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.

Protein-mediated B12 uptake by isolated rat liver mitochondria has been shown to be enhanced by plasma transcobalamin (TC-II) but not by salivary R binder in vitro. The process is enhanced by calcium and depends on active mitochondrial respiration. Following uptake, cyanocobalamin is converted to adenosyl and methylcobalamins and released from the mitochondria. TC-II appears to be required for both cellular and mitochondrial uptake of vitamin B12.

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