 

 lated only by solvent extraction invariably had a higher vitamin A content 

 than did those in which the oils could be separated by steaming. 



Von Euler and Karrer,^i as well as Lovern,'^^ have pointed out that the 

 liver oil of the halibut (Hippoglossus hippoglossus) is 50 to 100 times richer 

 in vitamin A than is a good grade of cod liver oil, although the authors 



« C. D. Robeson and J. G. Baxter, Nature, 155, 300 (1945). 

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

 ^8 E. Lederer, Les CaroUnoides des Planles, Paris, 1934. Cited by R. J. Williams, 

 ]'itatnins and Hormones, 1, 230 (1943). 



« A. Scheunert and J. Reschke, Deul. med. Wochschr., 57, 349-351 (1931). 



50 S. Schmidt-Nielsen and S. Schmidt-Nielsen, Biochem. J., 23, 1153-1157 (1929). 



"' H. V. Euler and P. Karrer, Naturwissenschaftcn, 19, 676 (1931). 



" J. A. Lovern, Nature, 129, 726 (1932). 



