Incorporation and Metabolic Conversion of Cyanocobalamin by Ehrlich Ascites Carcinoma Cells in Vitro and in Vivo

1. Cyano [57 Co] cobalamin bound to murine transcobalamin, associates with Ehrlich ascites carcinoma cells. Association was found to be dependent on temperature, and to require between 7.2 - 10-5 and 2 - 10-4 M ionized calcium. 2. Association was blocked by vinblastine and colchicine, but not cytochalasin, suggesting that microtubules may be involved in this phenomenon. 3. Although irreversible association of radioactivity with cells was observed within minutes, appearance of significant radioactivity associated with the intracellular B12 binder, and conversion of cyanocobalamin to methyl- and 5'deoxyadenosylcobalamin required more than 18 h of incubation. 4. A pool of free vitamin B12 was found in cell extracts. This was composed of metabolically-active cobalamins characteristic of the interior of the cell, and not of cyanocobalamin recently incorporated. 5. Incorporation of 57Co-labelled vitamin B12 by these cells involves two major processes: a rapid irreversible association of transcobalamin-B12 complex following reaction with a presumably calcium-dependent receptor, and a much later entry of vitamin into the cytoplasm to become exposed to enzymes and associated with an intracellular binder.