Temperature dependence of the exchange of monovalent anions in human red blood cells.

The temperature dependence of anion exchange across the red cell membrane was studied between 5 degrees C and 55 degrees C by measuring the rate of shrinkage of cells when transferred from a medium of pH 7.6 to one of pH 9.3 (as measured at 22 degrees C). The rates of shrinkage varied with the anion studied, the order being F-> Cl-> Br-> I-> SCN- but were faster in the presence of trace amounts of carbon dioxide than in its absence. NO3- was as fast as Cl- when carbon dioxide was present but comparable with I- when it was removed. Arrhenius plots of the rates were linear for all anions over the temperature range studied and gave the following apparent activation energies in kJ mol-1; F-, 67.7; NO3-, 68.4; Cl-, 70.2; Br-, 79.6; SCN-, 87.4 and I-, 95.1 in the presence of carbon dioxide. Inhibition of carbonic anhydrase with 5 microns ethoxzolamide and the removal of the carbon dioxide by degassing raised the activation energies to; F-, 124.8; NO3-, 129.0; Cl-, 141.5: Br-, 159.4; SCN-, 150.0 and I-, 185.6 kJ. mol-1. With the exception of F-, the apparent activation energies of the anions were linearly related to their thermochemical (dehydrated) radii in both cases. The relationship between the ionic radii and the energy of transfer suggests that anion exchange involves transfer through a hydrophobic pathway and that additional energy is required to overcome restrictions experienced in passing through the pathway. It is proposed that this, rather than a conformational change in the protein determines the activation energy of the process.