Uptake and metabolism of free cyanocobalamin by cultured human fibroblasts from controls and a patient with transcobalamin II deficiency

Metabolism. 1981 Mar;30(3):230-6. doi: 10.1016/0026-0495(81)90146-3.


We have investigated the uptake and metabolism of free cyanocobalamin (CN-Cbl; vitamin B12) by intact cultured human skin fibroblasts. Monolayers of control fibroblasts take up free CN-[57Co]Cbl via a saturable, calcium-independent process that is inhibited by sulfhydryl reagents, inhibitors of protein synthesis, and inhibitors of electron transport, but not by inhibitors of glycolysis. CN-Cbl taken up in this manner is converted to active cobalamin (Cbl) coenzymes (adenosylcobalamin and methylcobalamin) and becomes associated with intracellular Cbl-dependent apoenzymes (methylmalonyl CoA mutase and homocysteine:methyltetrahydrofolate methyltransferase). Since fibroblasts from controls were also found to synthesize transcobalamin II (TC II), a plasma protein shown previously to facilitate the cellular uptake of Cbl, it seemed possible that the observed uptake of free CN-Cbl was TC II-mediated. This thesis was rejected by demonstrating that cells from a patient with complete TC II deficiency took up free CN-Cbl as well as control cells did. Finally, we propose a mechanism by which an uptake process for free Cbl might serve a function in intracellular metabolism of Cbl.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Blood Proteins / deficiency*
  • Cells, Cultured
  • Cycloheximide / pharmacology
  • Ethylmaleimide / pharmacology
  • Fibroblasts / metabolism*
  • Humans
  • Kinetics
  • Potassium Cyanide / pharmacology
  • Transcobalamins / deficiency*
  • Vitamin B 12 / metabolism*


  • Blood Proteins
  • Transcobalamins
  • Cycloheximide
  • Potassium Cyanide
  • Ethylmaleimide
  • Vitamin B 12