Cobalamin metabolism in cultured human chorionic villus cells

Cobalamin (Cbl, vitamin B12) metabolism was analyzed in cultures of human chorionic villus (CV) cells obtained at 9-10 weeks of gestation. CV cells were shown to synthesize transcobalamin II (TCII) and to possess a high affinity receptor for that molecule. The cells bound and internalized radioactive cyanocobalamin (CN[57Co]Cbl) complexed to TCII. This internalized CN[57Co]Cbl was found to be converted to both methylCbl and adenosylCbl, the two intracellular coenzyme forms of Cbl, and bound to the two known intracellular Cbl requiring enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase. Both enzyme systems were found to be functional in the intact cell by demonstrating the incorporation of the radioactive label from both [14C]CH3-tetrahydrofolate and [14C]propionate into acid insoluble products. MS activity was also detected in lysed cell material. CV cells were shown not to be auxotrophic for methionine since they were able to utilize homocysteine in place of methionine for cell division. Since CV cells are capable of performing many of the complex events associated with Cbl metabolism, it may be possible to use these cells to diagnose genetic defects of Cbl metabolism.