Effects of 5-mercapto-2'-deoxyuridine on the incorporation of nucleosides into RNA and DNA in a primary lymphocyte culture system.

The effects of 5-mercapto-2′-deoxyuridine (MUdR) on DNA synthesis in a primary murine spleen lymphocyte culture system stimulated by phytohemagglutinin (PHA) were studied. Inhibition of thymidine incorporation into acid-insoluble nucleic acid material was 50% at 0.5 mm MUdR concentration, while inhibition of deoxyuridine incorporation into acid-insoluble nucleic acids was 50% at 0.01 mm MUdR. Time course studies, at 0.5 and 0.05 mm MUdR, showed that the magnitude of inhibition of incorporation for thymidine and deoxyuridine, respectively, increased from a time point after PHA stimulation when increased synthesis of thymidine kinase and thymidylate synthetase had leveled off. At 1 mm MUdR, total cellular DNA in cultures was decreased 43% at 42 hr after PHA stimulation. Neither the total number of cells nor the percentage of PHA-transformed cells was decreased in comparison to that of controls. MUdR therefore blocks the increase in DNA content of lymphocytes that is initiated during the S phase of the cell cycle. Millimolar levels of MUdR inhibited incorporation of uridine, adenosine, and cytidine into acid-insoluble material in PHA-stimulated primary murine lymphocyte cultures. Total cellular RNA synthesis was inhibited at these levels of MUdR, with no differential effects on 4, 18, or 28 S RNA species observed. Uptake of these nucleosides into the total cellular acid-soluble material was not blocked. Uptake of different labeled nucleosides into cellular, acid-soluble pools occurs at different rates. Thus, choice of a suitable minimum pulse time to achieve saturation for different labeled nucleosides must relate to this consideration. Thymidine kinase from whole-cell sonic extracts of PHA-stimulated lymphocytes was inhibited 65% by 1 mm MUdR at 24 and 48 hr after stimulation. Uridine kinase extracted from the PHA-stimulated cells was also significantly inhibited by 1 mm MUdR at 24 hr (56%). Exogenous guanosine incorporation into lymphocyte acid-insoluble material is increased by MUdR. This increased utilization of exogenous nucleoside is apparently the result of MUdR inhibition of conversion of adenosine to guanine nucleotides within the lymphocytes and a consequent diminution of the total intracellular guanine nucleotide pool size. The active inhibitory compound is the deoxyribonucleoside or deoxyribonucleotide. Comparison with the riboside analog 5-mercaptouridine showed that MUdR was a more efficient inhibitor of nucleoside incorporation.