Effects of cobalamin, cobalamin analogues and cobalamin binding proteins on P388D1 mouse leukemic cells in culture.

Cobalamin-deficient P388D1 mouse leukemic cells were created by propagation in a cyanocobalamin-free medium in which the original fetal bovine serum was replaced by bovine serum albumin. These cobalamin-deficient cells gradually ceased to multiply when the medium contained 5-methyltetrahydrofolate. The growth of cells that had been cultured with this coenzyme was recovered following the addition of cyanocobalamin (CNCbl), at concentrations above 37 pM. In contrast to the effect of CNCbl, cobinamide, and cobalamin analogues prepared from hydroxy cobalamin by reaction with ascorbic acid, did not have a growth-inducing effect on these cells, nor did these analogues inhibit CNCbl-dependent growth. Transcobalamin II-cobalamin complex had a remarkably stimulating effect on cell growth. The growth inducing effect became apparent with a cobalamin concentration of only 0.37 pM. This was about 1/100th the level of free cobalamin required for cell growth. However, no growth-inducing effect was seen at an R protein-bound cobalamin concentration of 37 pM, indicating that once cobalamin has been bound to R protein, it loses its growth-promoting effect on these cells in culture.