The Heterogeneity of the High Molecular Weight B₁₂ Binder in Serum

The binding of vitamin B₁₂ by serum proteins was studied by separating Co⁵⁷B₁₂-enriched serum by Sephadex gel filtration, column chromatography with DEAE-cellulose, and paper electrophoresis. Each method of separation yielded two discrete B₁₂-binding fractions. However, the analysis of each serum by all three separation technics indicated that one of the fractions was, in each case, bipartite.

The "high" molecular weight B₁₂-binding fraction defined by Sephadex gel filtration consisted of transcobalamin I and just part of the transcobalamin II fraction. The remaining portion of transcobalamin II was eluted from Sephadex gel in a "low" molecular weight fraction. Thus, transcobalamin II, equivalent to the β-globulin B₁₂-binder, consisted of both "high" and "low" molecular weight components.

This suggests that there are at least three serum proteins that can bind vitamin B₁₂: two β-globulins, together comprising the transcobalamin II fraction and differing in molecular weight; and transcobalamin I.