(6R)-5,10-Dideaza-5,6,7,8-tetrahydrofolic Acid Effects on Nucleotide Metabolism in CCRF-CEM Human T-Lymphoblast Leukemia Cells1

(6R)-5,10-Dideaza-5,6,7,8-tetrahydrofolic acid |(6R)DDATHF| is a folate antimetabolite with activity specifically directed against de novo purine synthesis, primarily through inhibition of glycinamide ribonucleotide transformylase. This inhibition resulted in major changes in the size of the nucleotide pools in CCRF-CEM cells. After a 4-h incubation with l ¿IM(6R)DDATHF, dramatic reductions in the ATP and GTP pools were observed, with almost no effect on CTP, DTP, and deoxyribonucleotide pools. When the incubation was continued in drug-free medium, recovery of ATP and GTP pools was protracted. ATP did not return to normal until 24-36 h, and GTP pools were only partially repleted by 48 h. The ATP and GTP pools were not affected when the initial 4-h incubation with (6R)DDATHF was conducted in the presence of 100 /J.Mhypoxanthine. Addition of hypoxanthine to the medium after a 4-h incubation with (6R)DDATHF caused rapid recovery of the ATP and GTP pools. Similar effects were seen when the purine precursor aminoimidazole carboxamide was used in place of hypoxanthine. The effect of (6R)DDATHF on nucleotide pools and the capability of hypo xanthine or aminoimidazole carboxamide to prevent or reverse this phenomenon correlated directly with the inhibition of cell growth. Pre sumably as a consequence of the decrease in purine nucleotide triphosphate levels, the conversion of exogenous!) added uridine, thymidine, and deoxyuridine to nucleotides was markedly decreased. These effects were protracted for almost 48 h and were also reversed by hypoxanthine. Differential repletion of ATP and GTP pools after (6R)DDATHF pre treatment demonstrated that diminished precursor phosphorylation is primarily a consequence of GTP rather than ATP starvation.