The utilization of nucleotides by animal cells

We have extended our studies on the toxicity of several arabinosyl nucleotides to L cells. In contrast to a short-term toxic effect of 9-beta-D-arabinofuranosyladenine (araA), both 9-beta-D-arabinofuranosyladenine 5'-phosphate (araAMP) and 9-beta-D-arabinofuranosyladenine 3',5'-cyclic phosphate produced sustained killing of the fibroblasts. 9-beta-D-Arabinofuranosyladenine 2',5'-cyclic phosphate and 9-beta-D-arabinofuranosyladenine-N1-oxide-5'-phosphate were not toxic, whereas 9-beta-D-arabinofuranosylhypoxanthine 5'-phosphate and adenosine 3',5'-cyclic phosphate had relatively slight effects on cell growth. 2',3'-Dideoxyadenosine 5'-phosphate was lethal to L cells, whereas 2',3'-dideoxyadenosine inhibited growth only slightly. The survival time of mice bearing Ehrlich ascites carcinoma was prolonged beyond controls equally well by treatment with either araAMP or araA. To explore the mechanism of nucleotide toxicity, we studied the metabolism of exogenous [3H, 32P]-araAMP. The double-labeled compound entered the cells very slowly and accumulated in both acid-soluble and acid-insoluble materials. 32P and 3H were associated with the adenine nucleotide fractions in ratios similar to that of the exogenous compound. After dephosphorylation of the acid-soluble fraction, greater than 85% of the 3H was associated with araA. Following degradation of the adenine-containing triphosphates with apyrase, all of the 32P and 97% of the 3H were associated with araAMP. Only small quantities of each label were associated with RNA and DNA. Following enzymatic degradation of DNA to 5'-monophosphates, most of both 3H and 32P were associated with the dAMP fraction in a ratio that was similar to that of the exogenous araAMP. After dephosphorylation of the 5'-monophosphates, greater than 90% of the 3H was contained in arabinosyl nucleosides. DNA was also enzymatically degreded to 3'-monophosphates. 3H was found primarily in the 3'-adenine nucleotide fraction with a slight amount in the nucleoside, whereas 32P was distributed among all the 3'-mononucleotides. After dephosphorylation of the 3'-monophosphates, 93% of the 3H was associated with araA. These results indicate that a small amount of araAMP penetrated the cell as the intact nucleotide, was phosphyorlated to the triphosphate and was subsequently incorporated within DNA.