VI. OCCURRENCE 101 



class of the carotenoids. Naturally occurring provitamins X are a-, (3-, and 

 7-carotenes (here referred to mostly as carotene), cryptoxanthin, myxo- 

 xanthin, echinenon, aphanin, aphanicin, and torularhodin. It is not yet 

 proved whether loprotin has the biological potency of vitamin A.' 



Myxoxanthin is found in algae,- echinenon in the red fungus Hymenia- 

 cidon sanguina,^ ai)hanin and aphanicin also in certain algae,* torularhodin 

 in red yeasts,^ and leprotin in M ycobaclerium phlei^ (for further reference, 

 see Karrer and Jucker'). Beside these naturally occurring carotenoids there 

 are known a great number of synthetical compounds and of degradation 

 products of carotenoids, which are likewise biologically active.*- ^ Regarding 

 the formulas of vitamin A, of the natural provitamins A, and of the artificial 

 products which are physiologically active, one should recognize that their 

 potency depends on the existence of a jS-cyclocitral ring system and of a 

 side chain consisting of two isoprene residues. Only recently it has been 

 found that the vitamin A2 containing a dehydrocyclocitral ring also shows 

 vitamin A activity.^" 



In contrast to the provitamins A, vitamin A is found exclusively in the 

 animal organism; at least until now there has been no proof of the existence 

 of vitamin A in vegetable material, either free or esterified. We may there- 

 fore assume that vitamin A is a secondary product and that the animal 

 organism only is able to convert the provitamins A into vitamin A. It seems 

 to be certain that fish are able to convert not only provitamins A but also 

 other carotenoids (e.g., astaxanthin) into vitamin A." Fish liver oils repre- 

 sent the richest and most important source for this vitamin. 



iS-Carotene is most wddely distributed in nature, and since it invariably 

 accompanies chlorophyll it is found in all green parts of plants. It occurs in 

 abundance in carrot roots, paprika skin, sorb apples, oranges, spinach, 

 sweet potatoes, and pumpkins. Furthermore it is found in many of the lower 

 plants such as algae, in mushrooms, and also in bacteria^ (see Table XVII). 



Cryptoxanthin^ is found preponderantly in Indian corn. Herbivorous 

 animals receive their provitamin A exclusively from green plants. As do- 



1 Y. Takeda and T. Ohta, Hoppe-Seyler's Z. physiol. Chem. 267, 171 (1941). 



2 J. M. Heilbron and B. Lythgoe, Nature 136, 989 (1935); /. Chem. Soc. 1936, 1376. 



3 P. J. Drumm, W. F. O'Connor, and L. P. Renouf, Biochem. J. 39, 208 (1945). 



* H. Tischer, Hoppe-Seyler's Z. physiol. Chem. 251, 109 (1936); 260, 257 (1939); 281, 

 143 (1944). 



^ P. Karrer and J. Rutschmann, Helv. Chim. Acta 26, 2109 (1943); 28, 795 (1945); 

 29, 355 (1946). 



* Ch. Grundmann and Y. Takeda, Naturwissenschaften 25, 27 (1937). 



^ P. Karrer and E. Jucker, Carotinoide. Verlag Birkhauser, Basel, 1948. 



8 F. Bohlmann, Z. angew. Chem. 62, 4 (1950). 



" H. Vogel and H. Knobloch, Chemie und Technik der Vitamine. 3rd ed. p. 41, 



Ferdinand Enke, Stuttgart, 1950. 

 i» E. M. Shantz, Science 108, 417 (1948). 

 " J. A. Lovern, Chemistry & Industry 61, 222 (1942). 



