176 VITAMIN D GROUP 



activation may involve the addition of energy to the sterol molecule through 

 an electron displacement. This is an oversimplification of what happens, 

 although it is true that energy is required for the rupture of ring B with 

 the formation of a fourth double bond for the vitamin molecule. 



Estimations of the energy requirements of activation have been made 

 by several investigators,"*'^-^''^' 216-221 j^ost recently by Harris et al}" The lat- 

 ter workers-^'^ found that 7.5 X 10^^ quanta were required to produce one 

 U.S.P. unit of vitamin D2 from ergosterol. The U.S.P. unit official at the 

 time (1938) was nominally identical with the international unit, but ex- 

 perienced bioassayers were aware that the U.S.P. Reference Cod Liver Oil 

 was weak in terms of the international standard. Arnold^^^ found the U.S.P. 

 unit to be 20% weak (49.7 U.S.P.U. = 40 I.U.), a conclusion with which 

 the present author closely agrees. Nelson^-^ recognizes a deficiency of 6.6 %, 

 but this probably refers to a later and improved batch of reference oil. 

 Applying Arnold's correction, one figures that 9.3 X 10^^ quanta are re- 

 quired to produce one international unit of vitamin D2 from ergosterol. 



Bunker et al}^° found no significant differences in quantum efficiency for 

 the wavelengths within the region where ergosterol absorbs strongly, but 

 they got some slight evidence that wavelength 302.5 m/x, which is near the 

 edge of the region of absorption, is less efficient. Haman and Steenbock^''^ 

 had earlier found that line 313, where absorption is still less, is distinctly 

 less efficient. 



In the activation of the provitamin D of cholesterol, Hess and Anderson^"^ 

 demonstrated as early as 1927 that the efficiency of line 313 is very low. 

 Bunker et al.,^^^ working with 7-dehydrocholesterol itself, observed no ac- 

 tivation by line 313. They reported that wavelengths 248.3, 253.7, 265.2, 

 280.4, and 302.5 mju are substantially uniform per quantum of energy 

 applied. But they claimed^"^- 2^° that line 296.7 is a little more efficient 

 than the others, an observation which is difficult to reconcile with the fact 

 that the absorption spectra of 7-dehydrocholesterol and ergosterol are prac- 

 tically identical. 



The production of vitamin D directly in animals by irradiation may be 

 regarded as a special instance of the activation of 7-dehydrocholesterol. It 

 is complicated by the presence in the skin of substances exhibiting general 

 absorption, a form of light-filtering which usually becomes more and more 



216 S. K. Kon, F. Daniels, and H. Steenbock, /. Am. Chem. Soc. 50, 2573 (1928). 



217 R. S. Harris, J. W. M. Bunker, and L. M. Mosher, /. Am. Chem. Soc. 60, 2579 (1938). 



218 A. Knudson and F. Benford, J. Biol. Chem. 124, 287 (1938). 



219 J. R. Owen and A. Sherman, /. Am. Chem. Soc. 59, 763 (1937). 



220 J. W. M. Bunker, R. S. Harris, and L. M. Mosher, /. Am. Chem. Soc. 62, 1760 (1940). 



221 J. W. M. Bunker and R. S. Harris, New Engl. J. Med. 216, 165 (1937). 



222 A. Arnold, Proc. Soc. Exptl. Biol. Med. 63, 230 (1946). 



223 E. M. Nelson, J. Assoc. Offic. Agr. Chemists 32, 801 (1949). 



