454 VITAMIN Bi2 



in constructing a standard response curve to vitamin B12, from which the 

 potencies of the unknown samples are calculated. 



2. Assay with Rats 



Rats were used for the assay of factor X in natural materials by Gary and 

 Hartman and coworkers in their pinoeering investigations which were dis- 

 cussed on p. 421. These workers drew attention to the necessity of depleting 

 the maternal stores of vitamin B12 so that young rats could be obtained 

 which were sufficiently depleted to be used for the assay which was carried 

 out by observing their growth rate. The investigations of Zucker and Zucker 

 (p. 421) followed similar lines. The degree of depletion encountered in litters 

 of weanling rats was observed to vary with different mothers. Consequently, 

 it was important to subdivide the litters in assembling experimental groups 

 of young rats for the assay. 



The addition of desiccated thyroid or iodinated casein has been used by 

 various investigators to increase the deficiency of vitamin Bi2 in young 

 rats with a view to improving the assay.^'* One procedure was as follows: 

 Male weanling rats (Sprague-Dawley) weighing between 40 and 50 g. were 

 used. They were fed a basal diet of the following composition, supplemented 

 with vitamins A and D fed independently: 



To the basal diet was added 0.06 to 0.1 % of iodinated casein. Five rats 

 were used in each group, and the gain in weight was measured over a 2- 



"U. D. Register, W. R. Rucgamer, and C. A. Elvehjem, J. Biol. Chetn. 177, 129 

 (1949). 



