Uptake and metabolism of N 5 -formyltetrahydrofolate by L1210 leukemia cells.

A^-Formyltetrahydrofolate was accumulated by L1210 cells by a temperature-dependent process. Accumulation of intracellular radioactivity was inhibited by p-chloromercuribenzoate, iodoacetate, fluoride ion, and ouabain. The rate of influx of A^-formyltetrahydrofolate was decreased by the presence of methotrexate (MTX) or 5-methyltetrahydrofolate but by only high concentrations of folie acid. Radiolabel, accumulated by the LI210 cells from radiolabeled 5-formyltetrahydrofolate (f-FH4), was displaced by 5-methyltetrahydrofolate and MTX. No significant displacement of radioactivity was obtained when folie acid was tested at similar concentrations. Chromatography of accumulated intracellular radiolabels after incubation of the cells with radiolabeled f-FH4 demonstrated that the latter was metabolized rapidly to other folate coenzymes. The efflux of MTX was increased by the addition of f-FH4 to the media. These findings suggest that f-FH4 transport into the LI210 cells is carrier mediated and that the cellular metabolism of this compound is rapid. This carrier system appears to be utilized also by MTX and 5-methyltetrahydrofolate and, to a lesser extent, by folie acid. Additional mechanisms by which f-FH4 reverses the action of MTX (namely, competition for uptake and enhancement of MTX efflux) are suggested by these studies.