FIGURE 3: L-ascorbate inhibits UapA transport activity. The graph shows dose response curves of [³H]-xanthine uptake rate in strains expressing either wild-type UapA or the isogenic mutant UapA-R481G/T526M, in the presence of increasing concentration of non-radiolabeled L-ascorbate (0-56 mM). In both cases, L-ascorbate inhibited UapA transport activity, with K_i values estimated at 17 and 11 mM for UapA and UapA-R481G/T526M, respectively. Results are averages of three measurements for each concentration point. SD was < 18%. For details of measurements and K_i estimation see Materials and methods.