Rate limiting processes in the bohr shift in human red cells

Abstract

The rates of the Bohr shift of human red cells and some of its constituent reactions were studied with a modified Hartridge-Roughton rapid reaction apparatus using an O2 electrode to measure the progress of the reaction. The rate of the Bohr shift was compatible with the hypothesis that the transfer of H+ across the membrane by means of CO2 exchange and reaction with buffers is generally the rate-limiting step. When the Bohr off-reaction produced by a marked increase in PCO2 around the cells, the half-time at 37[degree]C was 0.12 sec. In this case CO2 was available initially to diffuse into the cells, the process being predominantly limited by the rate of intracellular CO2 hydration. When the Bohr off-shift was produced by an increase of [H+] outside the cell, PCO2 being low and equal within and outside the cells, the half time became 0.31 sec. In this case, even at the start, the H2CO3 formed by the almost instantaneous neutralization reaction of H+ and HCO3[long dash] had to dehydrate to form CO2 and this in turn had to diffuse into and react within the red cell before the [HbO2] could change. When a carbonic anhydrase inhibitor was added to slow the CO2 reaction inside the cell, the half-time rose to 10 sec. The Bohr off-shift in a hemolysed cell suspension produced by an increase in PCO2 appeared to be limited by the rate at which the CO2 could hydrate to form H+. The Bohr off-shift has an average Q10 of 2.5 between 42.5 and 28[degree]C with an activation energy of 8000 cal. The pronounced importance of the CO2-bicarbonate system for rapid intracellular pH changes is discussed in connexion with some physiological situations.