1. The dependence of V and V/Km for threonine transport into Trypanosoma brucei upon the external concentration of H+ was studied. 2. Two ionizing groups, the α-amino group of the substrate and a group at the substrate-binding site of the carrier, were found to influence the observed kinetic behaviour of transport. 3. The pK of the group at the substrate-binding site on the free carrier was found to be 6.95 at 30°C and to be temperature-dependent; its heat of ionization was −63.8kJ, which is outside the range for most proton dissociations and suggests a significant contribution from some other source, possibly the remainder of the carrier or the membrane environment. 4. Binding of substrate caused the pK of its α-amino group to shift to a higher value, whereas that of the carrier group shifted to a lower value (6.65 at 30°C). 5. The ionic interaction between substrate and carrier appeared to be involved in the stabilizing of the protonated substrate and the species of the carrier–substrate complex required for the membrane-translocation step. 6. The same ionic species of carrier–substrate complex is required for both substrate dissociation and translocation of the substrate through the membrane. 7. H+ symport or antiport did not occur during threonine uptake.

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