VI. OCCURUKXCE 105 



of \itainiii Aj iiilo Ai lias hccii ohsiM-Ncd up to now.''' \'itamin A is ahvay.s 

 acconiiianicd by iuM)\itamiii .\ ( 13 , ll-nioiio-r/.s \itaniin A). Accordiiiji; to 

 Harris and a.ssoriates ueoxitamin A shows in the storage tost (.sec j). 98) 

 a vitamin A potency of 75% and in the <ii'<>^vth tost a potency of 85%.''^ 

 The provitamins A too can ho accompanied hy stereoisomers, as was j)roved 

 hy Headle and Zscheile.^' Our knowledge of the carotenoid stereoisomers 

 relates principally to the ct)mprehensive and classical investigations of 

 Zeehmeister ct al:^^-^'- DeiieP''' ^* has stated that the stereomeric carotenes 

 are not fully potent hiologically. Work on this subject is still going on, and 

 further results must be awaited before a definite knowledge in regard to this 

 very complex and important problem is i-oachod. 



In wliale liver oil a sul)stance is found in considerable (iiiantities which 

 shows no physiological activity but which is changed into vitamin A by 

 heating to over 170°. This substance in the crystalline state is called 

 kitol.^^'^"^ An analogous substance has been isolated from fresh-water fish 

 oils. Through heating it is changed into vitamin A) and is termed kitol^.'"'^ 

 Nothing definite is known about the role which kitol plays in the organism. 



\'itamin A and its provitamins are chemically rather sensitive compounds 

 which saiffer irreversible changes under the influence of light and air.''^ 

 Thus special attention should be paid to the conservation of foodstuffs and 

 fodder containing these substances. Peroxides or peroxide-containing oils 

 instantaneously destroy vitamin A;*^^ also metal ions, especially copper and 



" E. M. Shantz and J. H. Brinkmann, ./. BloL ('hem. 183, 467 (1950). 



" P. L. Harris, S. R. Ames, and J. H. Brinkmann, /. Am. Chem. Soc. 73, 1252 (1951). 



" B. W. Beadle and F. P. Zscheile, J. Biol. Chem. 144, 21 (1942). 



« A. E. Gillam and M. S. El Ridi, Xature 136, 914 (1935) ; Riochem.J. 30, 1795 (1936); 



31, 251 (1937). 

 " A. Polgdr and L. Zochmoistor, ./. Am. Chem. Sue. 64, 1S56 (1942); 65, 1528 (1943); 



67, 108 (1945). 

 50 L. Zeehmeister and A. Polgdr, J. Am.. Chem. Sue. 66, 137 (1944). 

 " L. Zeehmeister, Chem. Revs. 34, 267 (1944). 

 52 F. P. Zseheile, Arch. Biochem. 5, 77, 211 (1944). 

 " H. J. Deuel, Jr., C. Johnston, E. Sumner, A. Polgdr, and L. Zeehmeister, Arch. 



Biochem. 5, 107 (1944); 6, 157 (1945); 7, 247 (1945). 

 "H. J. Deuel, Jr., C. Hendrick, E. Straub, A. Sandoval, J. H. Pinc-kard, and L. 



Zeehmeister, Arch. Biochem. 14, 97 (1947). 

 " N. D. Embree and E. M. Shantz, /. Am. Chem. Soc. 65, 910 (1943). 

 "H. Pritchard, H. Wilkinson, J. R. Edisbury, and R. A. Morton, Biochem. J. 31, 



258 (1937). 

 " H. Willstaedt and II. B. Jensen, Xature 143, 474 (1939). 

 58 F. B. Clougli. II. M. Kaschcr. ('. D. R<)I)eson, and J. G. Baxter, Science 105, 436 



(1947). 

 " N. D. Embree and E. M. Shantz, /. Am. Chem. Soc. 65, 910 (1943). 

 5» R. A. Bolomey, J. Biol. Chem. 169, 323, 331 (1947). 

 ^^ G. Carrara and A. Mandella, Vitaminologia 33 (1944). 



