Inhibition of membrane transport of 5-fluoro[6-3H]deoxyuridine into L5178Y mouse leukemia cells

The influx of 1.0 muM 5-fluoro[6-3H]deoxyuridine (5F[6-3H]dUrd) into L5178Y mouse leukemia cells followed a linear function with time from 2 to 10 min. Ammonium 5-bromodeoxyuridine 5'-methylphosphonate (BrdUrd-OPO2Me) inhibited the membrane transport of 5F[6-3H]dUrd into L5178Y cells. Influx of 5F[6-3H]dUrd into inhibited cells was observed from zero to 3 min; after 3 min the net rate of 5F[6-3H]dUrd uptake into the cells treated with 18 muM BrdUrd-OPO2Me was almost zero. The cellular uptake of 2'-deoxy[6-3H]uridine or 5-bromo[6-3H]deoxyuridine was inhibited by BrdUrd-OPO2Me. The L5178Y cells were grown for 96 hr in a medium that contained tritium-labeled BruDur-OPO2Me. An analysis of the labeled products in the growth medium showed that the ester linkage is not cleaved to separate the [3H]methylphosphonate group and the nucleoside moiety of BrdUrd-OPO2[3H]Me. The activity of thymidine kinase in a cell-free preparation from L5178Y cells was demonstrated. Although 37 muM 5-bromo-2'deoxyuridine produced an inhibition of approximately 45% in kinase activity, BrdUrd-OPO2Me had no effect on enzyme activity. The results indicate that BrdUrd-OPO2Me is an inhibitor of the cell membrane transport of the 5-fluoro and 5-bromo derivatives of 2'-deoxy[6-3H]uridine.