Abstract
1. The total and free vitamin B12 of the serum of 40 normal human subjects was measured. The results showed that the total vitamin level ranged from 160 to 812μμg per ml with a mean value of 415μμg and the free form level from 0 to 84μμg per ml.2. The bound form of vitamin B12 in human serum is bound to the proteins in the α-(chiefly α2) globulin fraction and the free form seems to be bound loosely to the protein in the β-globulin fraction.3. α- and β-globulins have the capacity to bind added vitamin B12 in excess of the quantity normally present. The former takes the binding type of the bound form, whereas the latter the binding type of the free form. The binding capacity of the former is stronger and greater, the saturation level being 1.0-1.7mμg/ml, whereas that of the latter 0.7-1.1mμg/ml.4. The physiological significance of the two types of vitamin B12 bound with proteins present in the serum and the relationship to liver function is discussed.
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