Dephosphorylation of Nucleotides of 1-β-D-Arabinofuranosylcytosine in Relation to the Different Drug Sensitivity in Tumor Cells

Dephosphorylation of nucleotides of 1-β-D-arabinofuranosylcytosine (ara-C) was studied to clarify the intracellular metabolism of ara-C and the mechanism of natural resistance in the rat ascites hepatomas. The apparent rate of phosphorylation of ara-C in the intact cells was the balance of the true phosphorylation and the dephosphorylation of nucleotides formed, which was visible after the cessation of the phosphorylation. Ara-C nucleotides formed in the intact cells degraded upon the addition of iodoacetate as a result of ATP depletion in the cells thereby interfering with the ara-C phosphorylation and some additional mechanisms. Decrease in ara-C nucleotides after the dilution of 3H-ara-C with unlabeled ara-C was similar to that after the washing-out of the substrate, and longer than after the addition of iodoacetate. Half-life of ara-C nucleotides was shorter in the more resistant tumors; i.e., the dephosphorylation of the nucleotides was active in AH109A, AH60C, and AH66F in the decreasing order in accord reciprocally with the intracellular level of ara-C nucleotides and their drug sensitivity. Therefore, the rate of the nucleotide dephosphorylation is important together with the phosphorylation of ara-C in the maintenance of the active form of the drug, ara-C triphosphate.