Exchange of HCO ⁻₃ for monovalent anions across the human erythrocyte membrane

Summary

A stopped-flow rapid reaction apparatus was used for measuring changes in extracellular pH (pH _o ) of red cell suspensions under conditions wheredpH _o /dt was determined by the rate of HCO ⁻₃ /X ⁻ exchange across the membrane (X ⁻=Cl⁻, Br⁻, F⁻, I⁻, NO ⁻₃ or SCN⁻). The rate of the exchange at 37°C decreased forX ⁻ in the order: Cl⁻>Br⁻>F⁻>I⁻>NO ⁻₃ >SCN⁻, with rate constants in the ratios 1∶0.86∶0.77∶0.55∶0.52∶0.31. When HCO ⁻₃ is exchanged for Cl⁻, Br⁻, F⁻, NO ⁻₃ or SCN⁻, a change in the rate-limiting step of the process takes place at a transition temperature (T _T ) between 16 and 26°C. In I⁻ medium, however, no transition temperature is detected between 3 and 42°C. AlthoughT _T varies withX ⁻, the activation energies both above and belowT _T are similar for Cl⁻, Br⁻, NO ⁻₃ and F⁻. The values of activation energy are considerably higher whenX ⁻=I⁻ or SCN⁻. The apparent turnover numbers calculated for HCO ⁻₃ /X ⁻ exchange (except forX ⁻=I⁻) at the correspondingT _T ranged from 140 to 460 ions/site ·sec for our experimental conditions. These findings suggest that: (i) HCO ⁻₃ /X ⁻ exchange for allX ⁻ studied takes place via the rapid anion exchange pathway; (ii) the rate of HCO ⁻₃ /X ⁻ exchange is influenced by the specific anions involved in the 1∶1 obligatory exchange; and (iii) the different transition temperatures in the Arrhenius diagrams of the HCO ⁻₃ /X ⁻ exchange do not seem to be directly related to a critical turnover number, but may be dependent upon the influence ofX ⁻ on protein-lipid interactions in the red blood cell membrane.