Previous studies from this laboratory demonstrated that marked suppression of thymidylate synthase activity is required to slow the rate of interconversion of tetrahydrofolate cofactors to dihydrofolate when dihydrofolate reductase is blocked by an antifolate. This finding is due to the high catalytic activity of thymidylate synthase within cells in comparison to the tetrahydrofolate cofactor pool size. In the present study, we assessed the rate of resumption of thymidylate synthase catalytic activity in terms of [3H]deoxyuridine incorporation into DNA and dihydrofolate generation from tetrahydrofolate cofactors following exposure of cells to fluorodeoxyuridine. Log phase L1210 leukemia cells, incubated with fluorodeoxyuridine to abolish thymidylate synthase catalytic activity, were suspended into drug-free medium. Resumption of [3H]deoxyuridine incorporation into DNA was negligible; by 4hr enzyme activity was still inhibited by ∼98%. However, this was sufficient to interconvert all available tetrahydrofolate cofactors to dihydrofolate (T 1 2 ∼ 2 hr) when dihydrofolate reductase was inhibited by the lipophilic antifolate trimetrexate. Interconversion of tetrahydrofolate cofactors to dihydrofolate correlated with a decline, then cessation, of purine synthesis as measured by the incorporation of [14C]formate into purine bases. These data suggest that an earlier than previously expected depletion of tetrahydrofolate cofactors with consequent inhibition of purine and other folate-dependent synthetic processes is likely to occur when antifolates are administered after a fluoropyrimidine.
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