Estimation of Membrane Potential and pH Difference across the Cristae Membrane of Rat Liver Mitochondria

Abstract

The cristae membrane or M phase of resting or respiring rat liver mitochondria becomes relatively permeable to K+ ions in presence of valinomycin. The equilibrium distribution of K+ ions across the membrane can therefore be used to estimate the membrane potential ΔΨ provided that precautions are taken to minimise swelling of the valinomycin‐treated mitochondria. The pH difference ΔpH across the M phase of anaerobic mitochondria has been estimated from the buffering powers of the inner and outer phases and from the change of pH observed on lysing the mitochondria with Triton X‐100. When the anaerobic mitochondria (State 5), in presence of β‐hydroxybutyrate, are brought to a state of steady respiration either in absence (State 4) or in presence (State 3) of phosphate acceptor, the changes of pH and of pK of the medium recorded with H+ ion‐sensitive and K+ ion‐sensitive electrodes can be used to estimate the changes of ΔΨ and ΔpH across the M phase. The absolute values of ΔΨ and ΔpH have been estimated from the values determined in State 5 and the change of these values in the transition from State 5 to States 4 and 3. The total protonmotive force Δp =ΔΨ– 59 ΔpH across the M phase of the mitochondria oxidising β‐hydroxybutyrate in State 4 at 25° in a 250 mM sucrose medium near pH 7 is estimated to be about 230 mV, of which the major component is ΔΨ when the effect of translocation of K+ ions across the M phase is minimised. Under conditions permitting accumulation of a relatively large quantity of cation (State 6), Δp is not significantly different from that in State 4, but the major component is −Z ΔpH. The effects of changing Δp in mitochondrial suspensions in State 4 with uncoupling agent and with pulses of acid, alkali, calcium salt and ADP have been found to be in accord with the chemiosmotic hypothesis. In particular Δp in State 3 (ADP and Pi present) is estimated to be about 30 mV less than in State 4.