Role of transcobalamins I, II, and III in the transfer of vitamin B12 to human bone marrow cells in vitro.

A study of the uptake of transcobalamin-bound 57Co-cyanocobalamin by suspensions of human bone marrow cells has indicated that these cells can take up vitamin B12 from all 3 transcobalamins (I, II, and III). Similar transport processes were involved in the uptake from the 3 transcobalamins; uptake was dependent on the presence of calcium ions, cellular respiration and free sulphydryl groups. These results suggest that contrary to current belief, all 3 transcobalamins play a role in the transfer of vitamin B12 to tissue cells.