Folate uptake in L1210 cells: mediation by an adenine transport system.

Uptake of folate by L1210 cells in mediated by a transport system whose primary substrate is adenine. This conclusion is based upon the following evidence: (a) Folate uptake is inhibited competitively by adenine; (b) The K_t for folate transport (430 μM) is comparable to the K_i (450 μM) for folate inhibition of adenine transport; (c) The K_t for adenine transport (21 μM) agrees with the K_i (17 μM) for inhibition of folate transport by adenine; (d) The adenine analogs, 1-methyl-3-isobutylxanthine and 6-mercapto-purine, each inhibit folate and adenine transport to a comparable degree; and (e) Rates of folate and adenine uptake vary in parallel fashion during growth of L1210 cells.