Metabolism of Cyanocobalamin by L-1210 Leukemic Lymphoblasts

Abstract

Conclusions

In our previous experiments (4) we demonstrated that L1210 leukemic lymphoblasts were capable of taking up vitamin B-12 when bound to transcobalamin II. Intracellular B-12 was also found to be bound to a protein with the characteristics of TCII. In the present study we have demon-strated that L-1210 cells do not themselves contain a TCII-like protein in the cytoplasmic soluble phase. We have further demonstrated that a portion of this protein-bound B-12 has been converted to the adenosyl and methyl forms and that such coenzyme forms may be countertransported to the exterior of the cell. These observations are consistent with our original suggestion that the entire B-12 complex crosses the plasma membrane. TCII may deliver B-12 to mitochondria where conversion to the coenzyme forms occurs. A portion of the converted vitamin appears to be transported out of the cell where it is free to recirculate.