Fig. 4.

Transport of [³H]-uracil by L. mexicana promastigotes. (A) Transport of 1 μM [³H]-uracil was linear over 120 s with a rate of 0.0030 ± 0.0002 pmol(10⁷ cells)⁻¹s⁻¹ (●, r² = 0.974; P < 0.0001) and was fully inhibited by 1 mM unlabeled uracil (□, not significantly different from zero, P = 0.73). (B) Transport of 0.5 μM [³H]-uracil was linear over 10 min with a rate of 0.0020 ± 0.0002 pmol(10⁷ cells)⁻¹s⁻¹ (●, r² = 0.981; P = 0.0011) and was 95% inhibited by 1 mM unlabeled uracil. (C) Transport of 0.5 μM [³H]-uracil was dose-dependently inhibited by uracil (●) and by 5-fluorouracil (□). The inhibition data for unlabeled uracil were converted to a Michaelis-Menten saturation plot (inset). (D) Inhibition of 0.5 μM [³H]-uracil transport by uridine (▲) and by 2′-deoxyuridine (○). The level of inhibition by 1 mM unlabeled uracil is indicated (▼). Unit for transport was pmol(10⁷ cells)⁻¹ for frames A and B, and pmol(10⁷ cells)⁻¹s⁻¹ in frames C and D; symbols represent the average of triplicate determinations in a single representative experiment, and error bars represent SEM.