Fig. 6.

Transport of 0.25 μM [³H]-uridine by L. major promastigotes. (A) Uptake of [³H]-uridine in the presence (□) or absence (●) of 1 mM unlabeled uridine. The rate of uptake at 0.25 μM uridine was determined as 0.0024 ± 0.0002 pmol(10⁷ cells)⁻¹s⁻¹ by linear regression (r² = 0.964; P = 0.0005), but was not significantly different from zero (P = 0.94) in the presence of 1 mM unlabeled permeant. (B) Sigmoid inhibition curves for uridine (●) and uracil (□). The former was converted to a Michaelis-Menten saturation curve (inset). (C) Conversion of the uridine inhibition data of panel B to a Lineweaver-Burk double reciprocal plot, showing separate linear regression lines for the high affinity (□, r² = 0.999) and low affinity (▲, r² = 0.981) components. Inset: zoom-in of main plot. (D) Sigmoid inhibition plots with inosine (□), adenosine (●) and adenine (▼). Also shown are individual points showing the level of inhibition with 1 mM uridine (◊) and with 1 mM adenine + 1 mM inosine (▲). (E) Time course of 0.25 μM [³H]-uridine transport over 20 min in the presence of 100 μM inosine (▼, r² = 0.967, rate significantly different from zero P = 0.017 (F-test)), in the presence of 1 mM uridine (□, rate not significantly different from zero, P = 0.22 (F-test)) or without any inhibitors (●, r² = 0.993, rate significantly different from zero P = 0.0035 (F-test)). Unit for transport was pmol(10⁷ cells)⁻¹ for frames A and E, and pmol(10⁷ cells)⁻¹s⁻¹ in frames B-D; symbols represent the average of triplicate determinations in a single representative experiment, and error bars represent SEM.