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

Abstract

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.