Contributions of the depletions of guanine and adenine nucleotides to the toxicity of purine starvation in the mouse T lymphoma cell line.

The importance of the depletion of guanine nucleotides in cellular toxicity was compared to that of the depletion of adenine nucleotides following the inhibition of early de novo purine bio synthesis in the mouse T lymphoma (S-49) cell line. Early de novo purine biosynthesis was blocked with 6-methylmercaptopurine ribonucleoside (6MMPR), while adenine and guanine nucleotide biosyntheses were individually inhibited using u-alanosine (alanosine) and mycophenolic acid (MA), respectively. Incu bation with either alanosine or MA depleted adenosine 5'-triphosphate or guanosine S'-triphosphate levels, respectively, to the same extent as those caused by 6MMPR. The effects of a 3-hr incubation with either 6MMPR or MA on nucleic acid syn thesis were similar: a partial inhibition of RNA synthesis; and a dramatic inhibition of DMA synthesis. However, a 3-hr incubation with alanosine did not significantly affect either RNA or DNA synthesis. Furthermore, the effect of exogenous sources of adenine or guanine nucleotides on adenosine 5'-triphosphate and guanosine 5'-triphosphate levels, and DNA synthesis rates in cells pretreated with 6MMPR for 3 hr were studied. Resump tion of DNA synthesis was dependent on the return of guanine nucleotide levels, but not of adenine nucleotide levels, to normal. Finally, MA and 6MMPR had the same effect on the progression of cells through the cell cycle, while the effect of alanosine was dramatically different. These results suggest that the biological consequences of purine starvation are primarily mediated by the depletion of guanine nucleotides rather than that of adenine nucleotides. We previously found that the depletion of guanine ribonucleotides rather than that of guanine deoxyribonucleotides is associated with the toxicity when guanine nucleotide biosyn thesis is inhibited and that the depletion is associated with the inhibition of DNA synthesis. These combined results indicate that a function of guanine ribonucleotides, probably in DNA synthesis, is the cellular process most sensitive to the inhibition of early de novo purine biosynthesis.