26 VITAMINS A AND CAROTENES 



It has already been stated in the previous section that the antixerophthal- 

 mic vitamins are produced in nature from the corresponding provitamins A. 

 Since the conversion is known to occur in the animal organism only, the 

 vitamins A are entirely of animal origin and are not found in plants. In 

 most cases, but especially in fishes, more than one vitamin A is found in 

 the same species. This is probably due to the ease of interconversion of the 

 various forms in the organism. Structurally, all vitamins A are closely re- 

 lated; some, like vitamin Ai and neovitamin A, are stereoisomers; others, 

 like \itamin A2, vitamin A aldehyde or retinene, and anhydrovitamin Ai, 

 are presumably dehydrogenation, oxidation, and dehydration products, re- 

 spectively, of vitamin Ai. Rehydro vitamin A is formed in the rat by the 

 addition of water to anhydrovitamin A.'^^ In addition to vitamins A iso- 

 lated from natural sources, several synthetic products which do not seem 

 to exist in nature have been prepared and found to exhibit appreciable 

 biological activity. The preparation of an antivitamin A has also been re- 

 ported recently .'^^ These and other developments will now be discussed in 

 the following sections. 



2. Vitamin Ai (Axerophthol) 

 a. Sources 



Vitamin Ai is the most commonly known and abundant of all the natur- 

 ally occurring vitamins A. It occurs in all land animals and in sea- and 

 fresh-water fishes. Frequently it occurs together with less common \dtamins 

 A. Although this vitamin is present in all organs of the body, skeletal tissues, 

 and blood serum, it is found stored in larger quantities in the liver and 

 viscera. The most important natural source of this vitamin is fish liver oils, 

 and, prior to the appearance of the synthetic vitamin A in the market, all 

 industrial and medicinal vitamin A was obtained from this source. The 

 great scarcity of vitamin A during World War II and its importance in 

 animal nutrition, pharmaceuticals, and fortification of foods, especially 

 margarine, stimulated an extensive search for new sources. Fish liver oils 

 and oils from viscera of numerous species were examined during this 

 period. '^^"^^ Table IX shows some of the more important sources of vitamin 

 A. The potency shown in the last column includes both Aitamin Ai and 

 neovitamm A, As it may be seen, several of the oils contain also vitamin A2. 



^* H. Vogel, Chemie und Technik der Vitamine, Erster Band: Die fettloslichen Vita- 

 mine, 3rd ed. (revised by H. Knobloch), pp. 18-150. Ferdinand Enke, Stuttgart, 

 Germanj^ 1950. 



75 E. M. Shantz, J. Biol. Chem. 182, 515 (1950). 



7« P. Meunier, J. Jounneteau, and R. Ferrando, Vunipt. rend. 230, 140 (1950); P. 

 Meunier, Fortschr. Chem. orq. NaUtrntoffe 9, 102 (1952). 



" W. S. Rapson and H. M. Schwartz, ./. Chem. hul. 63, IS (1944). 



" U. S. Kini and K. Chidambaram, J. Chem. Ind. 66, 2:« (1947). 



7» R. O. Sinnhumber and D. K. Law, Ind. Eng. Chem. 39, 1309 (1947). 



