Effect of Methylthioinosine on Nucleotide Concentrations in L5178Y Cells1

phosphate concentrations declined, indicating turnover of this compound. Incubation of LSI78Y cells in medium containing 0.5 ^M MMPR reduced intracellular concentrations of adenosine triphosphate, guanosine triphosphate, and adenosine diphosphate by about 70% and proliferation rates by about 30%. Upon transfer of such cells to M MPR-free medium, nucleotide levels and proliferation rates returned to control values. L5178Y cells incubated in medium containing 0.1 ÕÕM MMPR proliferated at control rates even though their purine ribonucleotide levels were reduced by as much as 40%. The reduction in cellular pools of purine nucleotides during culture in 0.1 J/M MMPR was accompanied by an increase in the pyrimidine ribonucleotide concentrations. Cellular concentrations of uridine triphosphate and cytidine triphosphate increased almost threefold during the first 2 hr of exposure to 0. l ¿/M MPR, then fell during the next 2 hr to a steady-state level of about 180% of control values.