Uptake and Subcellular Distribution of Vitamin B₁₂ in Mouse L1210 Leukemic Lymphoblasts

Protein-mediated B₁₂ uptake by L1210 lymphoblasts was shown to be calcium dependent and enhanced by TCII but not by TCI in vitro. Subcellular fractionation resulted in the majority of B₁₂ localized in the soluble phase with significant amounts in the mitochondria. All vitamin B₁₂ found in the soluble phase was bound to a protein similar in molecular weight to TCII. This protein was capable of delivering B₁₂ to fresh L1210 cells.