100 VITAMINS A AND CAROTENES 



tion of the biological comparison of j8-carotene and vitamin Ai acetate. 

 One vitamin A unit is to one provitamin A unit as 1 is to 2.^- ^ 



The standard /3-carotene shall have a melting point of 180° (corr.) and 

 show a light absorption at 465 m/i (in benzene) with an extinction of e 

 = 122,700 (^}ln. = 2290), or at 455 m^ (in cyclohexane) with e = 130,800 

 (-^i^m. = 2400). A solution of /3-carotene in a vegetable oil serves as stand- 

 ard solution. Thus, according to U.S. P., 5 mg. of such a solution (/3-carotene 

 in cottonseed oil) contain 0.6 7 of /3-carotene equal to 1 I.U.; this solution 

 is stabilized with hydroquinone. The standard vitamin A acetate shall have 

 a melting point of 57.8 to 59" and a light absorption at 325 mju with an 

 extinction of e = 50,000 (£'1^^. = 1525) inisopropanol. Vitamin Ai alcohol 

 shows an ^'l^m. = 1750 at 328 m/x (depending on the solvent, the values 

 for £^}^m. can vary slightly) . A solution of vitamin A acetate in a vegetable 

 oil is used as a standard solution. The stabilization is accomplished by adding 

 at least 0.1 % of tocopherol.^ The standard solution of U.S.P. contains 

 0.344 7 of vitamin A acetate in 0.1 mg. of solution (cottonseed oil). The 

 final fixation of the conversion factor (C.F.) — that is, the ratio of biological 

 activity to the value -El^m. (328 m/i) — ^was arranged at the same conference.^ 

 Since 0.344 7 of vitamin A acetate equals 1 I.U., it can be stoichiometrically 

 calculated that pure vitamin A alcohol contains 3.33 X 10^ I.U. per gram. 

 Based on the value of 1750 for E\\,^ (328 m/x) for vitamin Ai alcohol, a 

 conversion factor of 1900 was ascertained. The conference declared this 

 value to be obligatory. Before this final regulation the conversion factor 

 had been subject to great fluctuations. Because of the existence of the so- 

 called irrelevant absorption in fish liver oils, the value for El'^^i. (328 m/x) 

 was not clearly defined. In 1934 it was internationally set at 1600,' but in 

 the United States the value of 2000 was in general use. Consequently 1 

 U.S.P. unit equaled 0.8 I.U.^ With the international conversion factor fixed 

 at 1900, which is generally accepted, such errors are removed. But it must 

 be emphasized here that the conversion factor of 1900 can be correctly 

 used only if the absorption curve obtained from the spectroscopic analysis 

 is identical with that of the standard preparation (see also p. 90). 



VI. Occurrence 



H. H. INHOFFEN and H. POMMER 



The provitamins A occur in the vegetable and animal kingdoms in greater 

 quantities than vitamin A itself. Chemically they belong exclusively to the 



* R. A. Morton, J. Pharm. and Pharmacol. 2, 133 (1950). 

 « C. J. Koehn, Arch. Biochem. 17, 337 (1948). 

 7 See, e.g., E. M. Hume, Nature 151, 535 (1943). 



