57Co cyanocobalamin mixed with serum filters through sephadex G-150 in two fractions, corresponding to apparent molecular weights of about 120,000 and 36,000, respectively. Much of the native B12 also filters in these fractions, but some is excluded from the gel, and a large fraction filters in the position of free B12. When heparin is mixed with serum or plasma, most of the B12 filters with the 120,000 fraction. When EDTA is mixed with serum or plasma not containing heparin, less 57CoB12 filters in the 120,000 fraction, and more filters in the 36,000 fraction. The 120,000 and 36,000 fractions cannot therefore be assumed to correspond precisely with transcobalamin 1 and 2, respectively.

It is suggested that heparin binds transcobalamin 2 and that EDTA dissociates either TC2 complexes or some of the TC1.

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