Inhibition of Membrane Transport of 5 - Fluoro ( 6 - 3 H ] deoxyuridine into L 5178 Y Mouse Leukemia Cells 1 Paul

A methylphosphonate derivative of BrdUrd3 is an inhibitor of the cellular uptake of 5F[6-3H]dUnd and other nucleosides (19). BrdUnd-OPO2Me also produces a delayed inhibitory effect on the growth of culture cells (18). The preparation and chemical properties of BrdUrd-OPO2Me have been re ported (18); the methylphosphonate ester linkage is not attacked by the phosphomonoesterases (8). Membrane transport is probably the nate-limiting step in the uptake and incorporation of [6-3H]dUrd into cellular DNA (15). Nevertheless, the choice of a nonmetabolized influx substrate can facilitate the interpretation of transport experiments (11). The uptake of 5F[6-3H]dUrd was chosen as the best available reaction to determine the site of action of BndUnd-OPO2Me.The nucleotide of 5F[6-3H]dUnd is not a potent substrate for thymidylate kinase or thymidylate syn thetase and 5-fluonouracil is not incorporated into the DNA of living cells (7). Thus, all of the enzymes in the pathway from 5-fluorodeoxyunidine 5'-monophosphate to DNA-fluo nounacil can be eliminated from consideration as sites of inhibitory action. Furthermore, BrdUnd-OPO2Me is not an inhibitor of the thymidine kinase in cell-free systems (19). An inhibition of the uptake of 5F[6-3HJdUrd indicates, there fore, that the inhibition by BrdUrd-OPO2Me occurs at a membrane transport system.