Saturation of glutamate uptake. Effect of unlabeled L-glutamate on the uptake of L-[³H]-glutamate into (A) arsenite-treated uninfected human erythrocytes (RBCs) suspended in (glucose-free) PBS and (B-C) P falciparum–infected human erythrocytes suspended in the presence (B) and absence (C) of extracellular Na⁺ (with the extracellular solutions as described in Figure 2). In each case the uptake of L-[³H]-glutamate was measured for 10 minutes at 37°C, with the concentration of unlabeled L-glutamate in the extracellular solution ranging from 0 to 100μM. Uptake of L-[³H]-glutamate is expressed as a percentage of that measured in the absence of unlabeled L-glutamate in Na⁺-containing medium. The data are averaged from 3 independent experiments. (A) The inset shows the concentration-dependence of L-glutamate uptake, with the line drawn using the Michaelis-Menten equation (K_m of 55 ± 9nM; V_max of 17.2 ± 1.7 nmol/[10¹² cells/hour]). The error bars indicate ± SEM. (B-C)The insets are Eadie-Hofstee plots of the data, which illustrate the fact that both in the presence (B) and absence (C) of extracellular Na⁺ there is one or more high-affinity component (dotted line) and 1 or more low-affinity component (dashed line) to the uptake of L-glutamate into P falciparum–infected erythrocytes. For clarity, the data are shown without error bars, with the exception of the inset in panel A.