The Binding of Vitamin B12 by Serum Proteins in Normal and B12-Deficient Subjects

Endogenous B₁₂ in normal serum has been shown to be associated with α globulins (Pitney, Beard and Van Loon, 1954; Heinrich and Erdmann-Oehlecker, 1956; Mendelsohn, Watkin, Horbett and Fahey, 1958). When B₁₂ has been added to normal serum, however, most or all of the added B₁₂ has been associated with the β- and α₂-globulins (Miller, 1958). In the present investigation with radioactive ₅₇cobalt cyanocobalamin (B₁₂*) added to serum, it was possible to define at least two B₁₂-binding proteins (BP), a β-globulin, and an α₁ globulin. B₁₂* added to the serum of normal subjects migrated mainly with the β-globulins on electrophoresis, but when added to the serum of B₁₂-deficient subjects, a small fraction of the B₁₂* was also associated with the α₂-globulins.