The Electrochemical Proton Gradient in the Bloodstream Form of Trypanosoma Brucei is Dependent on the Temperature

The membrane potential and pH gradient over the plasma membrane of the protozoan parasite Trypanosoma brucei were measured with radioactive indicators in combination with the silicone oil centrifugation technique over a range of temperatures. At 37°C a small membrane potential and pH gradient of similar magnitude, but of opposite polarity, were measured. The resulting electrochemical proton gradient was almost zero. However, when the temperature was lowered from 37°C to 22°C, the internal pH was kept constant independent of the external pH and a membrane potential of between -- 100 and - 150 mV was measured, depending on the external pH. Measurements at various temperatures between 15°C and 37°C revealed that above 26°C the membrane potential collapsed and that this collapse correlated with a sudden increase in membrane fluidity. The uptake of 2-deoxy-d-glucose and of pyruvate, which are both mediated by facilitated diffusion carriers in the plasma membrane of the trypanosome, were also affected by this sudden increase in fluidity of the membrane. The overall rate of the conversion of glucose into its metabolites, which is independent of the plasma membrane, varied only gradually. We conclude (i) that major changes occur in the plasma membrane of T. brucei around 26°C, that affect all membrane related processes; (ii) that the electrochemical proton gradient plays a minor role in the energy metabolism of T. brucei when it resides in the bloodstream of the mammalian host at 37°C; and (iii) that below 26°C an electrochemical proton gradient is maintained over the plasma membrane.