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Purine and pyrimidine analogues irreversibly prevent passage of lymphocytes from the G1 to the S phase of the cell cycle.

Six-hour pulses of the purine analogue 8-azaguanine (8-AG) and the pyrimidine analogue 5-fluorouracil (5-FU) produced a novel irreversible effect on mouse and human lymphocytes. Cells treated with these analogues early during culture with concanavalin A and then washed in presence of excess natural base could pass normally through the various stages of blast formation (e.g., increased K+ transport, increase in nuclear and cytoplasmic volume, disaggregation of chromatin), but showed a severe inhibition of DNA synthesis when this was measured by [3H]thymidine incorporation at 48 h of culture; this was true irrespective of whether the 6-h pulse with analogue occurred at 0, 12, or 24 h of culture in the presence of mitogen. The analogue 6-mercaptopurine, which strongly inhibited DNA and RNA synthesis while present in the medium, had no irreversible effects, unlike 5-FU and 8-AG. The persistence of the effects of 5-FU in presence of excess thymidine in the medium suggested that inactivation of thymidylate synthetase was not responsible for the inhibition observed here. The effect was expressed in the presence or absence of protein synthesis; therefore, the observed inhibition of proliferation was not due to synthesis of a toxic protein, but to an effect on the formation or function of the DNA synthesizing system and (or) on its template, thus preventing the cells from passing from the G1 to the S phase of the cell cycle.