FOUR crystalline preparations of vitamin D have recently been described1 having approximately equal antirachitic activities (20,000–25,000 International Units per milligram) and similar absorption spectra, but differing widely in their optical rotations. It follows either that there are several substances having antirachitic activity or that none of these preparations is pure, and that the varying optical rotation is due to the presence of one or more impurities.
Askew et al., Proc. Roy. Soc., B, 107, 76; 1930: Angus et al., ibid., B, 108, 340; 1931: Reerink and van Wijk, Biochem. J., 23, 1294 ; 1929: 25, 1001; 1931: Windaus, Proc. Roy. Soc., B, 108, 568; 1931: and Liebigs Ann., 489, 252; 1931.
Windaus et al., loc. cit.
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Critical Reviews in Clinical Laboratory Sciences (2014)
Physiological Reviews (2013)
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The Journal of Nutrition (2012)
Archives of Biochemistry and Biophysics (2011)