Synthesis of cobalamin coenzymes by human lymphocytes in vitro and the effect of folates and metabolic inhibitors

Abstract

The uptake of57Co cyanocobalamin (CN Cbl) and its conversion to 5 deoxyadenosylcobalamin (Ado Cbl), methylcobalamin (Me Cbl), and hydroxocobalamin (OH Cbl) has been studied in phytohemagglutinin (PHA) transformed lymphocytes from normal subjects and patients with pernicious anemia. Uptake and conversion were much greater by PHA stimulated lymphocytes than by mature nontransformed lymphocytes. In normal cells, uptake of57Co CN Cbl and synthesis of the cobalamin coenzymes were approximately linear between 3 and 48 hr incubation. Ado Cbl was the major cobalamin formed, and after 72 hr the cells contained about twice as much Ado Cbl as Me Cbl. Uptake by lymphocytes from patients with untreated pernicious anemia (PA) was greater than that by normal lymphocytes, but the proportions of Ado Cbl and Me Cbl synthesized by each were similar. Folic acid and methyltetrahydrofolate enhanced synthesis of Me Cbl both in normal and in PA cells, while methotrexate and 5 fluorouracil depressed it. This depression was overcome by 5 formyltetrahydrofolate, suggesting that an uninterrupted folate cycle may play an important role in Me Cbl synthesis.