The dynamics of cobalamin utilization in L-1210 mouse leukemia cells: a model of cellular cobalamin metabolism.
The uptake and metabolism of cobalamin (Cbl) has been studied in L-1210 murine leukemia cells propagating in vitro. Extracellular Cbl (protein bound and free) and intracellular Cbl (protein bound and free) were determined after culturing L-1210 cells in the presence of [57Co]cyanocobalamin (CN-Cbl) bound to transcobalamin II (transcobalamin, TC). The intracellular pool of free [57Co]Cbl increased during the first 24 h of culture and a substantial fraction of this free pool was effluxed from the cell to the medium. Upon depletion of extracellular TC-[57Co]CN-Cbl, the intracellular concentration of free Cbl decreased as did the efflux of Cbl to the medium. Internalized [57Co]CN-Cbl was converted to hydroxocobalamin (OH-Cbl), methylcobalamin (Me-Cbl) and 5'-deoxyadenosylcobalamin. These Cbl forms were found in both soluble (cytoplasmic) and insoluble (membrane) fractions. Intracellular protein-bound [57Co]Cbl fractionated with methionine synthase (MS) and methylmalonyl-CoA mutase (MU) activity. The major form of Cbl associated with the two enzymes was OH-Cbl. Cells propagated in medium containing N5-methyltetrahydrofolate and homocysteine showed a substantial increase in MS activity which paralleled the increase in the intracellular concentration of Me-Cbl and the Cbl bound to the enzyme.
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CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL GENE-Y CHEMICAL GENE-Y GENE-N GENE-N GENE-Y GENE-Y GENE-Y GENE-Y CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL
CN-Cbl cobalamin [57Co]Cbl Cbl TC-[57Co]CN-Cbl Cbl Cbl [57Co]CN-Cbl hydroxocobalamin OH-Cbl Cbl methylcobalamin Me-Cbl 5'-deoxyadenosylcobalamin Cbl [57Co]Cbl methionine methylmalonyl-CoA cobalamin cobalamin MS OH-Cbl transcobalamin II transcobalamin TC methionine synthase MS methylmalonyl-CoA mutase MU N5-methyltetrahydrofolate homocysteine Me-Cbl Cbl Cbl Cbl [57Co]cyanocobalamin