Abstract
1) The total vitamin B_<12> concentration of forty normal sera was found to vary from 160 to 812 μμg/ml with a mean of 415 μμg/ml. The range of the free form of vitamin B_<12> in these sera was from 0 to 83 μμg/ml. 2) By the paper electrophoretic examination of the human sera it was clarified that the bound form of vitamin B_<12> is bound by α-globulin especially by α_2-globulin and the free form by β-globulin. 3) When excess vitamin B_<12> was added to the normal sera in vitro and unbound vitamin was dialysed off, the bound vitamin was found mainly in α-and β-globulins. The binding ability of α-globulin appears to be stronger and larger than that of β-globulin. In the saturated normal sera with vitamin B_<12> the amount of the vitamin in α-globulin is 1.5-1.7 mμg/ml and that in β-globulin is 0.7-1.1 mμg/ml. 4) The significance of the vitamin B_<12>-binding proteins in the serum and its relation to the liver functions were discussed.
