782 VIII. PROVITAMINS D AND VITAMINS D 



However, secondary standards are employed in the United States and in 

 Great Britain. In the United States we have the U.S. Pharmacopoeia 

 Reference cod hver oil, while in England a similar standard is used for the 

 M.R.C. Unit (Medical Research Unit). The reference cod liver oil used 

 here has been accurately standardized against the International Standard 

 by bioassays on rats, and its potency has been determined on the basis of 

 such tests. The U.S. P. units refer to those established by the use of the 

 U.S.P. Reference cod liver oil. Since birds, and especially chickens, are 

 unable to utilize vitamin D2 efficiently, the International Standard is not 

 of importance in establishing the potency of vitamin preparations used in 

 chick feeding. However, the standard employed in this case is the "A.O.- 

 A.C. Chick Unit," introduced by the Association of Official Agricultural 

 Chemists, which is defined^^^ as the potency of one unit of vitamin D of 

 the U.S.P. Reference cod liver oil when determined under standard condi- 

 tions. Since the vitamin D3 content of various cod liver oils varies in rela- 

 tion to its vitamin D2 content, it would be advisable to replace the chick 

 standard by one composed of crystalline vitamin D3. Still better, if the 

 International Standard were changed to crystalline vitamin D3 instead of 

 vitamin D2, the primary standard would serve equally well for the rat and 

 for the chicken. 



One International Unit is therefore equivalent to one U.S.P. Unit, one 

 M.R.C. Unit, one Coward Unit, 5-6 Poulsson Units, 6-8 Laquer Units, 

 2.6 Prophylactic Units, 3.25 ADMA Units, 1.66 Oslo Units, or 0.025 mg. of 

 crystalline vitamin D2. 



4. Structures of the Vitamins D 



The provitamins and the corresponding vitamins have closely related 

 structures; each pair has the same empirical formula, and thus it is obvious 

 that they must represent isomers. Moreover, the several D vitamins dif- 

 fer from each other only in the side chain; the sterol structure of the nu- 

 cleus is the same in all cases for the provitamins, and the modified structure 

 derived from the steroid nucleus after activation of the provitamins is the 

 same for all of the vitamins. 



Although the structural formulas have not been worked out for all the 

 known D vitamins, very complete proof is available in the case of vitamin 

 D2. It has been possible to demonstrate by certain key reactions that the 

 general structure of vitamin D3 (exclusive of the side chain) is identical 

 with that of vitamin D2. In the case of the other D vitamins, the formulas 

 have been deduced by analogy. 



2*' Association of Official Agricultural Chemists, Methoda of Analysis, 5th ed., 1940, 

 pp. 371-373. 



