Pterins and the regulation of lymphocyte activation on the mode of xanthopterin action

Some intermediates of pterin anabolism amplify the lectin-induced lymphocyte stimulation while the catabolites xanthopterin and isoxanthopterin terminate their proliferation (Ziegler, I. et al., Cancer Res. 43, 5356 (1983). In the present investigation, we analysed the effect of xanthopterin on total RNA synthesis and on DNA synthesis in both concanavalin A-stimulated lymphocytes and in the lymphoblastoid cell line L 1210. The time courses at various inhibitor concentrations indicated that xanthopterin inhibits RNA synthesis prior to DNA synthesis. Further analysis of the RNA species was performed by double-labeling and subsequent polyacrylamide-gel electrophoresis. Pulse and pulse-chase experiments revealed that an inhibition of 45 S pre-RNA is closer to the target of xanthopterin inhibition than is DNA synthesis.