Cellular fluxes of vitamin B12

Blood. 1977 Nov;50(5):877-88.


Incorporation of vitamin B12 into L1210 cells requires the protein binder transcobalamin II (TCII). The process is saturable, follows Michaelis-Menten kinetics (Km = 2.5 X 10(-9) M at 37 degrees C), is both temperature and calcium (K50 - 1 X 10(-6) M) dependent, and is inhibited by apo-TCII, indicating the presence of a TCII specific receptor on the cell membrane. B12 also leaves the cell by a calcium-independent pathway bound to either TCII or to a protein with chromatographic properties similar to those of TCIII. Since intact TCII-B12 can be found in the cytosol and can promote B12 uptake by mitochondria, it is proposed that the B12 released from the cell bound to the TCIII-like protein is derived by mitochondrial processing of incorporated TCII-B12. The slower time course of release of the latter B12 is consistent with this postulate.

Publication types

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

MeSH terms

  • Animals
  • Kinetics
  • Leukemia L1210 / metabolism
  • Receptors, Drug
  • Transcobalamins / metabolism
  • Vitamin B 12 / metabolism*


  • Receptors, Drug
  • Transcobalamins
  • Vitamin B 12