Polyglutamylation as a Factor in the Trapping of 5-Methyltetrahydrofolate by Cobalamin-Deficient L1210 Cells

Abstract

Cobalamin-deficient L1210 mouse leukemia cells, developed by propagation in a medium from which cyanocobalamin was omitted and fetal bovine serum was replaced by bovine serum albumin, grew normally on excess folate or 5-formyltetrahydrofolate. These cells responded poorly, however, to 5-methyltetrahydrofolate unless exogenous cobalamin was added. A cobalamin dependency was also observed when low levels of folate or 5-formyltetrahydrofolate were used. With 5-methyltetrahydrofolate, optimal stimulation of growth was observed with free and transcobalamin II-bound cobalamin at 4,000 and 2 pM, respectively. Under cobalamin-replete conditions, the average cobalamin content was ca. 3,000 molecules/cell, and in the deficient state this value declined to < 10 molecules/cell. Optimal replication on 5-methyltetrahydrofolate required ca. 180 molecules/cell.

Preliminary accounts of these data were presented at the VI International Symposium on the Chemistry and Biology of Pteridines (La Jolla, California) and the Third European Symposium on Vitamin B₁₂ and Intrinsic Factor (Zurich, Switzerland). This work was supported by Grants CA06522 and AM07097 from the National Institutes of Health, and CH-31 from the American Cancer Society.